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Evidence of Nitrate Attenuation in Intertidal and Subtidal Groundwater in a Subterranean Estuary at a Cape Cod Embayment, East Falmouth, Massachusetts, 2015–16

Thomas G. Huntington, Kevin D. Kroeger, Timothy D. McCobb, J.K. Böhlke, John A. Colman, Thomas W. Brooks · U.S. Geological Survey

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usgs, geological survey, united states, science, government publication, usgs scientific investigations report, 2024, 5100, evidence, nitrate, attenuation, intertidal, subtidal, groundwater, subterranean, estuary, cape, cod, embayment, east, falmouth, massachusetts, 2015

Environmental Health Program Coastal/Marine Hazards and Resources Program Prepared in cooperation with the U.S. Environmental Protection Agency, Office of Research and Development and Region 1 (New England)

Evidence of Nitrate Attenuation in Intertidal and Subtidal Groundwater in a Subterranean Estuary at a Cape Cod Embayment, East Falmouth, Massachusetts, 2015–16

Scientific Investigations Report 2024–5100

U.S. Department of the Interior U.S. Geological Survey

Cover. Scientists collecting samples on the shore of the Eel River, looking southwest in Falmouth, Massachusetts. Photograph by Timothy D. McCobb, U.S. Geological Survey.

Evidence of Nitrate Attenuation in Intertidal and Subtidal Groundwater in a Subterranean Estuary at a Cape Cod Embayment, East Falmouth, Massachusetts, 2015–16 By Thomas G. Huntington, Kevin D. Kroeger, Timothy D. McCobb, J.K. Böhlke, John A. Colman, Thomas W. Brooks, and Beata Szymczycha

Scientific Investigations Report 2024–5100

U.S. Department of the Interior U.S. Geological Survey

U.S. Geological Survey, Reston, Virginia: 2024

For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment—visit https://www.usgs.gov or call 1–888–392–8545. For an overview of USGS information products, including maps, imagery, and publications, visit https://store.usgs.gov/ or contact the store at 1–888–275–8747. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner. Suggested citation: Huntington, T.G., Kroeger, K.D., McCobb, T.D., Böhlke, J.K., Colman, J.A., Brooks, T.W., and Szymczycha, B., 2024, Evidence of nitrate attenuation in intertidal and subtidal groundwater in a subterranean estuary at a Cape Cod embayment, East Falmouth, Massachusetts, 2015–16: U.S. Geological Survey Scientific Investigations Report 2024–5100, 45 p., https://doi.org/10.3133/sir20245100. Associated data for this publication: Huntington, T.G., Colman, J.A., McCobb, T.D., Kroeger, K.D., Bohlke, J.K., Brooks, T.W., and Syzmczycha, B., 2024, Geochemical data supporting analysis of fate and transport of nitrogen in the nearshore groundwater and subterranean estuary near East Falmouth, Massachusetts, 2015–2016: U.S. Geological Survey data release, https://doi.org/10.5066/P13LGNTT. ISSN 2328-0328 (online)

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Acknowledgments The support and cooperation of the Seacoast Shores Association and the residents near the study site are deeply appreciated. The laboratory and field support of Mark White, Warren Boothman, Jim Latimer, Jerry Keefe, Mary Garren, and Daniel Granz of the U.S. Environmental Protection Agency, and Marcel Belaval, Douglas Kent, Denis LeBlanc, Jason Sorenson, Jennifer Savoie, Phillip Woodford, Adrian Mann, and Janet Hannon of the U.S. Geological Survey are gratefully acknowledged.

Contents Acknowledgments��iii Abstract��1 Introduction��2 Geographic, Geologic, and Hydrologic Setting��4 Previous Investigations and Conceptual Flow Model��4 Seacoast Shores Groundwater System��4 Cape Cod Subterranean Estuary��6 Conceptual Model of the Subterranean Estuary Flow��6 Fate of Nitrogen in the Subterranean Estuary at Seacoast Shores Peninsula��8 Well Installation��8 Measurement of Hydraulic Head and Interpretation of Flow Direction��11 Water-Quality Sampling and Laboratory Analyses��11 Determination of Nitrogen Attenuation��12 Hydrogeologic and Geochemical Observations��12 Hydraulic-Head Gradients during Low, High, and Mean Tides��13 Chemical Characteristics of Shallow Groundwater in the Nearshore Subterranean Estuary��13 Specific Conductance��13 Dissolved Oxygen��14 pH��14 Alkalinity��14 Nitrate Plus Nitrite��26 Ammonium��26 Total Kjeldahl Nitrogen and Organic Nitrogen��27 Dissolved Organic Carbon��27 Concentrations and Ratios of Nitrogen and Argon Gases��27 Evaluation of Nitrate Reduction to Nitrogen Gas��27 Patterns and Controls of Nitrogen Transport and Attenuation��34 Summary��37 References Cited��39

Figures 1.

Map showing coastal embayments in southern Falmouth, regional water-table contours, and location of the study area at the Eel River and Seacoast Shores peninsula in East Falmouth, Massachusetts��5 2. Schematic diagram showing conceptual model of the subterranean estuary at Waquoit Bay, East Falmouth, Massachusetts��6 3. Section B–B′ showing distribution of specific conductance and measured nitrate plus nitrite in groundwater and selected features of the conceptual model of nitrogen transport and attenuation, including estimated nitrate loss at selected points, along transect B–B′ beneath the Eel River, Seacoast Shores peninsula, East Falmouth, Massachusetts, September 5–25, 2013��7

4. 5.

6. 7.

10.

11.

12.

13.

14.

Map showing locations and types of monitoring sites near the western shore of the Eel River, Seascoast Shores peninsula, East Falmouth, Massachusetts��9 Vertical transects showing locations of monitoring sites, land surface, and approximate altitudes of mean high tide and mean low tide for transects A–A′, B–B ′, C–C ′, and D–D ′ near the eastern shore of the Eel River, Seacoast Shores peninsula, East Falmouth, Massachusetts��10 Vertical sections showing water-level altitudes measured in drive-point wells and in the Eel River at low tide, high tide, and mean tide, October 24–26, 2006��15 Vertical transects showing distributions of specific conductance, dissolved oxygen, pH, alkalinity, nitrate plus nitrite, ammonium, total Kjeldahl nitrogen, dissolved organic carbon, nitrogen gas, argon gas, and ratio of nitrogen gas to argon gas in groundwater along transect A–A′ beneath the Eel River, Seacoast Shores peninsula, East Falmouth, Massachusetts, June 15–16, 2016��16 Vertical transects showing distributions of specific conductance, dissolved oxygen, pH, alkalinity, nitrate plus nitrite, ammonium, total Kjeldahl nitrogen, dissolved organic carbon, nitrogen gas, argon gas, and ratio of nitrogen gas to argon gas in groundwater along transect B–B′ beneath the Eel River, Seacoast Shores peninsula, East Falmouth, Massachusetts, November 9–10, 2015��19 Vertical transects showing distributions of specific conductance, dissolved oxygen, pH, alkalinity, nitrate plus nitrite, ammonium, total Kjeldahl nitrogen, dissolved organic carbon, nitrogen gas, argon gas, and ratio of nitrogen gas to argon gas in groundwater along transect D–D′ beneath the Eel River, Seacoast Shores peninsula, East Falmouth, Massachusetts, June 14–15, 2016��22 Graphs showing tidal stage and shallow nearshore groundwater sampling days during neap tide during November 2015 sampling, June 2016 sampling, and October 2016 sampling��25 Graph showing relation between nitrogen gas and argon gas concentrations from transects A, B, C, and D in the nearshore subterranean Eel River estuary adjacent to the Seacoast Shores peninsula, East Falmouth, Massachusetts��29 Graphs showing relations between salinity and nitrogen gas concentration (N2), argon gas concentration (Ar), N2/Ar ratio, calculated concentration of microbial N2 (N2MIC), and N2/Ar ratios in samples with N2MIC concentrations greater or less than the reporting limit of 25 µmol/L in groundwater from transects A–A′, B–B′, C–C ′, and D–D′ in the nearshore subterranean Eel River estuary adjacent to the Seacoast Shores peninsula, East Falmouth, Massachusetts, November 2015–October 2016��31 Mixing plots showing measured nitrate concentration, calculated nitrate loss assuming denitrification, and estimated initial nitrate concentration, versus salinity in samples that were analyzed for argon and nitrogen gas from all transects beneath the Eel River, Seacoast Shores peninsula, East Falmouth, Massachusetts��32 Vertical transects showing distribution of salinity and measured nitrate plus nitrite in groundwater including estimated nitrate loss at selected points, along northernmost transect A–A′, middle transect B–B′, and southernmost transect D–D′ beneath the Eel River, Seacoast Shores peninsula, East Falmouth, Massachusetts��35

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Table 1.

Summary statistics for dissolved gas concentrations and derived quantities in “Fresh,” “Saline,” and “Mixed” sample groups��33

Conversion Factors International System of Units to U.S. customary units

Multiply

To obtain

Length centimeter (cm)

0.3937

inch (in.)

millimeter (mm)

0.03937

inch (in.)

meter (m)

3.281

foot (ft)

kilometer (km)

0.6214

mile (mi)

Area square meter (m2)

0.0002471

acre

hectare (ha)

0.003861

square mile (mi2)

square kilometer (km2)

0.3861

square mile (mi2)

Volume liter (L) milliliter (mL)

0.2642

gallon (gal)

0.03381

ounce, fluid (fl. oz)

Flow rate centimeter per year (cm/yr)

0.3937

inch per year (in/yr)

cubic meter per day (m3/d)

264.2

gallon per day (gal/d)

millimeter per year (mm/yr)

0.03937

inch per year (in/yr)

gram (g)

0.03527

ounce, avoirdupois (oz)

2.205

pound avoirdupois (lb)

Mass kilogram (kg)

Hydraulic conductivity meter per day (m/d)

3.281

foot per day (ft/d)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows: °F = (1.8 × °C) + 32. Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows: °C = (°F – 32) / 1.8.

Datum Vertical coordinate information is referenced to the National Geodetic Vertical Datum of 1929 (NGVD 1929). Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83). Altitude, as used in this report, refers to distance above the vertical datum.

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Supplemental Information Specific conductance is given in millisiemens per centimeter at 25 degrees Celsius (mS/cm at 25 °C) or microsiemens per centimeter at 25 degrees Celsius (µS/cm at 25 °C). Specific conductance values were converted to practical salinity units (psu) by using the GSW (Gibbs Sea Water Functions) (version 3.05) package in R, http://www.teos-10.org/pubs/gsw/html/gsw_ SP_from_C.html, accessed September 29, 2022. Concentrations of chemical constituents in water are given either in micromoles per liter (µmol/L), microequivalents per liter (µeq/L), practical salinity units (psu), milligrams per liter (mg/L), or micrograms per liter (µg/L). Concentrations of nitrate plus nitrite (NO3 + NO2) are reported as micromoles per liter (µmol/L) nitrate-nitrogen (NO3-N) unless noted otherwise because nitrite concentration was usually small. Excess air concentrations are reported in units of cubic centimeters of gas at standard temperature (0 °C) and pressure (1 atmosphere) per liter of water (ccSTP/L). Site identifiers use shorthand “F” for “MA–FSW” unless noted otherwise. MA refers to Massachusetts and FSW refers to stations in Falmouth, Massachusetts.

Abbreviations δ15N

delta nitrogen-15

anammox anaerobic ammonium oxidation Ar

argon gas

ASW

air-saturated water

CaCO3

calcium carbonate

ccSTP/L

cubic centimeters of gas at standard temperature and pressure per liter of water

DNRA

dissimilatory nitrate reduction to ammonium

dissolved oxygen as O2

DOC

dissolved organic carbon

DSD

deep saltwater discharge

DSI

deep saltwater infiltration

DSW

deep saltwater wedge

excess air

Fe(II)

ferrous iron

FGD

fresh groundwater discharge

gas chromatography

ISC

intertidal saltwater cell

ISD

intertidal saltwater discharge

ISI

intertidal saltwater infiltration

MHT

mean high tide

MIC

microbially produced

MLS

multilevel sampler

MLT

mean low tide

nitrogen gas

N2-NMIC

concentration of microbially produced N2 gas, as N

N2O

nitrous oxide

N2MIC

concentration of microbially produced N2 gas, as N2

nitric oxide

NO3 + NO2 nitrate plus nitrite NWIS

National Water Information System

oxygen gas

psu

practical salinity units

PVC

polyvinyl chloride

SpC

specific conductance

STP

standard temperature and pressure

TMDL

total maximum daily loads

USGS

U.S. Geological Survey

Abstract Nitrogen dynamics in intertidal and nearshore subtidal groundwater (subterranean estuary) adjacent to the Seacoast Shores peninsula, Falmouth, Massachusetts, were investigated during 2015–16 by the U.S. Geological Survey. The peninsula is a densely populated residential area with septic systems and cesspools that are substantial sources of nitrogen to groundwater. The study area is in the Eel River, an estuarine saltwater embayment connected to the ocean adjacent to the western shore of the peninsula, that was the subject of an earlier study by Colman and others (2018, https://doi.org/ 10.3133/sir20185095) on nitrogen transport and transformations in groundwater between onshore and offshore locations. The previous study documented the distribution of nitrate concentrations and nitrate attenuation reactions in fresh groundwater beneath the peninsula and the estuary. The current study extended those observations with more detailed sampling and analysis of shallow groundwater from wells near discharge sites beneath the estuary. The current field investigation included sampling of existing wells and installation and sampling of clusters of wells and temporary sampling points in the subterranean estuary, including (1) shallow transects 0.3 to 1.2 meters (m) deep extending from 1 to 13.5 m offshore and (2) deeper wells (from 1.83 to 4.88 m deep) extending from 4.3 to 14.3 m offshore. Measurements of hydraulic-head gradients 2–5 m below the sediment/water interface in the intertidal and nearshore subtidal zones indicated that groundwater flow generally was upwards (towards the estuary) under all tide conditions in October 2016. The magnitude of the gradient was greatest during low tide conditions, indicating that groundwater discharge likely decreased during high tides. Measurements of specific conductance in shallow groundwater in the subterranean estuary in three transects perpendicular to shore were consistent with the existence of saltwater flow cells (infiltration of overlying saline water, mixing with fresh groundwater, and discharge to the overlying saline water) in the intertidal and nearshore subtidal regions. The size of these flow cells was variable in space and time and

dependent on the elevation of the tide (spring or neap). At this location in the Eel River subterranean estuary, and offshore to at least 13.5 m, offshore flow of fresh groundwater apparently prevented a deeper saltwater wedge from discharging to the surface. Nitrate concentrations in shallow groundwater (30 to 122 centimeters [cm] depth) were variable in space and time, ranging from not detectable to 600 micromoles per liter (µmol/L) (8.4 milligrams per liter as N), and were highest in June 2016 at depths from 61 to 122 cm below the sediment/ water interface and from 4 to 9 m offshore. Nitrate generally was not detectable in saline shallow groundwater at 30-cm depth or at any depth from 30 to 122 cm from 10 to 13.5 m offshore. Dissolved oxygen concentrations were suboxic (less than 16 µmol/L) in 60 percent of the sampled subterranean groundwater beneath the intertidal and subtidal zones. In the remaining sites, the range of dissolved oxygen concentrations was from 18 to 272 µmol/L and the median concentration was 43 µmol/L. Evidence for microbial nitrate reduction (denitrification and possibly anammox) was provided by the distribution of the reaction product nitrogen gas (excess N2, or N2MIC), as determined from analysis of the dissolved nitrogen gas and argon gas (Ar) concentrations in groundwater samples. Excess nitrogen gas provided evidence for nitrate reduction in shallow groundwater below the subtidal and, to a lesser extent, intertidal zones adjacent to the Seacoast Shores peninsula. These zones, where evidence for nitrate reduction was detected, were in fresh and brackish groundwater near subtidal or intertidal saltwater cells where discharging fresh groundwater mixed with infiltrating saline water. Infiltrating seawater may have supplied organic carbon, one of several potential electron donors that are required for denitrification. Other potential electron donors, such as organic carbon, iron, manganese, hydrogen, methane, ammonium, elemental sulfur, or sulfide phases, may have been supplied by the estuarine sediments. Drainage from surface runoff near the shore also may have supplied organic carbon to fresh groundwater near the intertidal saltwater cell.

2 Evidence of Nitrate Attenuation in Intertidal and Subtidal Groundwater in a Subterranean Estuary at a Cape Cod Embayment The highest amounts of nitrate converted to excess nitrogen gas were estimated to be in the range of 230 to 430 µmol/L in nearly fresh groundwater near the subtidal saltwater cell at depths of 61 to 122 cm below the sediment/ water interface and from 10 to 13.5 m offshore. Evidence of denitrification within 10 m of the shore was sparse (generally limited to less than 50 µmol/L of N2-N) despite the presence of high nitrate concentrations. The spatial distribution of estimated nitrate reduction in the intertidal and nearshore subtidal fresh and brackish groundwater may be related to local variability in the distribution of reactive electron donors in those zones. Variations in the amount of nitrate reduction to nitrogen gas were not clearly related to potential aqueous electron donors such as dissolved organic carbon, nor to potential reaction products such as alkalinity, but may have been controlled by combinations of aqueous and solid-phase reactants. The distribution of relatively shallow fresh groundwater containing nitrate could indicate potential nitrate discharge areas in the lower intertidal zone and uncertain locations farther offshore; however, the data did not extend all the way to the sediment/water interface or to the offshore freshwater limit. This study confirmed substantial loss of nitrate from some of the fresh and brackish groundwater in shallow subestuarine sediments prior to discharge but did not quantify how much nitrate eventually discharged to the estuary.

Introduction Human-caused eutrophication in coastal ecosystems and the need to understand the fate of excess nitrogen is a complex global problem that is influenced by ongoing climate change and changes in atmospheric deposition of nitrogen (Paerl and others, 2014; Cloern and others, 2016; Valiela and others, 2016). There is substantial interest and urgency in understanding the fate and transport of nitrogen that is released from septic systems in high-density residential neighborhoods on Cape Cod that subsequently infiltrates the nearshore groundwater and subterranean estuaries in multiple embayments. This nitrogen is a pollutant that at sufficient concentrations and under certain conditions can cause eutrophication, macroalgal blooms, and associated environmental problems. Nitrogen loading to coastal embayments on Cape Cod, Massachusetts, is one of the more pressing environmental challenges faced by coastal communities in the northeastern United States (Valiela and others, 1992; Nixon, 1995; Short and Burdick, 1996; Valiela and others, 2000; Howarth and others, 2002; Valiela and others, 2016). External nitrogen loading increases the growth of phytoplankton and macroalgae in coastal marine systems in the northeastern United States (Howarth, 1988; Valiela, 2006). In recent decades, the relative contribution to nitrogen loading from wastewater inputs has increased while the

contribution of atmospherically deposited nitrogen has decreased (Valiela, Collins, and others, 1997; Valiela, McClelland, and others, 1997). Increased supply of nitrogen causes decreased water clarity from increased algal growth and encourages the growth of epiphytes on submerged aquatic vegetation that reduces light penetration to the detriment of more desirable plant species, for example, eel grass, and damages ecosystems favoring finfish and shellfish (Thomsen and others, 2012; Rasmussen and others, 2013; Nelson, 2017). Nitrogen inputs to coastal ecosystems also are partially responsible for ocean acidification through biogeochemical processes associated with eutrophication (Cai and others, 2011; Gledhill and others, 2015; Rheuban and others, 2019) that can harm vulnerable calcifying species that are important fisheries in the northeastern United States (Ekstrom and others, 2015; Hare and others, 2016). Nitrogen additions also can stimulate microbial decomposition of organic matter in coastal marsh sediments, potentially reducing their capacity for carbon sequestration (Bulseco and others, 2019). Because tourism, retirement, and fishing are the mainstays of the Cape Cod economy (Cape Cod Chamber of Commerce, 2013), degradation of the coastal aquatic ecosystems is an important issue on Cape Cod. In 2000, recognizing the importance of nitrogen in eutrophication of Cape Cod waters, the Massachusetts Department of Environmental Protection established the Massachusetts Estuaries Project to evaluate nitrogen loading and capacity in 89 Cape Cod embayments (Massachusetts Department of Environmental Protection, 2018). In approximately one-half of the embayments to date, these evaluations have established regulatory numeric nitrogen loading limits, or total maximum daily loads (TMDLs) (Massachusetts Department of Environmental Protection, 2018). Currently, Massachusetts towns covered by the Massachusetts Estuaries Project are implementing nitrogen remediation actions that would meet the TMDL requirements. On Cape Cod, principal sources of nitrogen are from onsite disposal of domestic sewage (through cesspools and septic systems) and lawn fertilizer (Valiela and others, 2000). Nitrogen transport from these sources to coastal water is largely through groundwater because streams are uncommon on the sand deposits of the narrow peninsulas. In much of the Cape Cod aquifer, where subsurface conditions are aerobic and the predominant form of nitrogen is nitrate, transport is likely to be conservative. Aerobic onsite wastewatertreatment systems (septic system leaching fields) have been shown to be inefficient at removing inorganic nitrogen (Costa and others, 2002; Van Cuyk and others, 2001). Along the flow path from onshore groundwater to submarine groundwater discharge sites, some portion of inorganic nitrogen commonly is attenuated by conversion to gaseous forms but a substantial fraction can be transported through the subsurface to the estuary (Valiela and others, 1992; Paerl, 1997; Charette and Sholkovitz, 2002). For example, nitrate reduction can occur where groundwater encounters onshore or offshore anaerobic sediments containing organic carbon or

Introduction 3 other electron donors (originating from infiltrating seawater or decomposition of particulate phases). Nitrogen attenuation reactions are not considered in the conservative assumptions used in determining TMDLs (Massachusetts Department of Environmental Protection, 2018) and are less well documented in the subterranean estuary than in many other groundwater settings affected by agricultural and wastewater nitrogen inputs. Where subsurface conditions are anaerobic, nitrate, nitrite, and ammonium can be attenuated, that is, converted to gaseous forms (nitric oxide [NO], nitrous oxide [N2O], and nitrogen gas) through the bacterially mediated processes of denitrification and anaerobic ammonium oxidation (anammox) (Thamdrup and Dalsgaard, 2002; Strous and Jetten, 2004). Nitrate also can be microbially converted to ammonium through the process of dissimilatory nitrate reduction to ammonium (DNRA) (Giblin and others, 2013). Recently, it has been shown that bacterial denitrification occurs in sandy coastal sediments where tidal cycles resulted in fluctuating oxic to anoxic conditions (Marchant and others, 2017). Denitrification generally refers to microbial respiration, whereby nitrate nitrogen is reduced to gaseous forms of nitrogen (N2O and N2) in response to the oxidation of an electron donor such as organic matter, sulfide, methane, or reduced iron oxides (Knowles, 1982). Nitrogen gas can be released to the atmosphere or reduced to ammonia through nitrogen fixation in coastal sediments (Newell and others, 2016). One of the intermediate gaseous products of bacterial denitrification, nitric oxide that is produced from nitrite, can be chemically denitrified in the presence of Fe(II) or denitrified by fungi; in both cases, it is converted to N2O that can subsequently be released to the atmosphere (Wankel and others, 2017). Denitrification is common in aquatic anaerobic or suboxic environments in freshwater, wetlands, and estuarine and marine waters, as well as groundwaters where geochemical conditions are suitable (Smith and others, 1991; Korom, 1992; Seitzinger and others, 2006). Denitrification was reported to be an important nitrate reducing mechanism in a nitrate-contaminated aquifer on Cape Cod (Smith and Duff, 1988; Smith and others, 2004). In DNRA, microbes oxidize organic matter and use nitrate as an electron acceptor for respiration, reducing it first to nitrite and then to ammonium. Denitrification and DNRA compete for nitrate. Denitrification is usually the dominant nitrate reduction process; however, certain factors can favor DNRA (Giblin and others, 2013). DNRA may be favored when concentrations of organic matter are high and concentrations of nitrate are low (Tiedje and others, 1983; Dong and others, 2011) or when salinity increases (Giblin and others, 2010). DNRA retains inorganic nitrogen in the sediment as ammonium, which could later be converted back to nitrate in an aerobic environment. In anammox, ammonium and nitrite are converted into nitrogen gas, and nitrite is used as a terminal electron acceptor (Thamdrup and Dalsgaard, 2002; Strous and Jetten, 2004). The anammox reaction does not require

organic carbon, but it does require nitrite that, in anaerobic environments, would most likely be available if nitrate was present and anammox was coupled with ongoing denitrification or DNRA (where nitrite was produced). Similarly, fungal-mediated denitrification or abiotic (Fe(II)-mediated chemodenitrification) reactions (Wankel and others, 2017) converting nitrite to N2O require nitrate conversion to nitrite. Gaseous forms of nitrogen are released to the atmosphere and therefore are unavailable to algae, potentially mitigating eutrophic conditions. Conditions conducive to attenuation and losses of inorganic nitrogen have been documented in landfill plumes and in large plumes formed by land disposal of municipal sewage (Smith and others, 1991; Christensen, and others, 1994; DeSimone and Howes, 1995; Repert and others, 2006) and in many other aquifer settings. Denitrification and anammox processes have been documented in a wastewater-contaminated plume in the Cape Cod aquifer system (Smith and Duff, 1988; Smith and others, 2004, 2015). The current study was conducted to address questions that arose in a previous study at the Eel River, a coastal embayment in East Falmouth, Massachusetts, on western Cape Cod (Colman and others, 2018). The goal of the previous study was to identify pathways of nitrogen transport and areas of potential nitrogen attenuation in the subterranean estuary through bacterially mediated denitrification or anammox. Colman and others (2018) found evidence for nitrate reduction in some portions of the subterranean estuary but concluded that further study was needed to determine the distribution of nitrate reduction in shallow groundwater below the intertidal and nearshore subtidal zones. The goal of this follow-up project was to investigate nitrogen attenuation in the subterranean estuary below the intertidal and nearshore subtidal zones at the same study site to refine the conceptual model of nitrogen attenuation at the Eel River subterranean estuary. This study includes geochemical and hydraulic information in the subterranean environment of the intertidal and adjacent subtidal zones with a focus on the sediment/water interface and shallow offshore zones where fresh and saline water mix. This report includes details of field investigations, analytical methods, results, and discussion of geochemical indicators of nitrate reduction. The results presented in this report may help inform the design of future nitrogen remediation investigations in peninsular, coastal areas with high-housing density adjacent to saline surface water that may be targeted for sewer installation to prevent coastal eutrophication.

4 Evidence of Nitrate Attenuation in Intertidal and Subtidal Groundwater in a Subterranean Estuary at a Cape Cod Embayment

Geographic, Geologic, and Hydrologic Setting The Seacoast Shores neighborhood occupies a peninsula that extends southward from the mainland area of Cape Cod towards Vineyard Sound (fig. 1). The peninsula is about 2.6 kilometers (km) long and as much as 615 meters (m) wide and is bordered by saline tidal surface-water bodies that include the Eel River on the west and Eel Pond and the Childs and Seapit Rivers on the south and east. Land-surface altitudes generally decrease from about 10 m on the north to less than 1 m on the south. The topography generally is flat, with steep bluffs along the sides of the peninsula and a few low-lying areas in the interior. The peninsula and its adjacent narrow saltwater bays are similar to other peninsulas and bays along the southwestern shore of Cape Cod near East Falmouth. The bays occupy valleys that were cut into the glacial outwash plain at the end of the Pleistocene Epoch. The bays were subsequently drowned when sea level rose, leaving the intervening areas of the outwash plain as long, narrow peninsulas interspersed with coastal embayments. The aquifer materials are glacial outwash sand and gravel that generally grades downward into fine-grained glaciolacustrine sand, silt, and clay (Oldale, 1992; Masterson and others, 1997; Hull and others, 2019). The unconsolidated sediments are about 90 m thick and lie on crystalline, granitic bedrock (Fairchild and others, 2013). The Seacoast Shores peninsula is a part of the Sagamore flow lens, a part of the Cape Cod aquifer system consisting of several hydrogeologic units (outwash plains, terminal and ground moraines, and ice contact deposits) primarily composed of sand and gravel, with some silt and clay. The units were deposited during the late glaciation of New England. The regional aquifer is a shallow, unconfined hydrologic system in which groundwater flows radially outward from the top of a water-table mound that underlies western Cape Cod (Masterson and others, 1997; Walter and Whealan, 2005; Walter and others, 2019). The horizontal hydraulic conductivity of the sand and gravel is estimated to be about 60–110 meters per day and that of the interbedded fine sands and silts in the aquifer less than 15 meters per day (Walter and Whealan, 2005; Hull and others, 2019). The study area is located along the southern coastline, where the regional groundwater system discharges to Vineyard Sound. The peninsular freshwater aquifer is bounded at the top by the water table, at the bottom by the freshwater/saltwater interface, and at the lateral boundaries by the tidal saltwater bodies. The freshwater aquifers beneath the peninsulas are seaward extensions of the much larger regional groundwater-flow system that also discharges to the tidal saltwater bodies. The Seacoast Shores peninsula receives an estimated 114 centimeters per year (cm/yr) of precipitation. Slightly more than one-half of the precipitation recharges the aquifer at the water table (LeBlanc and others, 1986); the remainder is lost to evapotranspiration. Surface runoff is negligible owing to

the sandy soils and low topographic relief of the area. Water supply is public water obtained from wells and a lake inland from the peninsula. Data from the Cape Cod Commission (2015) indicated that about 1,000 dwellings on the peninsula account for about 10.5 cm/yr of wastewater disposal averaged over the total 1.2 square kilometer (km2) area of the peninsula (Colman and others, 2018). The concentration of nitrate in domestic wastewater reaching groundwater on Cape Cod is estimated to be about 3,000 micromoles per liter (µmol/L) (Eichner and Cambareri, 1992). Colman and others (2015) estimated that the volume-averaged concentration of dissolved nitrogen in groundwater in the Seacoast Shores peninsula based on wastewater inputs and dilution by recharge from precipitation should be about 500 µmol/L. These estimates indicate an annual loading of 3,675 kg N/km2 to the aquifer below the Seacoast Shores peninsula. Colman and others (2018) summarized the historical residential development on the Seacoast Shores and the legacy of onsite domestic wastewater disposal that led to contamination of the subterranean aquifer with nitrogen.

Previous Investigations and Conceptual Flow Model Previous studies on Cape Cod have examined the Seacoast Shores groundwater system, the Cape Cod subterranean estuary, tidal mixing at Waquoit Bay, and the fate of nitrogen in the subterranean estuary at the Seacoast Shores peninsula.

Seacoast Shores Groundwater System In a previous study, the Cape Cod Commission installed monitoring wells and collected sediment and groundwater samples at three sites along an east-west transect across the Seacoast Shores peninsula to examine nitrogen concentrations in shallow groundwater in an area served by onsite wastewater disposal (Thomas Cambareri, Cape Cod Commission, oral presentation for Waquoit Bay National Estuarine Research Reserve Research Exchange Day, 1994). Details of that investigation were reviewed by Colman and others (2018). The borings encountered mostly medium to coarse sand. The altitude of the water table at three sites was approximately 0.27 meters (m) above sea level (National Geodetic Vertical Datum of 1929 [NGVD 1929]). The freshwater lens overlying saltwater was about 14 m thick near the western side and middle of the peninsula where the present study was conducted and about 8 m thick at the eastern site. Elevated nitrate concentrations, as high as 1,200 micromoles per liter (µmol/L), were measured in the deepest fresh groundwater (about 11 m below the water table and immediately above the freshwater/saltwater interface in an onshore well about 20 m from the shore) (Colman and others, 2018, figs. 3 and 5).

Previous Investigations and Conceptual Flow Model 5 73°20'

70°00'

Atlantic Ocean

42°30'

MASSACHUSETTS

Cape Cod 41°40'

20 MILES

0 10 20 KILOMETERS

70°35'

70°30'

18 16 14 12 10

41°35'

Chi lds Riv it R er ive r

East Falmouth

Eel River

Study area

Seacoast Shores Waquoit Bay 2

Eel Pond

41°33'

Bournes Pond

EXPLANATION

Green Pond

Little Pond Great Pond

Base from U.S. Geological Survey and Massachusetts Office of Geographic Information digital data, 1:25,000 scale, 1991 National Geodetic Vertical Datum of 1929 North American Datum of 1983

Vineyard Sound

Simulated water-table altitude from Walter and others (2019), in feet above National Geodetic Vertical Datum of 1929—Interval is 2 feet 0.5

0 0

0.5

1 MILE 1 KILOMETER

Figure 1. Map showing coastal embayments in southern Falmouth, regional water-table contours, and location of the study area at the Eel River and Seacoast Shores peninsula in East Falmouth, Massachusetts.

pem24-0030_fig01

6 Evidence of Nitrate Attenuation in Intertidal and Subtidal Groundwater in a Subterranean Estuary at a Cape Cod Embayment

Cape Cod Subterranean Estuary The subterranean estuary is a zone near the coastline and below the sediment/water interface where freshwater and saltwater mix and where biotic and abiotic reactions can occur (Moore, 1999). Groundwater must pass through the subterranean estuary to move from land to offshore surface water. Past investigations of the subterranean estuary on Cape Cod focused primarily on the upper end of Waquoit Bay, Falmouth, Massachusetts, and Salt Pond, Eastham, Massachusetts, and were briefly reviewed by Colman and others (2018). Of these investigations, only the work at Waquoit Bay included estimation of nitrogen loss in the subterranean estuary, and results there varied. Where nitrate attenuation was observed (Kroeger and Charette, 2008), it occurred in regions of mixing between nitrate-bearing, suboxic, fresh groundwater and ammoniumbearing near-anoxic fresh or saline groundwater. Whether nitrate attenuation occurs in a subterranean estuary may depend on specific geochemical and hydrologic conditions that are spatially, seasonally, and interannually variable.

Conceptual Model of the Subterranean Estuary Flow The physics of tidal mixing have been investigated at Waquoit Bay by using two-dimensional computer models calibrated by seepage-meter, subsurface-salinity, and hydraulic-head measurements (Michael and others, 2003, 2005; Abarca and others, 2013). The modeling efforts from previous investigations at Waquoit Bay have led to a conceptual flow model for the subterranean estuary (fig. 2). In the conceptual model, fresh or mixed brackish groundwater

discharge (FGD) is focused in the lower intertidal zone. Intertidal saltwater discharge (ISD) from an intertidal saltwater cell (ISC) formed by intertidal saltwater infiltration (ISI) occurs just landward of the FGD. Deep saltwater discharge (DSD) from a deep saltwater wedge (DSW) occurs seaward of the FGD. Farther offshore, deep saltwater infiltration (DSI) occurs seaward of the deep saltwater discharge. The ISC is tidally driven such that the cell is filled by ISI during high tides and discharges saltwater (ISD) as the tide recedes. This tidally driven recirculation is sensitive to the amplitude of the tides, being greatest during spring tides and waning or disappearing during neap tides (Abarca and others, 2013). Colman and others (2018) showed that the conceptual model for Waquoit Bay does not fully explain groundwater flow at the current study location in the Eel River embayment. In the Eel River embayment, Colman and others (2018) found evidence for FGD in the intertidal zone and just seaward of the intertidal zone; however, they did not find evidence for a deep saltwater wedge (DSW) driven by DSI and DSD (fig. 3A). Instead, their measurements indicated broader distribution of deeper fresh groundwater beneath the estuary that extended offshore as far as they sampled (about 18 m offshore), consistent with geophysical transient electromagnetic surveys along the entire length of the Eel River (Lane and others, 2020). Lane and others (2020) reported fresher groundwater extending from the surface sediment to at least 10 m depth underlain by an apparent transition toward more saline groundwater along a north-tosouth track line near the center of the river at the current study location. These findings and other analyses, which are described in the section “Hydrogeologic and Geochemical Observations,” suggest a different conceptual model for fresh groundwater flow in the Eel River subterranean estuary, as illustrated in figure 3.

Intertidal saltwater infiltration (ISI)

Water table

Intertidal saltwater discharge (ISD)

High tide Low tide

Freshwater

Intertidal saltwater cell (ISC)

ter/ F r esh wa

salt wate

Deep saltwater discharge (DSD) Deep saltwater infiltration (DSI)

r mix Deep saltwater wedge (DSW) Saltwater

Schematic not to scale Figure 2. diagram, Schematic diagram showing conceptual model of the subterranean estuary at Waquoit Bay, East Falmouth, Massachusetts (modified from Abarca and others, 2013; copyrighted; used with permission). DSD, deep saltwater discharge; DSI, deep saltwater infiltration; DSW; deep saltwater wedge; FGD, fresh groundwater discharge; ISC, intertidal saltwater cell; ISD, intertidal saltwater discharge; ISI, intertidal saltwater infiltration; arrows indicate inferred directions of groundwater flow.

Previous Investigations and Conceptual Flow Model 7 A. Specific conductance B

EAST

WEST

F724

Land surfa

Altitude, in meters, NGVD 29

F726 93

171 185

975 1,547

204

167

266

−5

225

−10

209 219

134

255

215

323

528 35,100

35,600

277

? 159

182

>30,000

186

197 240

162

205 230 6,680 24,800 29,300

875

145 169

25,600

20,000

178

?176

−5

10,000 5,000 1,000 <100 Hypothetical groundwater flowpath

? −10

1,120

175

Sampling point and specific conductance value

−15

41,300 Water-table altitude on August 19, 2014, in meters above NGVD 29. Mean high tide (MHT) and mean low tide (MLT) from July 20 to October 23, 2013, in meters above NGVD 29.

−20 −40

Specific conductance, in microsiemens per centimeter at 25 degrees Celsius

175

182 176

1,086

−15

F743 MHT MLT Eel River

F733

Water table

EXPLANATION

F732

−30

−20

VERTICAL EXAGGERATION × 2

−10

−20

Distance from reference point on shoreline, in meters B. Nitrate plus nitrite B

EAST

WEST

F724

Land surfa

Altitude, in meters, NGVD 29

119 167

−5

−10

F726

Water table 323

621

148

56 248 294

71 345

921

642

654

1,035

329

735

1,149

1,064 135 344 270 0

1,706

1,649

>600 500 400 300 200 100 0

576

−5

496

550

388

638

293

771

456

142 0

−10

392

553

578

587

Nitrate plus nitrite, in micromoles per liter

522

449

540

1,649

−15

Eel River

<40 12 102

1,121

F743 MHT MLT

F733

<40

1,385

EXPLANATION

F732

587

Sampling point and nitrate plus nitrite value

Evidence of nitrate reduction and estimated nitrate loss, in micromoles per liter Saltwater; no nitrate present

−15

No evidence of nitrate reduction No circle indicates not evaluated

−20 −40

VERTICAL EXAGGERATION × 2

−30

−20

−10

−20

"F" is shorthand for MA-FSW

Distance from reference point on shoreline, in meters

Figure 3. Section B–B′ showing distribution of A, specific conductance, and B, measured nitrate plus nitrite in groundwater and selected features of the conceptual model of nitrogen transport and attenuation, including estimated nitrate loss at selected points, along transect B–B′ beneath the Eel River, Seacoast Shores peninsula, East Falmouth, Massachusetts, September 5–25, 2013. (Modified from Colman and others, 2018, fig. 3A).

8 Evidence of Nitrate Attenuation in Intertidal and Subtidal Groundwater in a Subterranean Estuary at a Cape Cod Embayment

Fate of Nitrogen in the Subterranean Estuary at Seacoast Shores Peninsula An assessment of potential nitrogen attenuation in the subterranean estuary near the Eel River embayment, adjacent to a densely populated coastal community (Seacoast Shores peninsula, Falmouth Massachusetts), was conducted in 2013 and 2014 (Colman and others, 2018). A network of onshore and estuarine nearshore wells was established to track dissolved nitrogen and identify flow paths, recharge areas, and zones of nitrogen attenuation. The primary goals of that study were to obtain information on the occurrence of nitrogen attenuation and how nitrogen losses could affect loading of wastewater nitrogen to the estuary. Water levels and hydraulic gradients indicated that the water table generally sloped downwards and northwest towards the Eel River from the center of the peninsula and that fresh groundwater was flowing upward on the western side of the peninsula and discharging into the Eel River. Colman and others (2018) determined that two of the necessary requirements for nitrate reduction by denitrification or anammox (anaerobic environment and relatively high nitrate concentrations) are widespread in groundwater beneath the peninsula. They measured geochemical indicators for evidence of nitrate reduction, including concentrations of the reaction product nitrogen gas, stable isotope ratios of nitrate and nitrogen gas, and changes in alkalinity. Evidence for attenuation of nitrate inputs to the Eel River by nitrate reduction was limited to two vertical intervals. A shallower nitrate reduction interval was identified near the water table in fresh groundwater onshore and, also, at one location offshore. Nitrate reduction in this interval may be related to particular recharge conditions at nearby sources, or to an interval of fine-grained sediments at about the same altitude (−1 to −6 m relative to the NGVD 1929) where flow is slower and reactive electron donors such as solid organic carbon, iron, or sulfide phases may be present to drive the reduction (Di Capua and others, 2019). A deeper nitrate reduction interval was identified near the freshwater/saltwater transition zone. Nitrate reduction in this interval may be related to mixing of freshwater containing nitrate and saltwater containing dissolved organic carbon (DOC) and ammonium, or to fine-grained sediments near the transition zone. The shallow and deep nitrate reduction intervals were separated vertically by a 7- to 8-m-thick zone of freshwater containing nitrate but with no evidence of nitrate reduction (Colman and others, 2018, fig. 32). Colman and others (2018) concluded that the absence of evidence for nitrate reduction in the intermediate depth interval may result from the lack of reactive electron donors in the water or aquifer material. Colman and others (2018) observed that a shallow intertidal saltwater flow cell was formed during a spring tide by saltwater infiltration during tidal run-up on the beach. They found variable nitrate concentrations in shallow groundwater beneath intertidal and subtidal zones of the estuary and speculated that nitrate reduction might occur if nitrate-containing fresh groundwater passed through reducing sediments or mixed with saltwater containing DOC before discharging in these

areas. However, they did not evaluate shallow groundwater beneath the intertidal or subtidal zones systematically for evidence of nitrate reduction. They suggested further studies in shallow offshore groundwater could improve understanding of the fate of septic-derived inorganic nitrogen near potential discharge areas in the estuary. The current study was undertaken to investigate whether nitrate reduction was occurring in these shallow offshore groundwater environments that were not fully characterized in the previous study.

Well Installation Pushpoint samplers (MHE Products, East Tawas, Michigan) for collection of water samples were installed vertically into the bottom sediments beneath the Eel River estuary to the desired depth to sample groundwater along transects A–A′ and D–D′ (fig. 4). Each sampler consisted of a stainless steel 3.175-cm-diameter, 15.24-cm-long, 0.010-slot well screen connected to 152.4-cm lengths of 3.175-cm-diameter stainless steel pipe. Multiple pushpoints at 12 approximately evenly spaced locations from 1 m onshore to 13.5 m offshore were installed along these transects. The positions of offshore locations are reported relative to the shoreline. The depths and horizontal distances from the shoreline of all well points in transects A–A′, B–B′, C–C ′, and D–D′ are shown in figure 5 and reported in Huntington and others (2024). The latitude and longitude of the location of the shoreline for each transect are listed for the points described as 0.0 feet along each east-west transect (Huntington and others, 2024). The distances and directions from the shoreline referred to in this report correspond to the values on the x-axes for all relevant figures. Four pushpoints were installed in each of the 12 clusters. The samplers were installed so that the bottoms of the well screens were positioned at 30, 61, 91, or 122 cm below the sediment/ water interface. Three additional permanent Solinst well point clusters (transect C–C ′, fig. 4) were installed parallel to and about 3 m northeast of transect D–D′. These clusters were located 4.3, 9.8, and 14.3 m from the shore. The three Solinst well points in these clusters were installed so that the bottom of the well screens were positioned at depths of 1.83, 3.35, and 4.88 m below the sediment/water interface (transect C–C ′, fig. 4) by using a slide-hammer or an air-driven post driver. Another transect of well clusters that was installed in the earlier study also was sampled during this study. That transect (B–B′, fig. 4) consisted of 12 well clusters of stainless steel pushpoint samplers (MHE Products Inc.) and permanent well points (AMS Inc., American Falls, Idaho), from the onshore site (F741) spaced evenly and extending 13.7 m offshore (F745) parallel to the other transects and midway between transects A–A′ and D–D′. The deepest samplers in each cluster were AMS permanent well points, and the remainder were MHE pushpoints. These well clusters each included four wells installed with the bottom of the well screens positioned 30, 61, 91, or 122 cm below the sediment/water interface. The installation of those wells is described in Colman and others (2018).

Well Installation 9 70°32'33"

70°32'32"

! 41°34'05.4"

A’ F777-A11 F777-A09 F777-A10 F777-A07 F777-A08 F777-A05 F777-A06 F747-A F777-A03 F777-A04 F777-A01 F777-A02 A

F777-A12

Eel River B’

C’

F775

F745-A01

F746-A01 F747-A01

41°34'04.8"

F734 F736 !

F735

F774

D’ F776-A12

F733

F739

F773

F776-A09

F738

F737

F776-A11 F776-A10

! !

F740 !

F741

F776-A07

F776-A08

F776-A06

F776-A05

F776-A04

F776-A03

F776-A02

F776-A01

Base from U.S. Geological Survey and Massachusetts Office of Geographic Information digital data North American Datum of 1983

20 FEET

6 METERS

EXPLANATION A

A’

Line of transect Mean low tide (MLT), July–September 2016 Mean high tide (MLT), July–September 2016

F773 F741

Solinst well-point cluster and site identifier !

F776-A01

Well point and site identifier Temporary pushpoint and site identifier

Tide gage “F” is shorthand for MA-FSW

Figure 4. Map showing locations and types of monitoring sites near the western shore of the Eel River, Seascoast Shores peninsula, East Falmouth, Massachusetts. Locations of transects A–A’, B–B’, C–C’, and D–D’ are shown.

10 Evidence of Nitrate Attenuation in Intertidal and Subtidal Groundwater in a Subterranean Estuary at a Cape Cod Embayment EAST 1.0

WEST

A' A01

Eel River

A02

0.5

A03

A04

A05

A06

A07

MHT MLT

A08

EXPLANATION

0.5

Sampling point by screen type

MHE temporary pushpoint

−0.5

AMS permanent well point

−1.0

Solinst permanent well point

A09

A10

A11

A12

−1.5

−1.5 VERTICAL EXAGGERATION ×2

−2.0 1.0

F740

Eel River

F739

F738

F737

F736

F735

F734

MHT

F733

F747

F746

1.0 0.5

MLT

F745

−0.5

−1.0

−1.5

−1.5 VERTICAL EXAGGERATION ×2

−2.0 1.0

−2.0

Eel River F773

MHT MLT

F774

1.0 0

F775

−1.0

−2.0

−3.0

−4.0

−5.0

−6.0

−6.0 NO VERTICAL EXAGGERATION

−7.0 1.0

−7.0

D' A01

0.5

Eel River

A02 A03

A04

A05

A06

A07

A08

A09

A10

−0.5

A11

1.0

MHT

0.5

MLT

A12

−0.5 −1.0

−1.0 −1.5 −2.0

“F” is shorthand for MA-FSW

−2.0

F741

0.5

Altitude, in meters from NGVD29

1.0

−1.5 VERTICAL EXAGGERATION ×2 −1

−2.0 4

Distance from reference point on shoreline, in meters

Figure 5. Vertical transects showing locations of monitoring sites, land surface, and approximate altitudes of mean high tide (MHT) and mean low tide (MLT) for transects A–A’, B–B’, C–C’, and D–D’ near the eastern shore of the Eel River, Seacoast Shores peninsula, East Falmouth, Massachusetts. MHT and MLT determined from tide-stage data from July 13 to September 8, 2016. Locations of transects are shown in figure 4.

Water-Quality Sampling and Laboratory Analyses 11

Measurement of Hydraulic Head and Interpretation of Flow Direction Water-level altitudes were measured during October 24–26, 2016, in the nine drive-point wells at the three well clusters (transect C–C ′; sites F773, F774, and F776, fig. 4) at low, high, and mean tides to determine hydraulichead gradients. The measurements were made with HOBO dataloggers (Onset Computer Corp., Bourne, Massachusetts). The values were plotted in vertical cross sections relative to the bottom of the Eel River by using graphing software. Contour lines were hand-drawn at varying intervals, and arrows were added perpendicular to the contours to indicate direction of flow. A pressure-transducer tide gage was installed to monitor the water level in the Eel River near the study area, and hydraulic heads were measured during October 24–26, 2016. The gage consisted of an open-bottom polyvinyl chloride (PVC) standpipe that was secured to a permanent dock piling at a nearby private residence. The tide gage installation is described in Colman and others (2018, app. 1). Measurements of tidal elevation were not recorded at the Eel River during water-quality sampling in 2015 and 2016. Tide data for the sampling periods in this study were retrieved for the U.S. Geological Survey (USGS) tide gage for Popponesset Bay, Mashpee Neck Road, near Mashpee, Massachusetts (USGS site 413601070275800), from the USGS National Water Information System (NWIS) database (U.S. Geological Survey, 2018).

Water-Quality Sampling and Laboratory Analyses Well clusters in transect B–B′ were sampled once during November 9–10, 2015. Well clusters in transect D–D′ were sampled once during June 14–15, 2016. Well clusters in transect A–A′ were sampled once during June 15–16, 2016. Well clusters in transect C–C ′ were sampled once on October 24, 2016. Samples for water-quality analyses were collected by peristaltic pump and polyethylene tubing. Field water-quality parameters were measured by using parameter-specific meters for water temperature (in degrees Celsius [°C]), temperature-corrected specific conductance (SpC) (µS/cm), and pH. Methods for water temperature and pH are described in the USGS national field manual (U.S. Geological Survey, variously dated). Methods for SpC are described in a U.S. Geological Survey Techniques and Methods book chapter (U.S. Geological Survey, 2005). Dissolved oxygen concentration (DO as O2) in milligrams per liter (mg/L) was analyzed colorimetrically with the indigo Rhodazine-D method (less than 1 mg/L) (Chemetrics, Inc., 2015) or by using a dissolved oxygen probe and meter (greater than

1 mg/L) (U.S. Geological Survey, variously dated). Alkalinity, in microequivalents per liter (µeq/L), was analyzed by titration (Fishman and Friedman, 1989). Samples for analysis of nitrate plus nitrite, ammonium, total Kjeldahl nitrogen, and total phosphorus concentrations were filtered (0.45 micrometer), preserved with sulfuric acid to pH <2, refrigerated at <4 °C, and shipped to analytical laboratories. Nitrate plus nitrite was analyzed by hydrazine reduction and colorimetric analysis (EPA Method 353.1) (Wendt, 1995). Ammonium was analyzed by the alkaline phenol method and colorimetric analysis (EPA Method 353.1) (U.S. Environmental Protection Agency, 1983b). Total Kjeldahl nitrogen was analyzed by Kjeldahl digestion with a mercury catalyst followed by colorimetric analysis (EPA Method 353.2) (O’Dell, 1993). Dissolved organic nitrogen concentrations were determined from the difference between total Kjeldahl nitrogen and ammonium nitrogen concentrations. Total phosphorus was analyzed by Kjeldahl digestion with a mercury catalyst followed by molybdate colorimetric analysis (EPA Method 365.4) (U.S. Environmental Protection Agency, 1983a). DOC was analyzed with a Shimadzu TOC–VCPH analyzer using high temperature catalytic oxidation followed by nondispersive infrared detection of carbon dioxide (EPA Method 415.3) (Potter and Wimsatt, 2005). The median relative percent difference for duplicate samples analyzed for nutrients was 4.2 percent (range 0.6 to 32 percent). In some cases, analyses of constituent concentrations were reported as beneath the level of quantitation. In the associated data release (Huntington and others, 2024), these results were reported with a “<” symbol (for example, <0.019). Values that were beneath the level of quantitation were plotted as 0 in the figures in this report. Samples for analysis of dissolved nitrogen gas and argon gas concentrations were collected in 12.4-milliliter (mL) glass vials with a threaded cap and butyl rubber septum. Vials were filled from the bottom with a continuous stream of pumped groundwater, gently overflowed with greater than 5 vial volumes, capped quickly without bubbles or headspace, and stored under water at less than 4 °C. In the laboratory at the Woods Hole Oceanographic Institution, samples were analyzed by membrane inlet mass spectrometry (Kana and others, 1994; Young and others, 2013; Szymczycha and others, 2017). Sample water was pumped through a section of capillary-diameter, gas-permeable, silicon tubing with the external space under vacuum. Extracted gas passed through a liquid nitrogen trap to remove water vapor and carbon dioxide, through a copper reduction tube at 600 °C to remove oxygen, and to a quadrupole mass spectrometer. The quadrupole mass spectrometer signals representing dissolved gas concentrations were recorded for the three masses of nitrogen gas (28, 29, 30) and the primary mass for argon gas (40). A deionized water bath, held at constant temperature (10 °C) and circulated continuously with a head space at 100-percent relative humidity served as a calibration solution for the gases and was sampled repeatedly before and after each set of 15 to 20 field samples. Readings for field samples were corrected for

12 Evidence of Nitrate Attenuation in Intertidal and Subtidal Groundwater in a Subterranean Estuary at a Cape Cod Embayment minor instrument drift. The relative standard deviation of three replicated analyses of the calibration bath was 0.02 percent for 28N and 0.01 percent for 40Ar. Duplicate samples collected from 2 12 locations using the same collection method, with duplicates collected sequentially from a stream of pumped groundwater, resulted in an average relative standard deviation of 2.0 percent, with a range of 0.04 to 9.0 percent. Data from samples that degassed substantially during sample handling were excluded; minor degassing may have contributed to overall uncertainties of the evaluated dissolved gas data, as described in the section “Evaluation of Nitrate Reduction to Nitrogen Gas.” Additional information on methods is provided in the accompanying data release (Huntington and others, 2024). Reported concentration units for various parameters in the data release were converted for discussion in this report. Quality assurance procedures were previously described (Colman and others, 2018).

Determination of Nitrogen Attenuation The primary method for quantifying permanent removal of fixed nitrogen from the subterranean estuary in this study was to evaluate the distribution of nitrogen gas produced by microbial nitrate reduction ± anaerobic ammonium oxidation (denitrification ± anammox). Other microbial nitrogen transformation processes such as dissimilatory nitrate reduction to ammonium (DNRA) or assimilation are more difficult to quantify using porewater data where potentially large reservoirs of solid-phase ammonium and organic nitrogen also exist, and they are more likely to represent temporary fixed nitrogen sinks that can be oxidized, mobilized, and discharged subsequently. Measured concentrations of dissolved nitrogen gas (N2) and argon gas (Ar) were used to estimate concentrations of excess nitrogen gas produced by nitrate reduction that could indicate nitrogen attenuation. Interpretation of denitrification or other microbial metabolic pathways for nitrogen gas production in groundwater must be conducted in the context of conditions in which the groundwater was recharged that control “background” gas concentrations in the absence of microbial nitrogen gas production (for example, Böhlke and others, 2002; Griggs and others, 2003; Szymczycha and others, 2017). The following processes can contribute to dissolved nitrogen gas in groundwater: (1) equilibration with the atmosphere and soil gas during recharge to the aquifer, with solubility determined by temperature, salinity, and atmospheric pressure (elevation), (2) dissolution of air bubbles entrained during recharge, resulting in “excess air” (EA), and (3) nitrogen gas production by microbial processes (denitrification or anammox). Argon gas occurs in groundwater due to processes 1 and 2 and, depending on various assumptions, can be used as a tracer to estimate nitrogen gas concentration resulting from those processes. Dissolved nitrogen gas and argon gas concentrations were evaluated as follows (Böhlke and others, 2002):

[N2Sample]/[ArSample] =([N2ASW]+[N2EA]+[N2MIC])/([ArASW ]+[ArEA]),(1) where N2ASW and ArASW refer to “air saturated water,” or the molar concentrations of nitrogen gas and argon gas from equilibration with the atmosphere at recharge. N2MIC is the concentration of nitrogen gas produced by microbial processes. N2EA and ArEA refer to concentrations of nitrogen gas and argon gas added to the equilibrium values by dissolution of air bubbles (excess air). Observations commonly indicate that excess air results from complete or nearly complete dissolution of bubbles, resulting in an excess air composition that is similar to atmospheric air, thus allowing estimation of the excess air component of total nitrogen gas as follows (Böhlke and others, 2002): [N2EA]=([Arsample]–[ArASW])*([N2AMF]/[ArAMF]),(2) where N2AMF and ArAMF are atmospheric mole fractions of N2 and Ar, respectively. In this study, the concentration of microbial nitrogen gas (N2MIC) in each water sample was used to estimate the concentration of nitrate that was lost by nitrate reduction in the water parcel represented by the sample. The N2MIC was assumed to have accumulated after the water in the sample entered the saturated zone during recharge, and it was assumed initially that 1 mole of N2MIC (2 moles of N2-NMIC) were produced from 2 moles of nitrate, as in denitrification. The initial nitrate concentration in the water at the point of recharge was estimated as the sum of the measured nitrate concentration and the estimated concentration of nitrate lost (converted to nitrogen gas) in the water sample. Estimated concentrations of nitrate lost and initial nitrate could be smaller if nitrate reduction was coupled with ammonium oxidation (anammox), which could yield 1 mole of N2MIC (2 moles of N2-NMIC) from 1 mole of nitrate. Additional details of these calculations and assumptions are presented in the context of the dissolved gas data in the section “Evaluation of Nitrate Reduction to Nitrogen Gas.”

Hydrogeologic and Geochemical Observations Observations reported in this section include water levels measured at wells, hydraulic head gradients, field water-quality characteristics measured during sampling, and chemical analytical results from laboratory analyses. All well-location and water-level data collected in this investigation are stored in the National Water Information System database (U.S. Geological Survey, 2018), available at https://doi.org/10.5066/F7P55KJN. All water-quality data are listed in the data release associated with this publication (Huntington and others, 2024).

Hydrogeologic and Geochemical Observations 13

Hydraulic-Head Gradients during Low, High, and Mean Tides Water levels were measured from October 24 through October 26, 2016, during a neap tide at transect C–C ′. The magnitudes of the hydraulic gradients can be visualized as the differences in water levels between the drive-point wells and the estuary (figs. 6A–6C). Water-level contours indicated that the hydraulic gradients from about −4 to −2 m altitude generally were consistent with upward flow (toward the estuary) at low, high, and mean tides as shown by the arrows in figures 6A–6C. The gradients were steeper at low tide than at high tide, implying greater potential for upward flow at low tide in this depth interval. Although these data appear to be consistent with net fresh groundwater discharge to the estuary in this area, they may not indicate local flow conditions within shallower parts of the subterranean estuary (altitude greater than 2 m) represented by water-quality data from the other transects sampled in this study. Furthermore, Colman and others (2018) concluded from the distribution of dissolved oxygen that groundwater flow in this area may have been partly oblique to the sampled transects. Groundwater from a regional flow system may have been converging under the estuary near the study area and interacting with local groundwater flow from the peninsula, and flow directions may have been affected by aquifer sediment heterogeneity (Colman and others, 2018; Lane and others, 2020). Information about the physical properties of the sediments and stratigraphy is limited, and gradient observations were collected along one transect; therefore, direct inferences about groundwater flow are limited.

Chemical Characteristics of Shallow Groundwater in the Nearshore Subterranean Estuary This section includes results and discussion of observations of specific conductance, DO, pH, alkalinity, nitrate plus nitrite, ammonium, total Kjeldahl nitrogen, and DOC in the nearshore subterranean estuary. These observations are for samples collected in three shallow transects at various depths from 0.3 to 1.52 m below the sediment/water interface in November 2015 (transect B–B′) and June 2016 (transects A–A′ and D–D′), and for a smaller set of samples collected from approximately 2 to 5 m below the sediment/water interface in October 2016 (transect C–C ′).

Specific Conductance The vertical profiles for specific conductance in transects A–A′, B–B′, and D–D′ generally are consistent with the formation of an intertidal saltwater cell (ISC) where fresh groundwater mixes with infiltrating seawater from the estuary above (figs. 7A, 8A, and 9A). In the conceptual model of Abarca and others (2013) (fig. 2), an ISC develops in shallow

groundwater during high tides when saltwater infiltrates beach sands at the highest altitudes of the intertidal zone and then flows through the sand to discharge back into the estuary near the lowest altitude of the intertidal zone at low tide. The ISC in mid-June 2016 (figs. 7A, 8A, and 9A) appeared to be somewhat less well defined than the ISC characterized by Colman and others (2018). Samples were collected during neap tides in November 2015, June 2016, and October 2016 (fig. 10). The transect characterized by Colman and others (2018), where evidence for the ISC was more pronounced, was measured during a spring tide. The fact that the apparent strength of the ISC was more robust in 2013 than in 2015 and 2016 may be due to the height of the tides. Saline-water infiltration and strength of the ISC are expected to be greater during spring tides. This pattern is consistent with the conceptual model of tidal control of infiltration and potential absence of an ISC during a neap tide (Robinson and others, 2007; Abarca and others, 2013). The patterns observed in this study suggest fresh groundwater discharge in the intertidal zone, particularly in the lower intertidal zone. The pattern of more saline water at greater depths in the subtidal zone at transect D–D′ compared with transect A–A′ could indicate fresh groundwater inputs were stronger at transect A–A′ and saline water infiltration was stronger at transect D–D′ when the transects were sampled. The middle transect, B–B′, which was sampled in November 2015, was more like transect A–A′ in the subtidal zone from 8 to 10 m offshore and more like transect D–D′ from 11 to 14 m offshore. The SpC data from all three transects are consistent with a conceptual model of saline water infiltrating from the estuary above and mixing with freshwater in the subterranean estuary. There is evidence for an ISC in the intertidal zone, plus a subtidal shallow zone of saline water farther offshore, consistent with the findings of Colman and others (2018) (fig. 3). The overall pattern of SpC data in the intertidal and subtidal zones in transect B–B′ was approximately the same in 2015 (fig. 8A) as it was in 2013 (fig. 3). The subtidal shallow saline zone apparently does not represent the landward edge of a deeper upwelling saltwater wedge (fig. 2), as it is bounded in part by freshwater below and farther offshore. Instead, the source of the near-surface saline water more than 5 m offshore could include local saltwater infiltration from the estuary above. Although flow patterns in this subtidal area were not well characterized, it is referred to in this report as a “subtidal saltwater cell,” where infiltrating estuarine water overlies and mixes with underlying fresh groundwater. At this location in the Eel River subterranean estuary, and offshore to at least 14 m, a thick, fresh groundwater lens apparently prevents a deep saltwater wedge from discharging to the surface. The estuary is about 100 m wide in this area and it is possible that a deep saltwater wedge could be present farther offshore and that it is connected with the overlying estuary as in the Waquoit Bay model. Colman and others (2018) reported a zone of fresh groundwater within the vertical profile from approximately 2 to 10 m below the sediment/water interface and extending offshore to their farthest measurement

14 Evidence of Nitrate Attenuation in Intertidal and Subtidal Groundwater in a Subterranean Estuary at a Cape Cod Embayment location (18 m offshore) (fig. 3). Although no samples have been collected farther offshore, geophysical resistivity surveys indicate fresh groundwater may be widespread beneath the Eel River estuary (Lane and others, 2020). The subterranean estuary groundwater was sampled at 1.83- 3.35-, and 4.88-m depths below the sediment/water interface in October 2016 at three locations on transect C–C ′ (F773, F774, and F775) (fig. 4). The vertical profiles for SpC and other water-quality variables for transect C–C ′ are not shown in figures in this report because only nine points were sampled and all depths were below those sampled on the other transects. The data are available in the accompanying data release (Huntington and others, 2024). SpC measurements at these locations and sampling depths were consistent with observations reported by Colman and others (2018). Samples were brackish to saline, (10,040 to 35,620 microsiemens per centimeter at 25 degrees Celsius [µS/cm at 25 °C]) at 1.83 m, moderately brackish (593–5,327 µS/cm at 25 °C) at 3.35 m and fresh (343–955 µS/cm at 25 °C) at 4.88 m. Saline conditions at 1.83-m depth in transect C–C ′ were consistent with saline-water infiltration that extends from the sediment/water interface to approximately 2-m depth as was reported by Colman and others (2018) when transect B–B′ was sampled in September 2013. In two of the transects (A–A′ and D–D′) in June 2016, the vertical profiles at the farthest offshore sampling location indicated possible freshwater discharge at about 13–14 m offshore (figs. 7A and 9A) that is consistent with the SpC profile observed in 2013 (Colman and others, 2018). The groundwater sampling in 2013 extended about 20 m offshore and to 11 m below the sediment/water interface, and the deeper underlying saltwater wedge that was identified during that sampling did not discharge to the sediment surface as shown in the conceptual model by Abarca and others (2013).

Dissolved Oxygen Dissolved oxygen concentrations generally were suboxic in the subtidal zone in transects B–B′ and D–D′ (figs. 8B and 9B). Suboxic is defined here as dissolved oxygen concentrations less than about 0.5 mg/L (<16 µmol/L as O2) (Hinkle and others, 2007). Dissolved oxygen concentrations (as O2) less than 30 µmol/L are favorable for classical denitrification (Spiteri and others, 2008). Dissolved oxygen concentrations in the subtidal zone in transect A–A′ were more variable, ranging from suboxic to 71 µmol/L (fig. 7B). In the intertidal zone in transects A–A′ and D–D′ from 61-cm to 122-cm depths below the sediment surface, dissolved oxygen concentrations generally ranged from 18 to 86 µmol/L. The intertidal zone in transect B–B′ was suboxic (fig. 8B). For the three locations sampled in October 2016 along transect C–C ′, dissolved oxygen concentrations were less than 10 µmol/L at 1.8-m depth, 80–270 µmol/L at 3.3-m depth, and 36–126 µmol/L at 4.88-m depth (Huntington and others, 2024). Dissolved oxygen concentrations were similar to those measured at depths less than 2 m in 2013 (Colman and others, 2018) but were about two times higher than what was measured from 3- to 5-m depths in 2013.

pH Measured pH values ranged from about 5.0 to 7.7 (figs. 7C–9C) and generally were correlated positively with SpC, qualitatively consistent with differences between fresh groundwater recharge on land and saline recharge from the estuary. There was little difference in pH with depth in transect D–D′ and substantial variability with depth in the other transects, similar to the vertical trends in SpC with the exception of the farthest offshore profiles on transect A–A′.

Alkalinity Measurements of total alkalinity in groundwater were made to determine whether alkalinity gradients were consistent with organic carbon oxidation coupled with nitrate reduction (Abrams and others, 1998; Colman and others, 2018). Alkalinity is substantially higher in seawater than in fresh groundwater at the study site. Therefore, it can be difficult to detect minor effects of carbon oxidation on alkalinity in the subterranean estuary where estuarine saline waters infiltrate and mix with fresh groundwater. However, increases in alkalinity have been attributed to denitrification in high salinity groundwater in Waquoit Bay, Cape Cod, Massachusetts (Liu and others, 2017). Other processes can generate alkalinity, including organoclastic sulfate reduction, aerobic metabolism of organic matter, iron reduction, and DNRA (Rassmann and others, 2020; Meister and others, 2022); therefore, increases in alkalinity are only potential indicators of denitrification. Alkalinity was measured only at selected locations on the transects; however, a general pattern of higher alkalinity associated with higher specific conductance is apparent in transect B–B′ (fig. 8D). Alkalinity was somewhat elevated in the shallow (30-cm depth) groundwater beneath the intertidal zone in transect B–B′ (greater than 1,000 µeq/L) (fig. 8D) and transect D–D′ (greater than 500 µeq/L) (fig. 9D). In this zone, saline water is thought to infiltrate at high tide, mix with fresh groundwater, and discharge back to the estuary at low tide. Alkalinity was more highly elevated (1,800–2,500 µeq/L) in shallow groundwater (30-cm depth) in the subtidal zone in transect B–B′ (fig. 8D) and at two locations (A11 and A12) where it was measured at the 61-cm depth in the subtidal in transect A–A′ (fig. 7D). Despite a general trend indicating that elevated alkalinity was usually associated with saltwater infiltration, there was some evidence of alkalinity in excess of what was typically measured that may be from other sources, for example, at two sites farthest from shore in transect A–A′ (A11 and A12) (fig. 7D) and in the near intertidal zone at 61 to 122 cm below the sediment/water interface in transect D–D′ (fig. 9D). The elevated alkalinity in two samples at 61-cm depth from 12 to 14 m offshore in the subtidal region of transect A–A′ (fig. 7D) were associated with relatively high N2/Ar ratios (fig. 7K) (refer to section, “Concentrations and Ratios of Nitrogen and Argon Gases”) and relatively low SpC (fig. 7A) and could include components of alkalinity derived from carbon oxidation by nitrate or oxygen reduction. Alkalinities also were

Hydrogeologic and Geochemical Observations 15 somewhat elevated at the 91-cm depths at A12 (farthest offshore location) and the 61-cm depth at A09 on the transect A–A′ where the groundwater was near the freshest end of the mixed salinity zone (fig. 7A) and N2/Ar ratios also were elevated. Alkalinities

were lower at the 61-cm depth closer to shore (A05 to A07) in the mixing zone below saline waters (fig. 7A, 7D), indicating that infiltrating sea water may not be the only source of elevated alkalinities.

EAST

WEST

A. Low tide 1.0

F773 Low tide altitude = −0.12 m

F774

Eel River

−1.0 −2.0 −3.0

0.09

0.53

−5.0

0.15

0.50

0.51

−4.0

0.30

0.50

−5.0 −6.0

“F” is shorthand for MA-FSW

−6.0 −7.0

Contour interval 0.20 meter

−7.0

B. High tide

Altitude, in meters from NGVD29

1.0

F775

1.0

Eel River

F774

F773 High tide altitude = +0.52 m

0.00

−1.0 0.05

0.04

−2.0 −3.0

0.06

0.08

0.09

0.07 0.06

−4.0 −5.0

0.41

Mean high tide (MHT) and mean low tide (MLT) determined from tide stage data from July 13 to September 8, 2016

−1.0 −2.0

0.04

0.05 0.06 0.07

−3.0 0.08

−4.0

0.09

−5.0

−6.0 −7.0

Mean low tide Solinst permanent well point with difference between the tide and water-level altitudes, in meters— Positive value indicates groundwater level is greater than the tide level

0.41

−4.0

−3.0

Line of equal difference between the tide and water-level altitudes, in meters— Contour interval varies by panel Mean high tide

−2.0

0.07

0.10 0.30 0.50

0.51

0.10

−1.0

0.00 0.24

EXPLANATION

1.0

F775

−6.0 Contour interval 0.01 meter

−7.0

C. Mean tide 1.0

F774

F773 Mean tide altitude = +0.22 m

1.0

F775

Eel River 0.00

−1.0 0.09

−2.0 −3.0

0.05

0.22

0.15 0.25

−3.0

0.25

0.29

−2.0

0.04

0.05

0.28

−4.0 −5.0

−1.0

0.09

0.28

−4.0

0.28

−5.0 −6.0

−6.0 −7.0

Contour interval 0.10 meter −1

−7.0 4

Distance from reference point on shoreline, in meters

Figure 6. Vertical sections showing water-level altitudes measured in drive-point wells and in the Eel River at A, low tide, B, high tide, and C, mean tide, October 24–26, 2006. Arrows indicate inferred directions of groundwater flow.

16 Evidence of Nitrate Attenuation in Intertidal and Subtidal Groundwater in a Subterranean Estuary at a Cape Cod Embayment EAST

WEST

MA-FSW 777 A01–A12 (Transect A)

A. Specific conductance 1.0

1.0

A01

Eel River

A02

0.5

A03

3,180

3,360

153

145

152

−0.5

13,600

A04

12,980

177

174

173

149

A05

15,820

A08

A09

242

37,500

167

34,200

184

175

691

196

157

1,468

158

162

1,591

163

165

0.5

MLT

38,900

173

166

−1.0

A07

36,300

164

180

176

A06

MHT

A10 34,500

A11 37,100

6,510

723

860

4,660

−1.5

A12

−0.5

EXPLANATION Specific conductance, in microsiemens per centimeter >30,000 20,000

−1.0

3,910

1,203

3,140

410

2,100

1,016

−1.5

−2.0

10,000 100

B. Dissolved oxygen 1.0

A01

Eel River

A02

0.5

A03

A04

Altitude, in meters from NGVD 29

123

−0.5

161

189

−1.0

A05

MHT MLT

A06

A07

A08

A09

−1.5

A10

A11

1.0

EXPLANATION

0.5

Dissolved oxygen (O2), in micomoles per liter

0 A12

−0.5 −1.0 −1.5

300 200 100 0

−2.0

C. pH 1.0

1.0

A01

Eel River

A02

0.5

A03

5.66

−1.0

5.21

5.41

A07

5.9

5.33

5.43

5.68

A06

4.68

5.15

5.38

5.23

A05

4.75

5.67

5.1

6.32

−0.5

5.35

5.46

6.95

A04

5.03

MHT

A08

6.97

A09

A10

A11

7.03

7.08

6.81

6.71

7.52

5.52

6.03

6.04

5.12

5.88

5.15

4.93

5.67

4.93

5.01

5.05

−1.5

0.5

MLT A12

7.46 8.51

6.25

7.38

6.2

6.66

−0.5

7.62 7.82 7.33 6.28

EXPLANATION pH, in standard units 8.6 7.6 6.6

−1.0

5.6

−1.5

4.6

−2.0

D. Alkalinity 1.0

A01

Eel River

A02

0.5

A03

A04

A05

−0.5

117

129

136

118

113

−1.0

A06

A07

MHT MLT

A08

308

A09

A10

545

A11

2,320

1.0

EXPLANATION

0.5

Alkalinity, in microequivalents per liter

2,000

−0.5

1,500

−1.0

1,000

A12

2,239

117

487

−1.5

500 0

−2.0

−2.0 −1

Distance from reference point on shoreline, in meters VERTICAL EXAGGERATION ×2

Mean high tide (MHT) and mean low tide (MLT) determined from tide stage data from July 13 to September 8, 2016

Figure 7. Vertical transects showing distributions of A, specific conductance, B, dissolved oxygen, C, pH, D, alkalinity, E, nitrate plus nitrite, F, ammonium, G, total Kjeldahl nitrogen, H, dissolved organic carbon, I, nitrogen gas, J, argon gas, and K, ratio of nitrogen gas to argon gas in groundwater along transect A–A′ (F777) beneath the Eel River, Seacoast Shores peninsula, East Falmouth, Massachusetts, June 15–16, 2016. Location of transect is shown in figure 4.

Hydrogeologic and Geochemical Observations 17 EAST

WEST

MA-FSW 777 A01–A12 (Transect A)

E. Nitrate plus nitrite 1.0

1.0

A01

Eel River

A02

0.5

A03

0 −0.5

A04

98 216 261

146

434

−1.0

223 403 481 479

MHT

A05

A06

A07

261

512

538 590

577

576

553

460 482

581

545

602

A08

−1.5

0.5

MLT A09

A10

A11

A12

577

EXPLANATION Nitrate plus nitrite, in micromoles per liter 600 500

−0.5 −1.0 −1.5

400 300 200 100 0

−2.0

F. Ammonium 1.0

A01

Eel River

A02

0.5 0

−1.0

A06

A05

A04

−0.5

Altitude, in meters from NGVD 29

A03

A07

A08

MHT MLT A09

6 4

−1.5

1.0

EXPLANATION

0.5

Ammonium, in micromoles per liter

A10

A11

A12

−0.5 −1.0 −1.5

75 50 25 0

−2.0

G. Total Kjeldahl nitrogen 1.0

A01

Eel River

A02

0.5

A03

−0.5

A04

−1.0

A05

A06

15 11 9

A07

A08

−1.5

MHT MLT A09

A10

A11

109

14 10

1.0

EXPLANATION

0.5

Total Kjeldahl nitrogen, in micromoles per liter

150

−0.5

100

A12

−1.0 −1.5

50 0

−2.0

H. Dissolved organic carbon 1.0

A01

Eel River

A02

0.5

A03

153

0 −0.5

485

112

172

115

A04

127 75

−1.0

A05

A06

81 0 0 0

A07

A08

MHT MLT A09

A10

133

164

129

126

136

−1.5

1.0

EXPLANATION

0.5

Dissolved organic carbon, in micromoles per liter

A11

A12

137

−0.5 −1.0

−1.5

VERTICAL EXAGGERATION ×2

Figure 7.—Continued

Distance from reference point on shoreline, in meters

300 200 100 0

−2.0

−2.0 −1

400

Mean high tide (MHT) and mean low tide (MLT) determined from tide stage data from July 13 to September 8, 2016

18 Evidence of Nitrate Attenuation in Intertidal and Subtidal Groundwater in a Subterranean Estuary at a Cape Cod Embayment EAST

WEST

MA-FSW 777 A01–A12 (Transect A)

I. Nitrogen gas 1.0

A01

Eel River

A02

0.5

A03

611

667 633

A04

A06

A07

602

656

−0.5

A05

569

647

651

650

−1.0

644 625

638

626

639

641

A08

A09

538

565

633

663

734

626

633

−1.5

MLT

492

633

632

MHT

665

A10

A11

1.0

EXPLANATION

0.5

Nitrogen gas (N2), in micromoles per liter

A12

538

529

617

732

751

730

748

736

825

737

749

849

801

811

−0.5 −1.0 −1.5

−2.0

850 800 750 700 650 600 550 500 450 400

−2.0

Altitude, in meters from NGVD 29

J. Argon gas 1.0

1.0

A01

Eel River

A02

0.5

A03

15.4

17.2

16.7

16.7 16.8

−1.0

A05

A06

A07

15.3

16.9

−0.5

A04

16.6

A09

13.3

16.4

13.6

16.3

16.9

16.5

15.9

16.4

15.9

16.6

16.5

16.4

−1.5

0.5

MLT

12.8

16.7 16.7

A08

MHT

A10

A11

0 A12

13.3

13.1

13.3

14.8

14.2

14.3

14.7

14.8

16 .3

15.3

15.5

−0.5 −1.0

EXPLANATION Argon gas, in micromoles per liter 19 17 15 13

−1.5

−2.0

K. Ratio of nitrogen gas to argon gas 1.0

1.0

A01

Eel River

A02

0.5

38.1 38.7

A04

38.9

38.6 38.7

−1.0

A05

38.9

37.5

−0.5

A03

A06

A07

39.3

38.5

38.1

38.1 38.6

38.6

A08

MHT

A09

38.3

40.5

41.5

38.9

39.4

46.2

A10

A11

0 A12

40.4

46.3

49.4

51.5

52.3

50.1

38.5

38.3

47.1

38.5

39.2

49.1

49.6

52.2

−1.5

0.5

MLT

52.2

−0.5 −1.0

50.5 52.4

−1.5 −2.0

Distance from reference point on shoreline, in meters VERTICAL EXAGGERATION ×2

Figure 7.—Continued

55 50

−2.0 −1

EXPLANATION Molar ratio of nitrogen gas (N2) to argon gas

45 40 35 30 Mean high tide (MHT) and mean low tide (MLT) determined from tide stage data from July 13 to September 8, 2016

Hydrogeologic and Geochemical Observations 19 EAST

WEST

MA-FSW 734–MA-FSW 747 (Transect B)

A. Specific conductance 1.0

1.0

F741

Eel River

F740

0.5 0

1,831

18,570

419

2,377

335

5,906

−0.5

F739

F738

9,313

3,003

2,529

303

1,744 484

278

382 160

349

147

162

−1.0

F737

135

F736

MHT

F735

6,556

F734 F733 20,870 41,580 31,820

148

3,947

138 126

454

F741

A11

35,530

140

207

453

19,150

137

217 464

25,490

−1.5

0.5

MLT

38,110 32,660 35,950

35,300

12,850

A12

38,940

−0.5

38,960

EXPLANATION Specific conductance, in microsiemens per centimeter >30,000 20,000

−1.0

8,857 26,120

−1.5

36,940

−2.0

10,000 100

B. Dissolved oxygen 1.0

F741

Eel River

F740

0.5

238

Altitude, in meters from NGVD 29

−0.5

F739

318

203

142

158

77 126

F738

3 13

−1.0

F737 1

3 3

F736

F735

MLT

F734

−1.5

MHT

F733

F741

A11

1 1

EXPLANATION

0.5

Dissolved oxygen (O2), in micomoles per liter

0 A12

1.0

−0.5 −1.0

−1.5

300 200 100 0

−2.0

C. pH 1.0

1.0

F741

Eel River

F740

0.5 0

5.67

6.98

5.85

6.81

5.72

5.74

−0.5

F739

F738

6.95

5.87

6.51

5.4

5.56

5.64

5.77

5.29

5.6

−1.0

F736 6.74

5.48

5.23

5.37

F737

5.75

F735 5.79

F734

MHT

F733

F741

6.97

7.13

7.64

5.21

6.19

5.4

5.41

5.52

5.66

5.69

5.76

5.71

5.7

5.27

A11

−0.5

7.67 6.6

7.65

5.49

0 A12

7.69

5.47

6.74

5.85

−1.5

0.5

MLT

6.92 6.74

5.75

EXPLANATION pH, in standard units 8.6 7.6 6.6

−1.0

5.6

−1.5

4.6

−2.0

D. Alkalinity 1.0

F741

Eel River

F740

0.5 0

160

624

201

447

161

156

−0.5

152

F739

F738

1,156

198

338

148

168

153

155

128

148

154

153

−1.0

F737

197

F736

F735

MHT

886

484

F734 F733 1,865 1,947

150

149

154

185

166

165

191

173

174

MLT F741 2,209

145 133

EXPLANATION

0.5

Alkalinity, in microequivalents per liter

2,000

−0.5

1,500

−1.0

1,000

A12

2,468

2,211

2,138

619 279

1,214

1,738

658

−1.5

A11

1.0

201

1,793

−1.5

245

−2.0

−2.0 −1

Distance from reference point on shoreline, in meters VERTICAL EXAGGERATION ×2

500 0

1,790

Mean high tide (MHT) and mean low tide (MLT) determined from tide stage data from July 13 to September 8, 2016

Figure 8. Vertical transects showing distributions of A, specific conductance, B, dissolved oxygen, C, pH, D, alkalinity, E, nitrate plus nitrite, F, ammonium, G, total Kjeldahl nitrogen, H, dissolved organic carbon, I, nitrogen gas, J, argon gas, and K, ratio of nitrogen gas to argon gas in groundwater along transect B–B′ beneath the Eel River, Seacoast Shores peninsula, East Falmouth, Massachusetts, November 9–10, 2015. Location of transect is shown in figure 4.

20 Evidence of Nitrate Attenuation in Intertidal and Subtidal Groundwater in a Subterranean Estuary at a Cape Cod Embayment EAST

WEST

MA-FSW 734–MA-FSW 747 (Transect B)

E. Nitrate plus nitrite 1.0

1.0

F741

Eel River

F740

0.5 0

114

454 433

−0.5

F739 171

F738 381

299

203

396

536

146

−1.0

F736 21

105

403

406

F737

F735

MHT

F734

F733

F747

122

109

124

121

130

F746

125

107

111

−1.5

0.5

MLT

58 0

307

F745

430

174

−0.5 −1.0 −1.5

−2.0

EXPLANATION Nitrate plus nitrite, in micromoles per liter 600 500 400 300 200 100 0

−2.0

F. Ammonium 1.0

F741

Eel River

F740

0.5 0

F738 0

−0.5

Altitude, in meters from NGVD 29

F739 0 0

0 0 11

−1.0

F737

F736

F735

MHT MLT

F734

F733

F747

F746

40 0

5 20

−1.5

1.0

EXPLANATION

0.5

Ammonium, in micromoles per liter

F745

−0.5 −1.0 −1.5

50 25 0

−2.0

G. Total Kjeldahl nitrogen 1.0

1.0

F741

Eel River

F740

0.5 0

F739 0

−1.0

F737 9

−0.5

F738

F736

F735

MHT

F734

F733

F747

F746

63 0

−1.5

0.5

MLT

13 37

F745

−0.5 −1.0 −1.5

EXPLANATION Total Kjeldahl nitrogen, in micromoles per liter 150 100 50 0

−2.0

H. Dissolved organic carbon 1.0

F741

Eel River

F740

0.5 0

167

113

290

158

102 79

−0.5

F739 256

F738 87

109

−1.0

F737

F736

F735

MHT MLT

F734

F733

167

108

126

108

0 0

171

−1.5

F747

F746

112 0 87 88

124

1.0

EXPLANATION

0.5

Dissolved organic carbon, in micromoles per liter

0 F745

106

101

−0.5 −1.0 −1.5

Distance from reference point on shoreline, in meters VERTICAL EXAGGERATION ×2

pem24-0030_fig08efgh Figure 8.—Continued

300 200 100 0

−2.0

−2.0 −1

400

Mean high tide (MHT) and mean low tide (MLT) determined from tide stage data from July 13 to September 8, 2016

Hydrogeologic and Geochemical Observations 21 EAST

WEST

MA-FSW 734–MA-FSW 747 (Transect B)

I. Nitrogen gas F741

1.0

F740 F739

0.5 0

628

564

718

608

627

584

−0.5

F738

F737

665 638

560

695

650

589

619

700

675

656

717

745

−1.0

F736

715

F735

768

680

717

709

Eel River

MHT

F734

MLT F733

F747

563

610

600

692

704

785

538

706

719

716

700

687

708

701

F746

−1.5

0.5

F745

−0.5

610

698

−1.0

644

592

612

537

610

EXPLANATION Nitrogen gas (N2), in micromoles per liter

621

615

1.0

−1.5

579

−2.0

850 800 750 700 650 600 550 500 450 400

−2.0

Altitude, in meters from NGVD 29

J. Argon gas F741

1.0

F740 F739

0.5 0 −0.5

F738

14.8

16.6 18.2

16.2

15.1

F737

16.9 16.3

14.5

17.6

16.6

15.1

15.9

17.8

16.5

18.2

−1.0

F736

17.9

F735

Eel River

MHT

F734

MLT F733

F747

16.5

14.5

18.1

17.7

17.4

18.1

17.9

17.6

17.5

17.7

14.9

17.5

−1.5

15.1

F746

15 17.9 15.4 13.8

15.5

0.5 0 F745

14.5

14.3

14.6

15.9

13.6

14.4

12.8

−0.5 −1.0

EXPLANATION Argon gas, in micromoles per liter 19 17 15 13

−1.5

−2.0

K. Ratio of nitrogen gas to argon gas 1.0

F741

1.0

F740

0.5 0 −0.5

Eel River

F739

38.2

39.5

38.7

38.8

F738

F737

39.4

38.7

39.3

39.1

39.3

39.8

39.5 39.3 39.5

40.9

−1.0

F736

39.9

F734

F735

MHT

F733

F747

42.7

41.2

39.6

38.8

40.5

39.7

40.1

39.8

40.1

39.8

39.4

40.1

36.1

−1.5

0.5

MLT

39.4

F746 40.1 43.9 45.8 44.5

42.6

47.7

40.7

43.7

42.6

41.8

45.1

−0.5 −1.0 −1.5 −2.0

Distance from reference point on shoreline, in meters VERTICAL EXAGGERATION ×2

Figure 8.—Continued

55 50

F745

−2.0 −1

EXPLANATION Molar ratio of nitrogen gas (N2) to argon gas

45 40 35 30 Mean high tide (MHT) and mean low tide (MLT) determined from tide stage data from July 13 to September 8, 2016

22 Evidence of Nitrate Attenuation in Intertidal and Subtidal Groundwater in a Subterranean Estuary at a Cape Cod Embayment EAST

WEST

MA-FSW 776 A01–A12 (transect D)

A. Specific conductance 1.0

1.0

A01

0.5

24,600

1,663

415

192

191

186

−0.5

Eel River

A02

8,500

143

A03 12,450 216 147 132

−1.0

A04 14,680 150 116

A05

A06

228

14,940

1173

40,800

31,000

41,300

38,900

43,200

43,500

111

104

MHT

−1.5

A07

39,500

196 171

A08

39,800 41,800

7,240

0.5

MLT A09

39,900 43,800

A11 37,400 41,400 40,800 42,100

A12

22,300

3,450

−0.5

11,250

20,000

−1.0

32,200 24,200 39,200

EXPLANATION Specific conductance, in microsiemens per centimeter >30,000

29,000

−1.5

41,700

−2.0

10,000 100

B. Dissolved oxygen 1.0

A01

0.5

A03

12 6

0 −0.5

Altitude, in meters from NGVD 29

Eel River

A02

2 46 95

114

−1.0

A04 19 63 60 56

MHT

A05

A06

A07

−1.5

A08

MLT A09

1.0

EXPLANATION

0.5

Dissolved oxygen (O2), in micomoles per liter

A11

A12 21

−0.5

4 12

−1.0

−1.5

300 200 100 0

−2.0

C. pH 1.0

1.0

A01

Eel River

A02

0.5

5.88 5.81

6.07

6.02

6.12

5.97

6.30

−0.5

A03 5.85 6.04 6.06

5.8

5.84

−1.0

A04

A05

5.4 5.65 5.57 5.46

4.92 5.08 5.26 5.15

A06 5.64 6.39 6.1 5.78

A07 6.77

A08

MHT

A09

A11

7.3

6.74

7.59

7.43

7.5

6.74

7.42

7.32

7.63

6.92

7.63

7.54

7.57

−1.5

0.5

MLT A12

7.78 7.55

7.61

7.58

7.56

−0.5

7.49 6.9 7.02

7.55

7.06

EXPLANATION pH, in standard units 8.6 7.6 6.6

−1.0

5.6

−1.5

4.6

−2.0

D. Alkalinity 1.0

A01

Eel River

A02

0.5

532 533

568

A03 541

A04

A05

A06

A07

MHT MLT

1.0

EXPLANATION

0.5

Alkalinity, in microequivalents per liter

2,000

−0.5

1,500

−1.0

1,000

−1.5

500

−2.0

786

620

506

361

−0.5

359

475 462 351

−1.0

199

216

100

184 185

116 111

A08

A09

A11

A12

235 160

−1

2 3 4 5 6 7 8 9 10 11 Distance from reference point on shoreline, in meters VERTICAL EXAGGERATION ×2

Mean high tide (MHT) and mean low tide (MLT) determined from tide stage data from July 13 to September 8, 2016

Figure 9. Vertical transects showing distributions of A, specific conductance, B, dissolved oxygen, C, pH, D, alkalinity, E, nitrate plus nitrite, F, ammonium, G, total Kjeldahl nitrogen, H, dissolved organic carbon, I, nitrogen gas, J, argon gas, and K, ratio of nitrogen gas to argon gas in groundwater along transect D–D′ (F776) beneath the Eel River, Seacoast Shores peninsula, East Falmouth, Massachusetts, June 14–15, 2016. Location of transect is shown in figure 4.

Hydrogeologic and Geochemical Observations 23 EAST

WEST

MA-FSW 776 A01–A12 (transect D)

E. Nitrate plus nitrite 1.0

A01

0.5

1.0 Eel River

A02 0

46 105

−1.0

A05

A04

−0.5

A03

119

61 103

A06

A07

431

A09

A10

385

A11

−0.5

344 376

−1.0

A12

0.5

MLT

A08

318

−1.5

MHT

−1.5

−2.0

EXPLANATION Nitrate plus nitrite, in micromoles per liter 600 500 400 300 200 100 0

−2.0

F. Ammonium 1.0

A01 6

0.5

Altitude, in meters from NGVD 29

−1.0

A05

A04

−0.5

A03

Eel River

A02 A06

A07

−1.5

A08

MHT MLT A09

1.0

EXPLANATION

0.5

Ammonium, in micromoles per liter

A10

A11

A12

−0.5

−1.0

−1.5

75 50 25 0

−2.0

G. Total Kjeldahl nitrogen 1.0

A01

Eel River

A02

0.5

A04

−0.5

A03

−1.0

A05

A06

7 0 0 0

A07

A08

MHT MLT A09

A10

A11 15

106

104

109

106

−1.5

100

1.0

EXPLANATION

0.5

Total Kjeldahl nitrogen, in micromoles per liter

150

−0.5

100

A12 15

−1.0 −1.5

50 0

−2.0

H. Dissolved organic carbon 1.0

A01

Eel River

A02

0.5

278

145

192

188

A04

268

361

321

−0.5

A03

370

342

129

162

−1.0

109 62 61 65

A05

A06

56 44 0 0

A07

A08

MHT MLT A09

147

140

120

142

119

107

125

106

116

102

112

−1.5

A10

A12

123

101

126

0.5

−0.5

111

EXPLANATION Dissolved organic carbon, in micromoles per liter

A11

131

1.0

−1.0

61 70

−1.5

VERTICAL EXAGGERATION ×2

Figure 9.—Continued

Distance from reference point on shoreline, in meters

300 200 100 0

−2.0

−2.0 −1

400

Mean high tide (MHT) and mean low tide (MLT) determined from tide stage data from July 13 to September 8, 2016

24 Evidence of Nitrate Attenuation in Intertidal and Subtidal Groundwater in a Subterranean Estuary at a Cape Cod Embayment EAST

WEST

MA-FSW 776 A01–A12 (transect D)

I. Nitrogen gas 1.0

A01 605

0.5

Eel River

A02 602

A03 651

0 660

684

−0.5

A05

588 650

628

−1.0

A04

652 650

632 629 651

A06

A07

A08

634

491

563

502

481

679

655

561 567

MHT MLT A09

A10

532

541

A11

652 624

484

557 596

538

641

0.5

−0.5 −1.0

554 539

−1.5

EXPLANATION Nitrogen gas (N2), in micromoles per liter

A12

553

1.0

−1.5

−2.0

850 800 750 700 650 600 550 500 450 400

−2.0

Altitude, in meters from NGVD 29

J. Argon gas 1.0

1.0

A01 15. 6

0.5

Eel River

A02 15. 2

A03 16. 5

0 17

17. 2

−0.5

A04

A05

15. 3 17

16. 7

−1.0

17. 1 17. 1

16. 4 16. 5 16. 9

A06

A07

14. 1

13. 1

13.1

12. 9

17. 2

A08

14. 4 14. 9

MHT

0.5

MLT A09 13. 5

14. 1

A10

A11

0 A12

13. 7

14. 5

12. 9

13. 7

−0.5

15. 1 15. 4

−1.0

14. 9

13. 9

−1.5

14. 8

13. 8

−1.5

EXPLANATION Argon gas, in micromoles per liter 19 17 15 13 11

−2.0

K. Ratio of nitrogen gas to argon gas 1.0

1.0

A01

Eel River

A02

0.5

38. 9 39. 7

A03 39. 4

0 38. 8

39. 8

−0.5

A05

38. 3 38. 1

37. 5

−1.0

A04

38. 1 38. 1

38. 5 38. 2 38. 4 38. 5

A06 42. 3 39. 8

A07 37. 8 38.4

39. 4

A08 38. 3

A09 39. 5

38. 3

A10

A11 44. 9

37. 5

40. 5

38. 6

2 3 4 5 6 7 8 9 10 11 Distance from reference point on shoreline, in meters VERTICAL EXAGGERATION ×2

Figure 9.—Continued

−0.5

41. 5 38. 6

−1.0

43 37. 4

−1

A12

40. 5

−1.5 −2.0

0.5

MLT

37. 3 39. 1

MHT

−1.5 39. 1

−2.0

EXPLANATION Molar ratio of nitrogen gas (N2) to argon gas 55 50 45 40 35 30 Mean high tide (MHT) and mean low tide (MLT) determined from tide stage data from July 13 to September 8, 2016

Hydrogeologic and Geochemical Observations 25 A. October 15–December 15, 2015

1.2 1.0 0.8 0.6 0.4 0.2 0 −0.2

Sampling

−0.4 −0.6

Gage height, in meters from NGVD 29

1.2

October

2015

November

December

B. June 1–July 30, 2016

1.0 0.8 0.6 0.4 0.2 0 −0.2

−0.6

1.2

Sampling

−0.4 1

June

2016

July

C. October 1–November 30, 2016

1.0 0.8 0.6 0.4 0.2 0 −0.2 Sampling

−0.4 −0.6

October

2016

November

Figure 10. Graphs showing tidal stage and shallow nearshore groundwater sampling days (red arrows) during neap tide during A, November 2015 sampling, B, June 2016 sampling, and C, October 2016 sampling. Tide data for U.S. Geological Survey site 413601070275800, Popponesset Bay, Mashpee Neck Road, near Mashpee, Massachusetts. NGVD 1929, National Geodetic Vertical Datum of 1929.

26 Evidence of Nitrate Attenuation in Intertidal and Subtidal Groundwater in a Subterranean Estuary at a Cape Cod Embayment

Nitrate Plus Nitrite The vertical profiles of nitrate plus nitrite concentrations differed substantially among transects. In transect A–A′, relatively high nitrate plus nitrite concentrations (400–600 µmol/L) extended from the intertidal zone (A03) to 9 m offshore into the subtidal zone (A08) from 61 to 122 cm below the sediment/ water interface (fig. 7E). This zone of elevated nitrite plus nitrate is notable in that these concentrations in the shallow subterranean estuary beneath the intertidal zone were five times higher than the concentrations measured in 2013 at the same depths below the intertidal zone in transect B–B′. There was a smaller spatial extent and lower elevated concentration zone in transect B–B′ in 2015, compared with the zone in transect A–A′ extending from onshore to the middle of the intertidal zone with the highest nitrate plus nitrite concentrations at the 91-cm depth below the sediment/water interface at F740 and F739 (fig. 8E). Additionally, nitrate plus nitrite concentrations were moderately elevated at 1.5-m depth at F733 and at 61-cm depth at F745 (fig. 8E). The nitrate plus nitrite concentrations were low in transect D–D′ but detectable in most of the intertidal zone but were substantially higher at the offshore end of the intertidal zone at 91-cm and 122-cm depths (A06) and the nearest subtidal (A07) locations and at 30-cm and 61-cm depth at the farthest offshore location (A12) (fig. 9E). Below the 30-cm depth and from the intertidal to near subtidal zone (A03 to A08), nitrate plus nitrite concentrations were substantially higher in the subtidal zone in the northernmost transect A–A′ than in transect D–D′. Nitrate plus nitrite concentrations in deeper zones (3.3-m and 4.88-m depth) in transect C–C ′ ranged from 300 to 700 µmol/L (Huntington and others, 2024) and were similar to the concentrations measured at this depth in transect B–B′ sampled in 2013 (fig. 3) (Colman and others, 2018). Nitrite concentrations were less than the reporting limit of 7 µmol/L in all but six samples with intermediate salinities. Reported nitrite concentrations ranged from 5 to 30 µmol/L, and nitrite/nitrate ratios were less than 0.1 in all but one sample (0.2 at the shallowest location at A11 in transect D–D′). In this report, nitrate (or NO3−) commonly is used to refer to nitrate plus nitrite.

Ammonium Dissolved ammonium concentrations generally were positively correlated with SpC. Ammonium was either not detected, or detected at very low concentrations, at all depths and in all transects in the intertidal region from the shore to 4 m offshore (figs. 7F, 8F, and 9F). In the subtidal region, 6–14 m offshore, ammonium concentrations generally were elevated (30–100 µmol/L) relative to fresh groundwater concentrations in the intertidal zone at the shallowest depth (30 cm) in transects A–A′ and B–B′ (figs. 7F and 8F). In transect D–D′, ammonium concentrations generally were elevated (30–53 µmol/L) at all depths in the subtidal zone 7.5–12 m offshore (fig. 9F). These ammonium concentrations and the

concentrations measured for the three well clusters sampled in October 2016 (transect C–C ′) that were sampled at 1.8-m, 3.3-m, and 4.88-m depths were similar to those measured in 2013, indicating persistence of somewhat elevated ammonium in the shallow subterranean estuary wherever relatively saline porewater was observed. In most locations and sampling depths within 4–5 m of the shore in transects A–A′ and B–B′, there was measurable nitrate plus nitrite but little or no detectable ammonium. Where ammonium and nitrate plus nitrite were both present in measurable concentrations, ammonium was typically a factor of 5 or 10 lower in concentration than nitrate plus nitrite. At the shallower sampling depths from about 7 to 13 m offshore, there were many instances where ammonium was present but nitrate plus nitrite was not detected. Ammonium concentrations tended to be highest in the most saline groundwater. It is unlikely that the ammonium in this zone originated from infiltrating surface water that mixed with the underlying freshwater flowing into this zone (fig. 3) because ammonium was less than detection limits in the Eel River when measured in 2013 (Colman and others, 2018). Ammonium likely originated in part from the anaerobic degradation of organic matter in the shallow subestuarine sediments (Reddy and others, 1984). Ammonium also could be produced by microbial dissimilatory nitrate reduction (Lam and Kuypers, 2011); however, concentrations of nitrate were low in infiltrating saline water. Furthermore, the low oxygen concentrations in saline groundwater would not have supported substantial ammonium oxidation to nitrite or nitrate. Ammonium also can sorb to or desorb from sediment ion-exchange sites. Mixing of nitrate-bearing fresh groundwater with ammonium-bearing fresh or saline shallow porewater could have supported anammox, the coupled anaerobic oxidation of ammonium and reduction of nitrite to produce nitrogen gas (Smith and others, 2015). It is not clear whether all of the ammonium was carried into the mixing zone by saline groundwater that transported ammonium produced through anaerobic metabolism in the subestuarine sediments, or if some of the ammonium was produced within the mixing zone; however, there is no evidence for substantial ammonium enrichment in mixed-salinity samples. Ammonium concentrations in saline groundwater below the freshwater lens (8 or more meters below our lowest sampling point) ranged from 89 to 119 μmol/L in groundwater 35 m onshore and from 4 to 62 μmol/L in the subterranean estuary at approximately 10 m offshore, as determined by Colman and others (2018). Ammonium concentrations in sample locations where evidence of nitrate reduction was observed (refer to section “Evaluation of Nitrate Reduction to Nitrogen Gas”) typically ranged from 5 to 10 µmol/L, whereas ammonium was largely absent from other areas, except the shallowest (30-cm) depth, where SpC indicated saltwater infiltration from the estuary where ammonium concentrations typically were 50–60 µmol/L (fig. 7F). This association between locations where there was geochemical evidence for denitrification and the presence of ammonium also was reported by Colman and others (2018).

Evaluation of Nitrate Reduction to Nitrogen Gas 27

Total Kjeldahl Nitrogen and Organic Nitrogen The distribution of dissolved total Kjeldahl nitrogen in transects A–A′ and D–D′ sampled in June 2016 generally were similar to the distribution of SpC. Total Kjeldahl nitrogen concentrations ranged from 40 to 130 µmol/L at the shallowest depth (30 cm) in the subtidal zone in transect A–A′ (fig. 7G) and at all depths in the subtidal zone from 7.5 to 11.5 m from shore in transect D–D′ (fig. 9G). Total Kjeldahl nitrogen concentrations generally were elevated (60–80 µmol/L) at the shallowest depth (30 cm) and ranged from 2 to 80 µmol/L at all depths from 61 to 122 cm in the subtidal zone of transect B–B′ (fig. 8G). The similar distributions of salinity and total Kjeldahl nitrogen in these mixing zones, and the fact that Colman and others (2018) reported minimal total Kjeldahl nitrogen concentrations (less than 10 µmol/L) throughout the freshwater subterranean estuary in this area, indicates a predominantly estuarine (surface water or sediment) source rather than an upwelling fresh groundwater source. Dissolved organic nitrogen concentrations (determined from the difference between total Kjeldahl nitrogen and ammonium nitrogen) in the subtidal zones of transects D–D′ and A–A′ ranged from 0 to 85 (median=17) μmol/L and 0 to 49 (median=8) μmol/L, respectively. Colman and others (2018) reported dissolved organic nitrogen concentrations in Eel River surface water of 22 μmol/L in August 2013. Dissolved organic nitrogen in the subtidal zone could be derived from infiltrating estuarine water or anaerobic metabolism in subestuarine sediments.

Dissolved Organic Carbon The patterns in the vertical profiles of DOC concentration (figs. 7H, 8H, and 9H) generally were similar to the patterns of SpC (figs. 7A, 8A, and 9A) in all transects, with the exception of a zone of high DOC concentrations (200–480 µmol/L as carbon) in onshore groundwater that generally decreased with depth and with increasing distance from the shore. DOC was moderately elevated (60–167 µmol/L as carbon) in the shallow subsurface (30-cm depth) in the subtidal zone from 7 to 14 m offshore. At depths of 61 to 122 cm in the intertidal and subtidal regions, DOC generally was below the level of detection in transect A–A′ and ranged from 50 to 120 µmol C/L in transects B–B′ and D–D′. Colman and others (2018) reported DOC concentrations ranged from 160 to 250 µmol/L in the Eel River surface water in August 2013. The vertical distributions suggest a terrestrial surface soil source for the zone of elevated DOC onshore and an estuarine source infiltrating the shallow sediments offshore that is variable in time. In general, DOC concentrations were relatively low in fresh groundwater containing high nitrate concentrations; thus, DOC in the subterranean estuary apparently was derived from sources other than onshore wastewater recharge.

Concentrations and Ratios of Nitrogen and Argon Gases The concentrations of dissolved nitrogen and argon in samples from transects A–A′, B–B′, and D–D′ are shown in figures 7I, 7J, 8I, 8J, 9I, and 9J, respectively. Argon and nitrogen gas concentrations generally were inversely correlated with SpC values because the solubilities of atmospheric gases in water are inversely related to salinity. Minor deviations from this relation commonly result from varying amounts of excess air in groundwater samples, whereas larger deviations can result from addition of excess nitrogen gas from nitrate reduction (N2MIC). In general, areas of the profiles with relatively high N2/Ar ratios are interpreted to contain substantial concentrations of N2MIC. The area of highest N2/Ar ratios is in relatively fresh groundwater in transect A–A′, 10 to 13.5 m from the shoreline at 61-cm to 122-cm depth below the sediment/water interface (fig. 7K). Other areas with elevated N2/Ar ratios include 9–10 m from the shoreline at the 30-cm depth in transect A–A′ (fig. 7K), 11–13.5 m from the shoreline at 30-cm to 122-cm depths in transect B–B′ (fig 8K), and 11–13.5 m from the shoreline at the 30-cm depth in transect D–D′ (fig. 9K). The N2/Ar ratios in groundwater samples collected on transect C–C ′ in October 2016 from locations F773, F774, and F775 at depths of 1.83, 3.35, and 4.88 m below the sediment/water interface with 343 < SpC < 2,560 µS/cm at 25 °C had relatively low N2/Ar ratios, suggesting relatively little nitrogen gas from nitrate reduction (Huntington and others, 2024). In contrast, two of the three wells with intermediateto-high salinity (10,040 < SpC < 35,620 µS/cm at 25 °C) showed evidence of microbial production of nitrogen gas, resulting in higher N2/Ar ratios. The two wells showing evidence for excess nitrogen gas were at the shallowest depth (1.83 m) and highest SpC (27,360–35,620 µS/cm at 25 °C) and farthest from the shore (F774 and F775). Nitrate concentrations were about 600 µmol/L in the relatively fresh groundwater beneath those wells.

Evaluation of Nitrate Reduction to Nitrogen Gas A common method for evaluation of nitrogen gas and argon gas concentrations in groundwater, and estimation of microbial nitrogen gas production, includes plotting the data as scatterplots of nitrogen gas in relation to argon gas (fig. 11). In this plot, the solid lines indicate theoretical dissolved gas concentrations in equilibrium with air (ASW, air-saturated water; Weiss, 1970) for water having salinities and recharge temperatures roughly bracketing those of the samples (0 and 30 practical salinity units [psu]; 10, 15, and 20 °C), and dotted lines indicate the effect of adding 2 mL (standard temperature and pressure, STP) of excess air per liter due to dissolution of bubbles during recharge or sample processing.

28 Evidence of Nitrate Attenuation in Intertidal and Subtidal Groundwater in a Subterranean Estuary at a Cape Cod Embayment The nitrogen and argon gas concentrations in the freshest (salinity <0.1 psu) endmember groundwater samples collected from transects A–A′, B–B′, and D–D′ are clustered in an array that resembles a hypothetical trajectory for air-saturated water (ASW) plus variable amounts of excess air (EA ≈ 0–3 ccSTP/L) at relatively constant recharge temperature (T ≈ 11–12 °C) (fig. 11). Average groundwater recharge temperatures commonly are similar to average air temperatures in humid temperate regions (Stute and others, 1995). Apparent recharge temperature can deviate significantly from average air temperature where the vadose zone is thin, and, because of warming through time of air and soil temperature, estimated recharge temperature can vary with date of recharge (Szymczycha and others, 2017). Thus, the observed scatter of points around the estimated recharge temperature and EA line is likely due to variations in recharge temperature as well as procedural and analytical errors. As the average air temperature at the nearby Barnstable airport from 1991 to 2020 was 11.1 °C (Northeast Regional Climate Center, http://www.nrcc.cornell.edu, accessed on April 11, 2022), the clustering of gas data indicating a similar recharge temperature with variable excess air (fig. 11) suggests that gas concentrations in the fresh groundwater endmember samples were largely controlled by physical processes, and that microbial nitrogen gas was relatively minor. In high-salinity groundwater samples (salinity ≥ 25 psu), gas concentrations plot in an array that is roughly parallel to the ASW curve for saline water (30 psu), with a relatively wide range of apparent recharge temperatures (fig. 11). Data suggest relatively little EA content, and little or no clearly resolvable N2MIC in the high salinity samples, consistent with general lack of nitrate in saline estuarine recharge. The pattern observed for samples with intermediate salinity (0.1 < salinity ≤ 25 psu) (fig. 11) contrasts strongly with the pattern for the fresh and saline endmember samples. Gas concentrations in many of these intermediate-salinity samples show evidence of substantial microbial production of nitrogen gas, resulting in a large shift to the right along the nitrogen gas axis (fig. 11). Previous studies have provided evidence for microbial removal of nitrate in subterranean estuaries on Cape Cod, Massachusetts, where nitrate conveyed by fresh groundwater approaches discharge areas near the land/sea margin where reducing, saline groundwater also is present (Kroeger and Charette, 2008; Colman and others, 2018). Quantification of nitrate losses, as freshwater parcels transit toward the estuary and mix prior to discharge with saline groundwater that has recharged through Eel River sediment, requires a different approach to that commonly used for fresh groundwater samples (Böhlke and others, 2002; Griggs and others, 2003; Szymczycha and others, 2017). In estuarine research, it is common practice to examine behavior of dissolved constituents across the salinity gradient, to determine whether concentrations deviate from conservative mixing between representative freshwater “endmember” concentrations and saline endmember concentrations. Concentrations above such mixing lines are evidence of

production of the constituent of interest during transit through the subterranean estuary, while concentrations below the conservative mixing line provide evidence for loss or consumption of the constituent of interest. In groundwater settings where dispersion is limited, substantial variability in constituent concentrations can occur at relatively small spatial scales, and there can be uncertainty regarding which freshwater and saltwater samples best represent the endmembers for any given mixed salinity sample. Those features present a challenge for analysis of conservative mixing. Nonetheless, the ranges of concentrations and concentration changes in groundwater environments commonly are large enough that chemical transformation can be readily detected, particularly where a large number of samples have been collected across each salinity zone (Kroeger and Charette, 2008). In the current study, we applied conservative mixing analysis for dissolved gas samples in the subterranean estuary beneath the Eel River (fig. 12A–12E). Samples were divided into groups representing a “fresh” endmember (<0.1 psu) that displayed evidence of varying excess air but no clear evidence of N2MIC, a “saline” endmember (>25.0 psu) that presumably was recharged from the estuary with little or no nitrate and also displayed little or no evidence of N2MIC, and “mixed” samples (≥0.1 to 25 psu) with evidence of varying N2MIC concentrations. Whereas the individual gas concentrations vary substantially because of varying physical factors such as recharge temperature and excess air, the N2/Ar ratios are less affected by those factors and relatively more sensitive to addition of microbial nitrogen gas. Data from the “fresh” and “saline” endmembers were used to determine a “baseline” relation between salinity and the N2/Ar ratio for samples most likely to have no N2MIC (fig. 12C). The N2/Ar baseline equation was constrained by the mean values of psu and N2/Ar in the fresh and saline endmembers and by the assumption that the mean values of calculated N2MIC concentrations in those groups should be 0: N2/Ar[baseline] = −0.02892*psu + 38.922.

(3)

Samples that plot substantially above this N2/Ar mixing baseline indicate excess (non-atmospheric) nitrogen gas that may be a result of nitrate reduction (that is, N2MIC). The apparent concentration of N2MIC in each sample was estimated from: N2MIC = Ar * (N2/Ar − N2/Ar[baseline]).

(4)

Calculated N2MIC concentrations are summarized in figure 12D, where small positive and negative apparent values (for example, −25 µmol/L < N2MIC < 25 µmol/L) are related in part to uncertainties in the data and assumptions, as described below. The relation between calculated N2MIC and salinity indicates that nitrate loss occurred over a wide range of salinity values from nearly fresh to greater than 24 psu (fig. 12D).

Evaluation of Nitrate Reduction to Nitrogen Gas 29 19

EXPLANATION Transect A (F777)—Fresh

Transect A (F777)—Mixed

Excess air (11.5 °C)

Transect B (F733-F747)—Fresh Transect B (F733-F747)—Mixed Transect B (F733-F747)—Saline Transect C (F773-F775)—Mixed

Argon, in micromoles per liter

psu=0 EA=0

Transect D (F776)—Mixed

psu=0 EA=2

Air-saturated water 0 m elev., 10–20 °C 0 and 30 psu 0 and 2 ccSTP/L EA

Transect D (F776)—Fresh Transect D (F776)—Saline Air-saturated water

Air-saturated water plus excess air "F" is shorthand for MA-FSW

Excess N2

20 10 14

Degassing

13 psu=30 EA=0

psu=30 EA=2

12 20

11 400

500

600

700

Nitrogen, in micromoles per liter

800

900

Figure 11. Graph showing relation between nitrogen gas (N2) and argon gas (Ar) concentrations from transects A, B, C, and D in the nearshore subterranean Eel River estuary adjacent to the Seacoast Shores peninsula, East Falmouth, Massachusetts. Locations of transects are shown in figure 4. Samples are grouped by salinity: “Fresh” indicates samples (pink symbols) with salinity <0.1 practical salinity units (psu) (450 microsiemens per centimeter), “Saline” indicates samples (red symbols) with salinity >25.0 psu (39,500 microsiemens per centimeter), and “Mixed” indicates samples (blue symbols) with salinity intermediate between the fresh and saline endmembers. The solid lines indicate theoretical dissolved gas concentrations in equilibrium with air (ASW, air-saturated water) for freshwater (0 psu) and saline water (30 psu), with recharge temperatures of 10, 15, and 20 degrees Celsius (°C) (solubility data from Weiss, 1970). Dotted lines indicate the effects of adding 2 cubic centimeters (cc) of excess air at standard temperature and pressure per liter (2ccSTP/L EA). Dashed arrows indicate hypothetical trajectories that could result from degassing, excess N2 (addition of N2mic), and addition of excess air at constant temperature.

30 Evidence of Nitrate Attenuation in Intertidal and Subtidal Groundwater in a Subterranean Estuary at a Cape Cod Embayment For the “fresh” samples (n=37), equations 3 and 4 yielded a mean value of 0±13 (stdev) µmol/L for N2MIC and was consistent with a mean recharge temperature of 11.5 °C and excess air concentration of 1.1 ccSTP/L. For the “saline” samples (n=10), these equations yielded a mean value of 0±8 (stdev) µmol/L for N2MIC, which was consistent with a mean recharge temperature of 13.8 °C and excess air concentration of 0.6 ccSTP/L. For the “mixed” samples (n=82), equations 3 and 4 yielded variable concentrations of N2MIC ranging up to 217 µmol/L. The mean estimated fresh groundwater recharge temperature was similar to the mean annual air temperature in the area (11.1 °C), as discussed above; whereas the slightly higher mean estimated saline groundwater recharge temperature may have been related to the fact that many of the shallow saline samples were collected when infiltrating surface water from the estuary would have been warmer than the mean annual air temperature (for example, June 2016). The apparent difference in endmember excess air concentrations was consistent with previous coastal groundwater studies indicating subaqueous (saline) recharge is less likely to incorporate excess air than terrestrial (fresh) infiltration recharge (Böhlke and Krantz, 2003; Kroeger and Charette, 2008; Colman and others, 2018). The current calculations differ slightly from the approach used by Colman and others (2018), which was based on the approximation that all samples had 1±1 ccSTP/L of excess air. Relatively high and variable excess air concentrations in fresh samples were indicated by an array of sample data in the upper part of the Ar-N2 diagram (fig. 11). For comparison with results of the N2/Ar baseline equation (eq. 3), N2MIC concentrations for individual fresh samples also were calculated by assuming they were all recharged at a constant temperature and with variable excess air. For a constant recharge temperature of 11.53 °C, the mean N2MIC concentration in fresh samples was 0±17 µmol/L, indicating more variability with this approach than with equation 3, for the same assumed mean value of 0 for N2MIC. For the saline samples, an alternative approach to equation 3 was based on the assumption that no excess air was incorporated during recharge beneath the estuary. In this case, N2MIC concentrations were mostly positive, with mean value of 12±10 µmol/L. Thus, some N2MIC may have been present in saline samples, but the calculated concentrations were not significantly different from zero and it is possible that trace amounts of excess air were introduced as a result of imperfect sample handling. For each sample, the concentration of N2MIC derived from equations 3 and 4 was used to quantify the concentration of nitrate that may have been reduced to nitrogen gas by denitrification, where 2 moles of nitrate are consumed to produce 1 mole of N2: nitrate attenuation (µmol/L) = N2-NMIC or 2×N2MIC.(5)

If anammox was an important process of nitrogen attenuation, then each mole of N2MIC could represent as little as 1 mole of nitrate loss, with concurrent consumption of ammonium. For plotting and interpretation of the data, results were considered to be insignificant if less than 25 µmol/L as N2MIC (figs. 12D and 12E) and 50 µmol/L as N2-NMIC, based on the standard deviations of the “fresh” and “saline” endmember values and additional uncertainties that could be related to sample handling (for example, air contamination, leakage, gas loss) or the calculations (for example, incompletely constrained assumptions). Calculation uncertainties were related to the fact that multiple variables affect the total concentration of nitrogen gas in a groundwater sample, including recharge temperature, salinity, amount of excess air incorporated, fractionation of excess air, production of excess nitrogen gas from denitrification (or anammox), amount of degassing, and fractionation during degassing; thus, measurements of Ar, N2, and salinity do not provide unique estimates of the nitrogen gas yield from nitrate reduction. Additional measurements, such as other noble gases, can help reduce those uncertainties in some situations (for example, Böhlke and others, 2007). For each sample with dissolved gas data, measured nitrate plus nitrite concentrations were combined with the calculated concentration of nitrate loss (assuming all N2-NMIC was from denitrification) to obtain an estimate of the nitrate concentration the sample would have had in the absence of nitrate loss (apparent initial nitrate concentration). For these calculations, zero values were assigned to censored data (less than 7 µmol/L for measured nitrate plus nitrite and less than 50 µmol/L for nitrate loss). When plotted against salinity, these results provide useful constraints on major factors affecting the distribution of nitrate in the shallow subterranean estuary (fig. 13). In figure 13A, measured nitrate plus nitrite (“NO3−”) concentrations in samples analyzed for argon gas and nitrogen gas plot within the triangular area defined by (1) saline water with no initial nitrate, (2) freshwater with no initial nitrate, and (3) freshwater with high initial nitrate (maximum near 700 µM). With few exceptions, calculated concentrations of N2-NMIC (nitrate loss) and initial nitrate (assuming denitrification) also plot within the same triangular area (figs. 13B and 13C). These patterns are consistent with the interpretation that freshwater was recharged on land with variable concentrations of nitrate (more or less contaminated by wastewater) and then the nitrate was variably reduced microbially and variably diluted by mixing with saline water in the subterranean estuary. These data do not indicate clearly where the nitrate reduction occurred (only that it had occurred between recharge and sampling), nor do they indicate a common factor that controlled the distribution of nitrate reduction (which could have been promoted by electron donors in the freshwater, saline water, or sediment). Some of the largest concentrations of N2-NMIC were in samples that were nearly fresh, indicating that aqueous electron donors in saline water likely were not the predominant controlling factor in those cases.

A. Nitrogen gas concentration Low T, excess air, N2MIC

800 700 600 500

High T, degassing

400

C. Nitrogen gas to argon gas ratio

Nitrogen/argon, in micromoles per liter

Argon, in micromoles per liter

900

N2MIC

45 40 35

E. Nitrogen/argon gas ratios in samples with N2MIC < or > 25 µmol/L

B. Argon gas concentration Low T, excess air

18 16 14

High T, degassing

12 10

250

NitrogenMIC, in micromoles per liter

Nitrogen, in micromoles per liter

Evaluation of Nitrate Reduction to Nitrogen Gas 31

D. Calculated concentration of microbial nitrogen gas

200 150 100 50 0 −50

0 ± 25

Graphs A and B

Fresh

Mixed

Saline

Graph D

Salinity (psu)

Graph C

Fresh

Endmember means

EXPLANATION

Salinity (psu)

Mixing baseline Graph E

Transect A

N2MIC <25

Transect B

N2MIC >25

Transect C

Mixing baseline

Transect D

Figure 12. Graphs showing relations between salinity and A, nitrogen gas concentration (N2), B, argon gas concentration (Ar), C, N2/Ar ratio, D, calculated concentration of microbial N2 (N2MIC), and E, N2/Ar ratios in samples with N2MIC concentrations greater or less than the reporting limit of 25 µmol/L in groundwater from transects A–A', B–B', C–C', and D–D' in the nearshore subterranean Eel River estuary adjacent to the Seacoast Shores peninsula, East Falmouth, Massachusetts, November 2015–October 2016. Locations of transects are shown in figure 4. Samples are grouped in three salinity groups: “Fresh” indicates samples with salinity <0.1 practical salinity units (psu), “Saline” indicates samples with salinity >25.0 psu, and “Mixed” indicates samples with salinity intermediate between fresh and saline. The dashed lines indicate theoretical concentrations and gas ratios in a conservative mixing scenario between mean “endmember” values for fresh and saline samples, while the vertical dashed arrows indicate various causes of variation of gas concentrations. The “mixing baseline” is described by equation 3. N2/Ar ratios significantly above the mixing baseline are interpreted as evidence of microbial nitrogen gas production through denitrification (± anammox) during groundwater transport through coastal sediments and mixing with saline groundwater.

32 Evidence of Nitrate Attenuation in Intertidal and Subtidal Groundwater in a Subterranean Estuary at a Cape Cod Embayment A

800 Line extends to 1,200

700 600 500 400 300 200 100

600 500 400 300 200

Salinity (psu) Line extends to 1,200

700

Nitrate-initial, in micromoles per liter

Line extends to 1,200

100

0 800

700

Nitrate-lost, in micromoles per liter

Nitrate-remaining, in micromoles per liter

800

EXPLANATION Transect A Transect B

600

Transect C 500

Transect D

400 300 200 100 0

Salinity (psu)

Figure 13. Mixing plots showing A, measured nitrate concentration (nitrate-remaining), B, calculated nitrate loss (N2-NMIC) assuming denitrification (nitrate-lost), and C, estimated initial nitrate concentration (nitrate remaining plus nitrate lost) (nitrate-initial), versus salinity in samples that were analyzed for argon and nitrogen gas from all transects beneath the Eel River, Seacoast Shores peninsula, East Falmouth, Massachusetts. Locations of transects are shown in figure 4. Zero values were assigned to censored data (less than 7 micromoles [µmol/L] for measured nitrate plus nitrite and less than 50 µmol/L for nitrate loss). Dashed lines indicate hypothetical mixtures of saline groundwater with no initial nitrate and fresh groundwater with either 600 or 1,200 µmol/L initial nitrate.

Some of the nearly saline samples with no measurable nitrate apparently had significant concentrations of N2-N and initial nitrate that plot outside the triangular area described above. Assuming no problems with sample handling, storage, and analyses, those data potentially could be consistent with either (1) dilution and denitrification of a freshwater component with initial nitrate concentration

substantially higher than implied by the other samples (for example, around 1,200 µmol/L) or (2) anammox rather than denitrification as the predominant nitrate reduction process in a freshwater component more similar to the other samples (maximum around 700 µmol/L). Option 1 could be consistent with nitrate concentrations up to 1,700 µmol/L reported previously for deeper fresh groundwater beneath the

Evaluation of Nitrate Reduction to Nitrogen Gas 33 study site (Colman and others, 2018), although flow paths consistent with this hypothesis have not been identified. Option 2 could be consistent with apparent ammonium deficits in some samples with moderate to high salinities, in comparison to conservative groundwater mixing (dilution) curves. If anammox were indicated as a predominant process in any of the samples, then it could have contributed partially to nitrate loss in other samples as well. Additional studies involving isotopic tracers, microbial data, and field experiments could help resolve these possibilities (for example, Smith and others, 2015). Results summarized in figure 13 were aggregated to yield mean values and standard deviations for the concentrations of measured nitrate plus nitrite (158±202), nitrate loss (60±110 µmol/L), and initial nitrate (219±191 µmol/L) for the complete sample set with reported nitrogen gas and argon

gas concentrations. These averages include saline samples that were recharged with little or no nitrate as well as fresh and mixed samples that were recharged with variable nitrate concentrations. The ratio of nitrate loss to initial nitrate in this evenly weighted sample set would be equivalent to a fractional nitrate loss of 27 percent. It is not possible to assess the accuracy and uncertainty of these aggregated data as representations of overall nitrogen removal in the subterranean estuary. Nonetheless, there is a high degree of certainty that a detectable quantity of land-derived groundwater nitrate was reduced to microbial nitrogen gas within the subterranean estuary before being discharged, as indicated by nitrogen gas concentrations that were commonly greater in the mixed salinity sample set than in the endmember sample sets (figs. 11 and 12C; table 1).

Table 1. Summary statistics for dissolved gas concentrations and derived quantities in “Fresh,” “Saline,” and “Mixed” sample groups. [n, number of samples in group; Ar, argon concentration; N2, nitrogen gas concentration; N2/Ar, molar ratio of N2 to Ar; N2MIC, estimated concentration of microbial N2; T, mean estimated recharge temperature; EA, mean estimated concentration of excess unfractionated air; psu, practical salinity unit; µmol/L, micromole per liter; ccSTP/L, cubic centimeter at standard temperature and pressure per liter; std dev, standard deviation; --, not applicable]

Variable

Fresh (psu<0.10) n=37

Saline (psu>25) n=10

Mean

Std. dev

Mean

Std. dev

Salinity (psu)

0.07

0.01

26.35

Ar (µmol/L)

17.09

0.56

13.67

N2 (µmol/L)

665

N2/Ar

38.90

0.77

N2MIC (µmol/L)

T (°C)

11.53

EA (ccSTP/L)

1.12

0.58

Mixed (0.10<psu<25) n=82 Mean

Std. dev

0.69

8.43

9.10

0.75

15.40

1.35

522

643

38.15

0.61

41.84

4.18

13.76

1As described in the text, calculations were based on the assumption that the mean concentrations of N 2MIC were zero in the fresh and saline endmember groups (eq. 3).

34 Evidence of Nitrate Attenuation in Intertidal and Subtidal Groundwater in a Subterranean Estuary at a Cape Cod Embayment

Patterns and Controls of Nitrogen Transport and Attenuation Some of the major processes affecting the distribution of nitrate can be resolved by comparing the distributions of salinity, measured nitrate, and nitrate lost along transects A–A′, B–B′, and D–D′ (fig. 14). The extent, or completeness, of denitrification at a given sampled location can be inferred from the proportion of estimated nitrate loss to initial nitrate (sum of nitrate measured and lost). In most locations in the region from A03 to A08 (from 2.5 to 8.5 m from the reference point on the shoreline), nitrate removal from fresh and brackish groundwater was less than 50 µmol/L and maximum nitrate concentrations (500 and 700 µmol/L) were similar to those associated with the underlying groundwater moving upwards in the interval from 2 to 10 m below the sediment/water interface measured by Colman and others (2018) (fig. 3). In contrast, from A09 to A12 (from 10 to 14 m from shore), fresh and brackish groundwater commonly had little or no measured nitrate because of partial or complete nitrate reduction. The dissolved gas data suggest the following broad patterns regarding nitrate reduction in groundwater sampled in the present study, as detailed in the following paragraphs and as shown in figure 14. Relatively little nitrate reduction was evident in the nearshore fresh groundwater, as indicated by gas concentrations that are largely explained by solubility and by bubble dissolution during recharge (fig. 11). Colman and others (2018) reported substantial N2MIC in shallow fresh groundwater in the upland more than 30 m from the shore (fig. 3). In the highest salinity zones, there may be weak evidence for minor nitrate reduction (24±20 stdev µmol/L, assuming no excess air); this could indicate a small nitrate source in estuarine surface water or estuarine sediment porewaters, possibly associated with coupled nitrificationdenitrification near the sediment/water interface. The major occurrences of nitrate reduction were in or near areas where nitrate-bearing terrestrial fresh groundwater encountered more reducing saline groundwater recharged beneath the Eel River estuary, where electron donors may have been supplied by infiltrating surface water and shallow subestuarine sediments (for example, Kroeger and Charette, 2008). Geochemical indicators of nitrate reduction were observed principally during the June 2016 sampling of transect A–A′, November 2015 sampling of transect B–B′, and October 2016 sampling of transect C–C ′. Samples with the highest concentrations of excess nitrogen were observed 9 m or more from shore. Estimated nitrate loss was greatest in the zone from 10 to 14 m offshore and depths of 61 to 122 cm in the subterranean estuary of transect A–A′ and ranged from 230 to 434 µmol/L of nitrate (fig. 14A). Estimated nitrate loss in the zone from 10 to 14 m offshore and depths of 30 to 122 cm in the subterranean estuary of transect B–B′ ranged from 54 to 275 µmol/L of nitrate (fig. 14B). Four of the 12 samples collected from these locations in transect B–B′ had detectable nitrate at concentrations ranging from 58 to 430 µmol/L

(fig. 8E). We cannot rule out the possibility that some fraction of the excess nitrogen gas was derived from anammox rather than denitrification. Almost all samples collected from transect D–D′ in June 2016 had estimated nitrate loss of less than 50 µmol/L (fig. 14C). Where estimated nitrate loss was less than 50 µmol/L (generally within about 8 m of the shoreline position at mean high water), there may have been some nitrate reduction but the concentrations of N2-NMIC were within the estimated combined uncertainties of the data and assumptions. A limited number of analyses in this study in transect C–C ′ showed evidence of partial to complete nitrate removal at 178 cm below the sediment/water interface in the subtidal zone from 2.5 to 14 m offshore. The nitrate concentrations in these samples were 66, 138, and 0 µmol/L at 2.75, 8.25, and 13.75 m from shore, respectively. Colman and others (2018) measured partial nitrate removal of 71 µmol/L and nitrate concentration of 346 µmol/L in a similar location to the 178-cm depth of transect C–C ′ at 152-cm depth and 9 m offshore (fig. 3). Dissolved oxygen concentrations generally were low (less than 25 µmol/L as O2) in the regions of the highest nitrate loss (figs. 7B and 8B), which is consistent with an environment suitable for denitrification activity. Alkalinity was elevated relative to conservative mixing of fresh and saline endmembers in some areas of high nitrate loss (figs. 7D and 8D), which could be consistent with potential denitrification activity because alkalinity is a biproduct of microbial oxidation of organic matter; however, the data are incomplete and appear to be somewhat inconsistent with simple reaction stoichiometries. In relatively saline groundwater from the shallowest sampled depths in the subterranean estuary (30 cm) and from 9 to 13.5 m offshore, ammonium concentrations (figs. 7F, 8F, and 9F) were elevated in comparison to those of either fresh groundwater or overlying saline surface water, based on ammonium measurements in the Eel River reported by Colman and others (2018). Elevated ammonium concentrations in saline groundwater likely were generated by anaerobic biodegradation of sedimentary organic matter. However, some mixed-salinity samples with detectable N2MIC appeared to be relatively depleted in ammonium when compared with conservative mixtures of fresh and saline groundwater endmembers, possibly indicating ammonium loss (for example, anammox) or sediment-water exchange processes. DOC concentrations were relatively low in fresh groundwaters that had high initial nitrate concentrations, including some that were denitrified and some that were not denitrified. This could be viewed as evidence for the importance of electron donors from solid phases, rather than aqueous species, as controls of localized nitrate reduction (as in the subtidal area of transect A–A′ (figs. 7H and 14A). Hydraulic gradients and vertical profiles of SpC indicate that fresh groundwater originating from recharge under the peninsula migrates towards the estuary as depicted in the conceptual dashed yellow lines in figure 3. Figure 3 is modified from the earlier study (Colman and others, 2018) to illustrate our current understanding of groundwater flow in this subterranean estuary and how that compares to the

Patterns and Controls of Nitrogen Transport and Attenuation 35 earlier conceptual model shown in figure 2. In two of the three transects in the current study (A–A′ and B–B′), nitrate concentrations in the near-shore intertidal zone generally were relatively low initially and may have been partially diluted by mixing with saltwater; this nitrate was minimally or not denitrified and some of it may have been discharging to the estuary. In the third transect (D–D′), nitrate concentrations were substantially lower. The nitrate in the intertidal zones of these transects was minimally or not denitrified and some of it may have been discharging to the estuary. Slightly farther offshore in a narrow region from 5 to 6 m from shore, generally there was little evidence of mixing with infiltrating saltwater and low nitrate concentrations and minimal evidence of denitrification; small amounts of nitrate may have been discharging to the estuary in this zone. Farther from shore

in the subtidal area from about 7 to 12.5 m offshore, salty porewater overlays the freshwater plume, indicating no freshwater discharge directly to the estuary and possible horizontal offshore movement of groundwater. Groundwater in this freshwater zone was depleted in nitrate and this is consistent with evidence for nearly complete nitrate reduction. Farther offshore, Colman and others (2018) reported fresh groundwater with relatively high nitrate concentrations from about 1.5 to 11 m below the sediment/water interface, but data were not available at shallower depths to indicate whether mixing, reaction, or discharge might have occurred there (fig. 3). Thus, there appears to be a large body of fresh groundwater containing substantial nitrate underlying the estuary farther offshore than our measurements, but there are insufficient data to determine where or how it discharges.

Altitude, in meters from NGVD 29

A. MA-FSW 777 A01–A12 (transect A) EAST

WEST

1.0

A01

Eel River

A02

0.5

A03

−0.5

A04

A06

−1.0

A05

A07

MHT MLT

A08

0.5

A10

A11

A12

A09

−1.5

2 2

−0.5 −1.0 −1.5

Salinity, in practical salinity units 29 20 10 0

Sampling point with label 2

Salinity

−2.0

Altitude, in meters from NGVD 29

EXPLANATION

1.0

A01

Eel River

A02

0.5 0

15 NM

<50

A03

A04

98 <50 216 NM

223 NM

A05

A06

A07

−1.0 −1.5

0.5

MLT

A08

<50 0 NM 84 <50 0 <50 0 60 32 <50 261 <50 403 <50 460 NM 512 <50 538 <50 146 <50 577 <50 481 <50 434 <50 482 NM 581 <50 590 <50 577 479 <50 <50 545 <50 602 <50 576 <50 553 <50

−0.5

MHT

A09

261

87 0 0 230 0 0 262 0

A10

A11

57 0 312 0 308 0

0 356 17 434 0

0 A12

57 0 357 0

200

331 0 409 0

342

−0.5

383

−1.0 −1.5

419

−2.0

−2.0 −1

Distance from reference point on shoreline, in meters VERTICAL EXAGGERATION ×2

Mean high tide (MHT) and mean low tide (MLT) determined from tide stage data from July 13 to September 8, 2016

EXPLANATION Nitrate plus nitrite, in micromoles per liter 600 500 400 300 200 100 0

Sampling point with labels Measured nitrate concentration

261

312

Estimated nitrate loss

Nitrate concentration and estimated nitrate loss in micromoles per liter; NM, not measured.

Figure 14. Vertical transects showing distribution of salinity and measured nitrate plus nitrite in groundwater including estimated nitrate loss at selected points, along A, northernmost transect A–A´ (MA–FSW 777, A01–A12, June 2016), B, middle transect B–B´ (MA–FSW 733—MA–FSW 747, November 2015), and C, southernmost transect D–D´ (MA–FSW 776, A01–A12, June 2016) beneath the Eel River, Seacoast Shores peninsula, East Falmouth, Massachusetts. Locations of transects are shown in figure 4.

36 Evidence of Nitrate Attenuation in Intertidal and Subtidal Groundwater in a Subterranean Estuary at a Cape Cod Embayment B. MA-FSW 733 MA-FSW 747 (transect B)

Altitude, in meters from NGVD 29

1.0

B' F741

1.0

F740

0.5

−0.5

F739

F738

F737

−1.0

F736

EXPLANATION

WEST

F735

Eel River

MHT

F734

MLT

F733

F747

22 11

−1.5

F746

16 22

0.5

−0.5

F745

Salinity, in practical salinity units

−1.0 −1.5

Sampling point with label 2

Salinity

−2.0

EXPLANATION Altitude, in meters from NGVD 29

1.0 0.5 0 −0.5 −1.0 −1.5

F741

1.0

F740

91 <50

Eel River

F739 F738

70 <50

MHT

F737

<50 F736 F735 F734 114 <50 171 28 <50 F733 <50 381 <50 203 <50 299 454 <50 139 21 39 <50 86 11 <50 0 <50 0 66 <50 396 105 <50 433 <50 536 72 <50 84 <50 <50 122 <50 125 <50 406 <50 403 73 <50 73 <50 96 <50 146 73 124 <50 111 <50 81 <50 109 <50 121 <50 130 <50

0.5

MLT F747

107 191 58 225 0 172

307 <50

F746

F745

0 137

125

0 275 0 148

430

−0.5 −1.0

174 120

−1.5

177

−2.0

−2.0 −1

Distance from reference point on shoreline, in meters VERTICAL EXAGGERATION ×2 Mean high tide (MHT) and mean low tide (MLT) determined from tide stage data from July 13 to September 8, 2016

Figure 14.—Continued

Nitrate plus nitrite, in micromoles per liter 600 500 400 300 200 100 0

Sampling point with labels Measured nitrate concentration

261

312

Estimated nitrate loss

Nitrate concentration and estimated nitrate loss in micromoles per liter; NM, not measured.

Summary 37

Altitude, in meters from NGVD 29

C. MA-FSW 776 A01–A12 (transect D) EAST

WEST

1.0

A01

0.5

5 1

1.0

−0.5

Eel River

A02

A03

A04

A05

−1.0

A07

MLT

A06

MHT

A08 A09

A10

−1.5

A11

−0.5

0.5

A12

EXPLANATION Salinity, in practical salinity units

−1.0 −1.5

Sampling point with label 2

Salinity

−2.0

Altitude, in meters from NGVD 29

EXPLANATION A01

1.0 0.5 0

Eel River

A02

MHT

<50

A09 6 <50 NM A10 94 <50 A11 A12 0 <50 15 NM 89 107 0 <50 0 <50 <50 0 <50 45 <50 119 NM 24 <50 0 62 22 184 16 NM 70 <50 0 <50 0 344 78 <50 0 NM 46 <50 105 <50 32 <50 0 <50 0 431 <50 0 NM 60 376 <50 0 <50 0 <50 61 <50 103 <50 0 <50 0 NM 318 NM 385 <50 59 133 0 NM 0 NM 0 NM 0 <50 16 <50

−0.5

1.0

−1.0 −1.5

<50

A03

A04

<50

A05

A06

A07

0.5

MLT

A08

0 −0.5 −1.0 −1.5 −2.0

−2.0 −1

Distance from reference point on shoreline, in meters VERTICAL EXAGGERATION ×2 Mean high tide (MHT) and mean low tide (MLT) determined from tide stage data from July 13 to September 8, 2016

Nitrate plus nitrite, in micromoles per liter 600 500 400 300 200 100 0

Sampling point with labels Measured nitrate concentration

261

312

Estimated nitrate loss

Nitrate concentration and estimated nitrate loss in micromoles per liter; NM, not measured.

Figure 14.—Continued

Summary This study was conducted to follow up on a previous study (Colman and others, 2018, https://doi.org/10.3133/ sir20185095) that investigated the fate and transformations of nitrogen in the groundwater below the Seacoast Shores peninsula. The previous study found evidence of nitrate reduction near the water table in fresh groundwater onshore and near the freshwater/saltwater transition zone beneath a 7–8 meter (m)-thick freshwater zone in the subterranean aquifer. That study documented variable concentrations of nitrate in fresh and brackish shallow groundwater beneath intertidal and subtidal portions of the estuary but did not assess the distribution of nitrate reduction in those areas. The current study focused on evidence for the distribution of nitrate reduction in shallow groundwater from 0 to 1.5 m beneath the intertidal and subtidal portions of the estuary. The distributions of fresh and saline groundwater in the subtidal area of the subterranean estuary under the Eel River are consistent with shallow intertidal and subtidal saltwater cells where infiltrating saline and underlying freshwater mix.

However, these distributions do not conform to the simplified conceptual model of groundwater flow in the subterranean estuary at Waquoit Bay, East Falmouth, Massachusetts, where a deep saltwater wedge discharges to the estuary. At the location of this study in the Eel River subterranean estuary, to a distance offshore of at least 18 m, lateral subsurface fresh groundwater flow apparently prevented a deep saltwater wedge from discharging to the surface near the shoreline (Colman and others, 2018). Studies elsewhere have documented similar offshore flow of fresh groundwater lenses beneath estuaries, in some cases extending hundreds of meters offshore (for example, Bratton and others, 2004, 2009; Russoniello and others, 2013). The distribution of salinity determined in this study, and in Colman and others (2018), although variable over time, indicates a nearshore intertidal cell and a subtidal saltwater cell where rising fresh groundwater mixes with infiltrating saline water, and an intermediate zone in the intertidal area closer to mean low water where freshwater discharges without mixing with saline water. Specific conductance data were collected to about 18 m offshore in the Colman and others (2018) study and to

38 Evidence of Nitrate Attenuation in Intertidal and Subtidal Groundwater in a Subterranean Estuary at a Cape Cod Embayment about 13.5 m in this study, and both studies seem to indicate fresh groundwater discharges into the estuary farther offshore beyond the shallow subtidal saltwater cell. Hydraulic-head gradients measured 2 to 5 m below the sediment/water interface in October 2016 indicated that fresh groundwater flow generally was upwards (towards the estuary) under all tide conditions, providing a mechanism for transport of nitrate from deeper nitrate-rich groundwater (that was documented in Colman and others, 2018) into the shallower subestuarine sediments where fresh and saline porewaters mixed. Nitrate-bearing fresh and brackish groundwater in some of the shallowest samples (less than 61 centimeters below the sediment/water interface) might indicate direct discharge of groundwater nitrate to the estuary in portions of the intertidal and subtidal zones. In other areas, fresh and brackish groundwater was overlain by nitrate-free saline water, indicating little or no direct local nitrate discharge. Variability of nitrate concentrations in shallow intertidal and subtidal groundwater was related to a combination of (1) variable original nitrate concentrations in fresh groundwater recharge, (2) dilution by mixing with nitrate-free saline water recharged beneath the estuary, and (3) nitrate reduction along groundwater flow paths. The primary geochemical indicator of nitrate reduction (excess nitrogen gas) indicated only minimal attenuation of nitrate by microbial denitrification or anammox in the intertidal zone. Other nitrate reduction processes such as dissimilatory nitrate reduction to ammonium and assimilation were not evaluated quantitatively, in part because they involve interactions with solid phases that were not sampled. There was no aqueous evidence for substantial ammonium production in mixed-salinity samples, and the distributions of initial nitrate, nitrate remaining, and concentration of microbially produced nitrogen gas, as N2 (N2MIC) could reasonably be explained as a result of mixing and local denitrification (± anaerobic ammonium oxidation [anammox]) of freshwater sources of nitrate. In the subtidal zone, data were consistent with complete removal of nitrate by denitrification in fresh groundwater in one of the transects measured in June 2016. Anammox was not measured directly and it cannot be ruled out as a potential microbial source for some of the excess nitrogen gas. Excess nitrogen gas, determined from analysis of dissolved gases (nitrogen gas and argon gas), indicated that microbial conversion of nitrate to nitrogen gas was likely occurring in the intertidal zone and especially in the subtidal zones. The estimated amounts of nitrate loss were less than 50 to 139 micromoles per liter (µmol/L) in the intertidal zone and less than 50 to 434 µmol/L in the subtidal zone. Estimated amounts of nitrate reduction were highest in subtidal area greater than 10 m from the shore in one of two of the transects sampled in June 2016 (A–A′), in one transect sampled in November of 2015 (B–B′), and in another sampled in October 2016 (C–C ′). In the highest salinity zones, there may be weak evidence for minor nitrate reduction (24 ± 20 standard deviation µmol/L, assuming no excess air). Although these values are considered to be within the uncertainties of the calculations, they could potentially indicate

a minor nitrate source in estuarine surface water or estuarine sediment porewaters, possibly associated with coupled nitrification-denitrification near the sediment/water interface. The primary zone where evidence for nitrate reduction was detected was in subtidal fresh groundwater that was overlain by a thin layer of saline and mixed groundwater. The fact that geochemical evidence for nitrate reduction was less pronounced in other areas of the subterranean estuary that were suboxic but rich in nitrate may indicate more reactive sources of electron donors in shallow subtidal sediments than in shallow intertidal sediments. Fresh groundwater with high initial nitrate concentrations had relatively low dissolved organic carbon (DOC) concentrations in areas with and without N2MIC, indicating that wastewater DOC may have been depleted by microbial reactions before reaching the shallow subterranean estuary. The concentration and chemical nature of DOC also are important variables that can limit nitrate reduction if the concentration is too low or if the carbon is resistant to decomposition (Devito and others, 2000; Rivett and others, 2008). The transport of nutrient-rich groundwater through a hydrogeologically complex subterranean estuary can be influenced by variable transmissivity due to the occurrence of heterogeneous confining layers that can restrict flow (Mulligan and others, 2007; Bokuniewicz and others, 2008; Michael and others, 2011; Russoniello and others, 2013), tidal and current dynamics (De Simoni and others, 2005; Rezaei and others, 2005) that affect mixing between fresh groundwater discharge and saltwater infiltration, and the biogeochemical processes that can attenuate nutrient transport. Previous geologic borings on the Seacoast Shores peninsula encountered fine sands and silts directly upgradient of the study area (Colman and others, 2018). Natural heterogeneities of the subsurface lithology combined with tidal and hydrologic effects, therefore, make it difficult to rely on a single conceptual model that describes groundwater flow in these complex subterranean estuaries in Cape Cod embayments. Further study could better quantify the seasonal, spatial, and tidal variation in excess nitrogen gas in the subterranean estuary adjacent to the Seacoast Shores peninsula, because large variations were observed in this and the previous study. Densely spaced vertical and longitudinal water-level measurements and sampling of groundwater along potential or suspected flow paths from septic sources to the nearshore subterranean estuary could be used to follow nitrate along these logical flow paths to better discern where transformations may occur. This information could be supplemented with isotopic and conservative tracers to confirm suspected flow paths and timing of transport. Further studies using molecular techniques (environmental DNA) or isotopic enrichment experiments could also be employed to determine the relative importance of denitrification, anammox, DNRA, or other nitrate attenuation processes. The ultimate fate of deep fresh groundwater and associated nitrate prior to discharge might be determined by extending such studies farther offshore and possibly all the way across the Eel River.

References Cited 39

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For more information about this report, contact: Director, New England Water Science Center U.S. Geological Survey 10 Bearfoot Road Northborough, MA 01532 dc_[email protected] or visit our website at https://www.usgs.gov/centers/new-england-water Publishing support provided by the Pembroke and Reston Publishing Service Centers

Huntington and others—Evidence of Nitrate Attenuation in Intertidal and Subtidal Groundwater in a Subterranean Estuary at a Cape Cod Embayment—SIR 2024–5100

ISSN 2328-0328 (online) https://doi.org/10.3133/sir20245100

Evidence of Nitrate Attenuation in Intertidal and Subtidal Groundwater in a Subterranean Estuary at a Cape Cod Embayment, East Falmouth, Massachusetts, 2015–16

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