1

Biogeochemistry of seepage face sediments down-gradient from NITREXTM

permeable reactive barriers

Maryann Tekverk, Haverford College, Haverford, PA

Collaborator: Kenly Hiller, Connecticut College, New London, CT

Advisor: Dr. Ken Foreman, Marine Biological Laboratories, Woods Hole, MA

2

Abstract

Nitrate loading is an increasing problem in Woods Hole and the Upper Cape, caused by

sewage and groundwater seepage into the areas streams, lakes and estuaries. One

possible solution to this problem is the installation of NITREXTM Permeable Reactive

Barriers at freshwater-saltwater interface zones throughout the Cape. These barriers are

known to remove nitrate from groundwater more effectively than the beach, but the

mechanisms of nitrate removal taking place within these barriers are not known. In

addition, these barriers may remove nitrate down-gradient from themselves, in

groundwater that has not passed through them, by creating a down-gradient environment

where microbes can more effectively denitrify. We investigated the nitrate removal

processes at the NITREXTM barriers by collecting well-point water and sediment core

samples down-gradient from a NITREXTM barrier at Waquoit Bay, western

Massachusetts. We tested these samples for NO3, NH3, PO4, S2-, SO4, N2, and Fe

concentrations. Data indicates that the more nitrate is removed at the barriers than at an

equivalent control site. Average nitrate concentration down-gradient from the barrier was

0.1 uM and at the control site it was ~30 uM. In addition, sulfide, iron, and phosphate

concentrations were higher down-gradient from the barrier, indicating dissimilatory

nitrate reduction. Anammox may also occur down-gradient from the barrier (???)

Keywords: denitrification, freshwater-saltwater interface, NITREXTM Permeable

Reactive Barriers, sewage remediation

Introduction

Nutrient Loading in Waquoit Bay

Nitrogen in the form of nitrate is found in increasing concentrations in Falmouth’s

groundwater from wastewater and fertilizer inputs. About 75% of the nitrogen that enters

the watershed is removed before reaching the ocean (Burgin and Hamilton 2007). The

key interfaces for this groundwater nitrate removal are large rivers, lakes, estuaries, and

marine ecosystems (Burgin and Hamilton 2007). When excess nitrate is introduced to

these interfaces, it degrades the quality of the water found there, sometimes causing

eutrophication in the form of harmful algal blooms, which can lead to fish kill and

ecosystem instability.

In order to address this problem, alternative sewage treatment systems have been

proposed, but these are very expensive and would take an unreasonable amount of time

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and money to put in place. In addition, the effects of these alternative sewage treatment

systems would not be seen for years. As a possible cheaper more direct means of

denitrification, wood chip barriers have been installed at Waquoit Bay. The barriers are

10-20 m long trenches 2-4 m wide and 2 m deep, filled with a combination of wood chips

and limestone (Vallino and Foreman 2008). Wood chips in the barriers provide a

substrate for micro-organisms, as well as a carbon source. In this way the barriers

enhance denitrification, decreasing the nitrate load entering Waquoit Bay. These barriers

produce an anoxic plume that is more depleted in nitrate than concentrations observed at

an equivalent control site without a barrier (Vallino and Foreman 2008).

In addition to denitrification within the barrier, the barriers may create an

environment down-gradient from the barrier which allows for accelerated denitrification.

If conditions for increased denitrification are created down-gradient from the barrier at

the groundwater-saltwater interface the implications are that shallow barriers could be

installed at seepage faces throughout Falmouth as a less expensive more direct means of

denitrification than alternative sewage treatment systems. If shallow barriers create

conditions down-gradient which enhance denitrification, than groundwater may be

denitrified at the NITREXTM permeable reactive barriers without having to pass directly

through them. Instead, groundwater that passes under the barriers but is exposed to the

down-gradient seepage face environment created by the barriers will undergo the same

processes as water that passes directly through the barriers. Little work has been done on

the seepage face at Waquoit Bay to determine whether denitrification occurs there or to

examine the specific processes of denitrification that occur there. Water samples and

sediment cores were collected and analyzed for relevant chemical constituents to assess

4

the types of denitrification occurring down-gradient from an experimental NITREXTM

permeable reactive barrier at Waquoit Bay.

Denitrification Mechanisms

Denitrification is has been shown to occur at the NITREXTM barriers, but the

specific denitrification processes occurring are not known (Vallino and Foreman 2008).

Multiple processes of denitrification are possible, depending on the micro-organisms

present at the barriers, the environmental conditions (oxic or anoxic), and the available

energy sources. The most common process taking place within the barriers is probably

respiratory denitrification. In this process microbes use nitrate as a terminal electron

acceptor, oxidizing organic matter to CO2 under anaerobic conditions (eq. 1). Organic

matter is the energy source.

OHOHNCONOOCH 432545 22232 Eq. 1

Microbially mediated dissimilatory nitrate reduction to ammonium (DNRA) may

also occur at the barriers, resulting in higher concentrations of ammonium, which is more

biologically available than nitrate. Because seawater infiltrates the NITREX barriers,

sulfate is introduced. This sulfate undergoes sulfide reduction, producing sulfide that can

be used as an energy source in DNRA (eq. 2). Sulfide is used as an electron acceptor in

the DNRA process, producing N2 and sulfate (SO4) (eq. 3).

OHSHCOSOOCHH 22242 2222 (reduced sulfur oxidation) Eq. 2

OHOHNHCONOOCH 434545 23232 (DNRA) Eq. 3

This process is chemolithoautotrophic, coupling nitrate reduction to reduced sulfur

oxidation. Free sulfide inhibits the final two reduction steps in this process, so sulfate

5

reduction to NH4 rather than N2 or N2O may occur if sulfide concentrations are too high

(Burgin and Hamilton 2007).

Iron and manganese may also be used as energy sources in denitrification,

resulting in iron oxides and manganese oxides at the fresh-salt water interface (eq. 4 and

5). Specifically, Fe2+ is converted to Fe3+ in this process and by a similar process Mn4+ is

converted to Mn2+. Thus, if excess Fe2+ is readily available at the seepage-face, iron-

driven denitrification can occur.

eFeFe 32

(iron reduction) eq. 4

eMnMn 242 (manganese reduction) eq. 5

ANNOMOX (Anaerobic ammonium oxidation) may occur at the barriers as well,

biologically mediated by microbes combining nitrite and ammonium and forming N2.

This process can be represented by eq. 6 (Shivaraman and Shivaraman 2003).

ANNAMOX is the only process which may form N2 from both nitrate and ammonium.

OHNNONH 2224 2

Eq. 6

In order to assess the processes of denitrification that are occurring at the Waquoit

Bay barrier, I tested well-point water samples for NH4, NO3, PO4, S=, H2S, total Fe,

Cl:SO4=, sulfate depletion, and N2:Argon. In addition, I collected particulate sediment

sample cores and analyzed them for sulfides and total iron.

Methods

Sampling Site Description

The Waquoit Bay National Estuarine Research Reserve is an 825 acre bay in

eastern, Massachusetts. Groundwater seeps into the bay when the tide is low. At the

experimental site, this groundwater passes through or under a NITREXTM Permeable

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Reactive Barrier. The barrier is a trench 10-20 m long and 2-4 m wide and 2 m deep,

filled with a combination of wood chips and limestone (Vallino and Foreman 2008).

Groundwater flows from a bluff through the barrier and then through the sand seepage

face and into the bay. At the equivalent control site (16-44 meters down the beach from

the barrier) groundwater flows from the bluff through the sand seepage face and into the

bay (Fig. 1).

Wellpoint Installation

I collected samples from two sites at Waquoit Bay: a control site and a barrier

site. I collected samples along 2 transects down-gradient from the barrier and two

transects at equivalent locations at the control site. One transect at each site was eight

meters from the bluff and one transect was eleven meters from the bluff near the low tide

line (Fig. 1). Along each transect I collected 5 water samples at 60 cm depth and 5

samples at 110 cm depth using a 1.0 cm diameter stainless steel well-point and a slide

hammer. A closed tubing system was attached to the end of the well-point and I

attempted to introduce as little air as possible into the water samples collected. I collected

2 water samples at each depth in two BOD bottles at each wellpoint, collecting a total of

70 samples. I added 25 uL of HgCL to one BOD at each site and stored it in a cooler of

cold water so that no air exchange took place. These bottles were used for N2/Ar

analysis.

Sample Processing

I sub-sampled for sulfide, nitrate, ammonia, phosphate, iron, and, and SO4:Cl

ratio from the same BOD bottle. For each sulfide sample, I pipetted 25 uL of sample and

225 uL of DI water to one scint vial with 6 mL of cold Zinc Acetate (ZnAc) already in it

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and 250 uL of sample to another scint vial with 6 mL o fcold ZnAc. I pipetted 5 mL of

sample and 5 mL of DI water to a scint vial for SO4:Cl ratios and bubbled each sample

with N2 for about ten minutes to react all of the sulfides. The rest of each BOD bottle

was filtered and divided into four separate scint vials for nitrate, phosphate, ammonium,

and iron analysis. Nitrate samples were frozen and phosphate, ammonium, and iron

samples were acidified with 15 uL of trace metal grade 6 N HCl.

Colorometric Analysis – Sulfide, Nitrate, Ammonia, Phosphate

Frozen nitrate samples were thawed and analyzed on a Latchat Flow Injection

Analyzer using a method adapted from Wood et al. (1967). I analyzed ammonium,

phosphate, and sulfide concentration using a spectrophotometer for colorometric analysis.

For ammonium, I used methods adapted from Strickland and Parsons (1972). For

phosphate I used methods adapted from Murphy and Riley (1962). To determine sulfide

concentration in groundwater I used methods adapted from Gilboa-Garber (1971).

Sulfate:Chloride

SO4:Cl ratio was determined using a Dionex DX-120 Ion Chromatograph.

Total Iron

Total iron concentration was determined in groundwater using an atomic

absorption spectrometer and methods adapted from Forstner and Salomons (1980).

N2:Argon

N2:Ar ratio was determined using a mass spectrometer.

Calculating NO3 Accounted for by Denitrification

To look at denitrification, I took an average nitrogen concentration and an average

argon concentration at the barrier and the control site along both transects. I subtracted

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the N2 concentration at the control site from that at the barrier to calculate the amount of

nitrogen present from barrier denitrification. I took this number and corrected it using the

excess argon value. From this calculation I got the Average excess N2 from barrier

denitrification which was 17.6 uM. I multiplied this by 2 because of the stoichiometric

relationship between N2 and nitrate.

Sediment Core Collection and Processing

I collected three samples of sediment from each transect using a corer and

subsampled each core in five intervals, from 0-2 cm, 2-4 cm, 4-6 cm, 6-10 cm, and 10-14

cm. I homogenized the samples from each depth interval (total = 30 samples) and used

them to determine the abundance of Fe and S2-.

Sediment Sulfides

To determine the abundance of sulfide in sediment samples, I added 1 mL of sediment

from each subcore to 6 ml of cold ZnAc. After the addition of diamine these samples

were centrifuged and I used methods adapted from Gilboa-Garber to determine sulfide

concentration in sediment (1971).

Iron in sediment

After homogenizing each subcore, I dried each sediment sample for 2 days at 60ºC. I

used ~5 g of sediment from each sample and performed acid digestions using methods

adapted from Forstner and Salomons (1980). Total iron concentration in sediment was

determined using an atomic absorption spectrometer and methods adapted from Forstner

and Salomons (1980).

Results

Anaerobic Barrier Environment

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Sulfide abundance in sediment was much higher down-gradient from the barrier

than it was at the control site (Fig. 2). At the control site sulfide abundance ranged

between 0.04 and 0.1 ug/g. At the barrier sulfide abundance decreased with depth in the

core, increasing from ~ 0.1 ug/g to 1.6 ug/g at 12 cm deep. Sulfide concentrations in

groundwater were slightly higher down-gradient from the barrier, ranging between 0.01

and 0.09 uM (Fig. 3). Iron abundance in sediment was similar at the control and the

barrier site (Fig. 4) ranging between 2 and 9 mg/g at the barrier and 2 and 12 mg/g at the

control site. At both sites, iron concentration decreased with core depth. Iron

concentrations in groundwater were also higher down-gradient from the barrier, ranging

between 0.2 and 2 mg/L at the barrier site and never reaching higher than 0.05 mg/L at

the control site (Fig. 5). Phosphate concentrations were consistently higher down-

gradient from the barrier than at the control site (Fig. 6). This trend was much more clear

along the 11 m transect than along the 8 m transect.

Comparison of sediment sulfide, dissolved iron, and groundwater phosphate

concentrations shows that higher iron concentrations correspond to higher phosphate

concentrations, but that higher iron concentrations do not necessarily correspond to

higher sulfide concentrations. In sediment, iron showed a trend opposite to that of

sulfide, decreasing with depth while sulfide increased.

Evidence for Denitrification

Nitrate concentrations were consistently lower down-gradient from the barrier

than at the control site, varying between 0.4 and 1.1 uM (Fig.7). At the control site,

along both transects, nitrate concentrations varied widely, decreasing drastically between

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32 to 44 m away from distance zero along both transects. Nitrate concentrations at the

control site ranged between 90 uM and 0.7 uM, consistently decreasing toward 44 m.

Ammonium concentrations showed no clear trend, varying between 0 and 16 uM,

with a 16 uM peak in ammonium at 40 m which showed up in both 8 m transect and the

11 m transect (Fig. 8).

Raw N2 concentration at the barrier was slightly higher on average than N2

concentration at the control, whereas argon concentration at the barrier did not differ

from argon concentration at the control (Table 1). Based on averaged concentrations at

the control and barrier sites, there is enough N2 on average at the barrier to account for 37

uM of NO3.

SO4:Cl

The sulfate:chloride ratio was consistent at the control and barrier site, remaining

around 0.05, except at two points which may have been contaminated (Fig. 9). Sulfate

depletion showed no trends, and at about half the sample sites sulfate depletion was

negative, meaning that there was excess sulfate compared to what would be expected.

Discussion

Evidence for Sulfate Reduction and an Anaerobic Barrier Environment

There is much more dissolved sulfide and sediment sulfide present at the barrier

site than at the control site, indicating that sulfate reduction occurs at the barriers,

producing this excess sulfide. This sulfide also indicates that the seepage face down-

gradient from the barrier is an anaerobic environment. This has implications for

denitrification as well as benthic infauna, which are sensitive to high concentrations of

sulfide as well as anaerobic conditions.

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Iron concentrations in the sediment are about the same at both sites, but iron

concentrations are much higher in groundwater at the barrier, because iron at the barrier

is in the form Fe2+ and the iron at the control is Fe3+. Only Fe2+ is soluble in water so

iron in groundwater is only present at the anaerobic barrier site. Because of the anaerobic

environment down-gradient from the barrier, phosphate concentrations are also higher at

the barrier site. At the barrier site phosphate is released from iron-oxides due to

anaerobic conditions and it is mobilized in the groundwater. At the control site this

phosphate is sorbed to the surface of iron oxides.

Because sulfate reduction occurs in the groundwater and the sediment at the

barrier seepage face, and high concentrations of iron and sulfide down-gradient from the

barrier are available, the environment down-gradient from the barrier is ideal for iron-

driven denitrification as well as sulfur-driven nitrate reduction. Based on iron and

sulfides alone, lower nitrate levels are expected at the barrier seepage face than at the

control site.

Evidence for Denitrification

Previous studies have found that nitrate concentrations from groundwater coming

out of the barrier are much lower than nitrate concentrations from groundwater at an

equivalent control site (Vallino and Foreman 2008). Water samples collected down-

gradient from the barrier at the seepage face echo this trend, indicating that nitrate

loading to the estuary is less significant at the barrier site than at the control site.

Water collected at the control site decreased in nitrate concentration with distance from

zero. This may be the result of a nitrate plume which is oriented diagonally along the

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beach. Nitrate concentration also peaks at the far end of the barrier site (at the zero

mark), indicating the end of the barrier’s influence.

Based on the results of N2:Argon ratios at the control and barrier sites, there is

enough N2 on average at the barrier site to account for denitrification of 37 uM of NO3.

The difference between the NO3 value at the control and that at the barrier was 28 uM,

indicating that if nitrate addition was equal across the beach, excess N2 at the barrier

should only account for these 28 uM of NO3. However, we know from Figure 7 that the

nitrate plume at Waquoit Bay is not even across the beach. Nitrate loading may be

elevated at the barrier site in comparison to the control site, leading to a higher N2

concentration than that predicted from the control site. That said, when you consider all

the variables involved in predicting denitrification, these numbers are in pretty good

agreement. Although this study is not detailed enough to quantify denitrification, the

data indicates that the differences seen in N2 and NO3 at the barrier seepage face are

caused by denitrification.

In order to address this denitrification question more effectively it would be useful

to set up an experiment examining the nitrate plume before it reaches the barrier, at the

barrier and below the barrier, so that denitrification could be quantified more accurately.

In addition it would be useful to analyze the N2 concentrations found in fresh

groundwater and those found in seawater because the seepage face has a mix of these two

end members.

SO4:Cl

From observation, it is clear that iron sulfides are present at the barrier in much

higher abundances than at the control site. In addition, iron was present in the acid

13

digestion products from the barrier and the control site, and sulfide abundance was higher

at the barrier site. The SO4:Cl data should also indicate that sulfate reduction occurs at

the barrier, through sulfate depletion, but this is not the case. Calculated sulfate depletion

was extremely variable at the control and the barrier sites, and indicated no trends.

Instead at many of the wellpoint sited sulfate was higher than expected from chloride

concentrations, rather than being depleted. Perhaps there is another sulfate source at

Waquoit bay, such as gypsum, which provides so much sulfate that the system is

saturated with respect to chloride. If this is the case then SO4:Cl will not be a useful

indicator of sulfate reduction.

Conclusions

Lower nitrate concentrations are present in groundwater down-gradient from the

barrier, meaning that differences seen in N2 and NO3 at the barrier are caused by

denitrification in the seepage face down-gradient from the barrier as well as within the

barrier. This down-gradient denitrification indicates that the barrier may create an

environment which stimulates denitrification in waters that are never in direct contact

with the barrier. Instead, groundwater rich in nitrate passes through the seepage face and

is denitrified through iron-driven denitrification and sulfur-driven DNRA. This means

that perhaps the installation of shallow barriers could be used as a method of sewage

remediation for a large volume of groundwater, not all of which would have to flow

directly through the barrier. This would be cheaper and easier than installing deeper

barriers which would interact directly with all of the groundwater passing into estuaries

in the Upper Cape.

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More research and a longer-term project are required before it can be determined

certainly that the NITREXTM Permeable Reactive Barriers are the right sewage

remediation plan for the Upper Cape, but this study suggests that denitrification in the

seepage dace at Waquoit Bay is directly attributable to the experimental barrier there.

Future research must include longer-term seasonal monitoring, up-gradient nitrate plume

data, and a study of the impacts of the barriers on benthic infauna and trophic

relationships within the estuary.

Acknowledgements

I thank my advisor Ken Foreman, my collaborator Kenly Hiller, Rich McHorney, Will

Daniels, and Maya Wei-Hass for help in the lab and the field, Jane Tucker for running my

N2/Argon samples, and Anne Giblin for helping to run my iron samples.

15

Figures and Tables

Figure 1: Study Sites

Bluff

Low Tide

Barrier Control8m

11m

4m16m

= wellpoint

= sediment core

16

Figure 7: Nitrate concentrations in groundwater at the 8 and 11 meter transects

Nitrate concentration in groundwater, 8 m transect

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

100.0

0 10 20 30 40 50

Distance from zero

[NO

3]

uM

60 cm deep

110 cm deep

Nitrate concentration in groundwater, 11 m transect

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

100.0

0 10 20 30 40 50

Distance from zero

[NO

3]

uM

60 cm deep

110 cm deep

17

Figure 8: Ammonium concentrations in groundwater at the 8 and 11 meter

transects

Ammonium concentration in groundwater, 8 m transect

0.00

2.00

4.00

6.00

8.00

10.00

12.00

14.00

16.00

18.00

0 10 20 30 40 50

Distance from zero

[NH

4+

] u

M

60 cm deep

110 cm deep

Ammonium concentration in groundwater, 11 m transect

0.00

2.00

4.00

6.00

8.00

10.00

12.00

14.00

16.00

18.00

0 10 20 30 40 50

Distance from zero

[NH

4+

] u

M

60 cm deep

110 cm deep

18

Figure 6: Phosphate concentrations in groundwater at the 8 and 11 meter transects

Phosphate in groundwater, 8 m transect

0.0

5.0

10.0

15.0

20.0

25.0

30.0

0 10 20 30 40 50

Distance from zero

[PO

4]

uM

60 cm deep

110 cm deep

Phosphate concentration in groundwater, 11 m transect

0.0

5.0

10.0

15.0

20.0

25.0

30.0

0 10 20 30 40 50

Distance from zero

[PO

4]

uM

60 cm deep

110 cm deep

19

Figure 5: Iron concentrations in groundwater at the 8 and 11 meter transects

Iron concentration in groundwater, 8 m transect

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

1.80

0 10 20 30 40 50

Distance from zero

[Fe

] (m

g/L

)

60 cm deep

110 cm deep

Iron concentration in groundwater, 11 m transect

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

1.80

0 10 20 30 40 50

Distance from zero

[Fe

] (m

g/L

)

60 cm deep

110 cm deep

20

Figure 3: Sulfide concentrations in groundwater at the 8 and 11 meter transects

Sulfide concentration in groundwater, 8 m transect

0.000

0.010

0.020

0.030

0.040

0.050

0.060

0.070

0.080

0.090

0.100

0 10 20 30 40 50

Distance from zero

[S2

-] u

M

60 cm deep

110 cm deep

Sulfide concentration in groundwater, 11 m transect

0.000

0.010

0.020

0.030

0.040

0.050

0.060

0.070

0.080

0.090

0.100

0 10 20 30 40 50

Distance from zero

[S2

-] u

M

60 cm deep

110 cm deep

21

Figure 4: Iron abundance in sediment at the control and barrier site

Iron abundance in sediment, control site

0

2

4

6

8

10

12

14

0.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00

Iron (mg/g)

De

pth

(cm

)

Control 1

Control 2

Control 3

Iron abundance in sediment, barrier site

0

2

4

6

8

10

12

14

0.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00

Iron (mg/g)

De

pth

(cm

)

Barrier 1

Barrier 2

Barrier 3

22

Figure 2: Sulfide abundance in sediment at the control and barrier site

Sulfide abundance in sediment, control site

0

2

4

6

8

10

12

14

0 0.5 1 1.5 2

[S2-] (ug/g)

De

pth

(cm

)

Control 1

Control 2

Control 3

Sulfide abundance in sediment, barrier site

0

2

4

6

8

10

12

14

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

[S2-] (ug/g)

De

pth

(cm

)

Barrier 1

Barrier 2

Barrier 3

23

Figure 9: Sulfate:Chloride ratio at the control and barrier site

SO4:Cl Ratio, 8 m transect

0.000

0.050

0.100

0.150

0.200

0.250

0.300

0.350

0.400

0 10 20 30 40 50

Distance from zero (m)

SO

4:C

l

60 cm deep

110 cm deep

SO4:Cl ratio, 11 m transect

0.000

0.100

0.200

0.300

0.400

0.500

0.600

0.700

0 10 20 30 40 50

Distance from zero (m)

SO

4:C

l

60 cm deep

110 cm deep

24

Table 1: Calculation of NO3 that can be accounted for by denitrification

Treatment Transect Average

N2 Average Argon

Difference in N2

(Barrier-Control)

Difference in Argon (Barrier- Control)

Excess N2 from

Air

Excess N2 from

Denitrification

Barrier 8 m 680.2 17.28 19.6 0.1 7.9 11.7

Control 8 m 660.6 17.18

Barrier 11 m 684.1 17.33 23.6 -0.02 0 23.6

Control 11 m 660.5 17.35

Average Control [NO3]: 28.5 µM

Average Barrier [NO3]: 0.7 µM

25

26

Figures and Tables

Figure 1 Study Sites

Figure 2 Sulfide abundance in sediment at the control and barrier site

Figure 3 Sulfide concentrations in groundwater at the 8 and 11 meter transects

Figure 4 Iron abundance in sediment at the control and barrier site

Figure 5 Iron concentrations in groundwater at the 8 and 11 meter transects

Figure 6 Phosphate concentrations in groundwater at the 8 and 11 meter

transects

Figure 7 Nitrate concentrations in groundwater at the 8 and 11 meter transects

Figure 8 Ammonium concentrations in groundwater at the 8 and 11 meter

transects

Figure 9 Sulfate:Chloride ratio at the control and barrier site

Table 1 Calculation of NO3 that can be accounted for by denitrification

27

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133.

Kroeger, K.D. and Charette, M.A. (2008). Nitrogen biogeochemistry of submarine

groundwater discharge. Limnology and Oceanography. 53(3): 1025-1039.

Murphy J. and Riley J.P. (1962) A modified single solution method for the determination

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Shivaraman, N. and Shivaraman, Geetha. (2003) Anammox – A novel microbial process for

ammonium removal. Current Science 84(125): 1507-1508.

Strickland, J.D.H. and T.R. Parsons. A practical handbook of Seawater Analysis 1972

Ottawa, Fisheries Research Board of Canada 2nd Ed.

Vallino, Joseph and Foreman, Ken. 2008. Effectiveness of Reactive Barriers for

Reducing N-Loading to the Coastal Zone. The NOAA/UNH Cooperative Institute for

Coastal and Estuarine Environmental Technology (CICEET). NOAA Grant Number(s)

NA04NOS190109.

Wood, E.D., F.A.G. Armstrong, and F.A. Richards. 1967. Determination of nitrate in

seawater by cadmium-copper reduction to nitrate. J.Mar. Biol. Assoc. U.K. 47:23.

Figure 1: Sample Sites at Waquoit Bay