Gra S300 No3study Poster

Evaluation of Sources and Fate of Ground Water Nitrate at a Semi-aridCatchment in the Central California Coast Ranges using Stable Isotopes

UCRL-POST-218111

Vic Madrid1, (madrid2@llnl.gov), H.R. Beller1, B. Esser2, G.B. Hudson2, M. Singleton2, W. McNab1, and S. Wankel3,Environmental Restoration Division1 Lawrence Livermore National Laboratory,

Chemical Biology and Nuclear Science Division2 Lawrence Livermore National Laboratory, P.O. Box 808, Livermore, CA, USA 94551-0808,Isotope Tracers of Hydrological and Biogeochemical Processes3, U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025

science for a changing world

LivermoreLivermore

TracyTracy

AlamedaCounty

SanJoaquinCounty

SanFrancisco

Site 300Site 300

NO

RTH

AlamedaCounty

Contra CostaCounty

SanFrancisco

Contra CostaCounty

SanJoaquinCounty

Scale: Miles

0 5 10

HE Process Area OUHE Process Area OU

NO

RTH

Site 300boundary

0 2,000 4,000Scale : Feet

Tnbs2

aquifer

We are conducting an interdisciplinary study to characterize the potential sources, distribution, and fate of nitrate in ground water at Lawrence Livermore National Laboratory (LLNL) Site 300, a high-explosives test facility in the Altamont Hills of central California. This site has been used since the 1950s to conduct a variety of experiments involving nitrogenous chemical explosives such as hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX). Site 300 ground water contains nitrate concentrations ranging from <0.5 to >200 mg NO3

-/L. Several lines of evidence strongly suggest that denitrification is naturally attenuating nitrate in the confined, oxygen-depleted region of the bedrock aquifer under study (Tnbs2): (a) both nitrate and dissolved oxygen (DO) concentrations in ground water decrease dramatically as ground water flows from unconfined to confined aquifer conditions, (b) stable isotope signatures (i.e., δ15N and δ18O) of ground water nitrate indicate a trend of isotopic enrichment that is characteristic of denitrification, and (c) dissolved nitrogen gas concentrations, the product of denitrification, were highly elevated in nitrate-depleted ground water in the confined region of the Tnbs2 aquifer. Long-term nitrate concentrations were relatively high and constant in recharge-area monitoring wells (typically 70 to 100 mg NO3

-/L) and relatively low and constant in the downgradient, confined region (typically <0.1 to 3 mg NO3

-/L), suggesting a balance between rates of nitrate loading and removal by denitrification. Chemolithoautotrophic denitrification with pyrite as the electron donor is plausible in the Tnbs2 aquifer, based on the: (a) low dissolved organic carbon concentrations (<1.5 mg/L) that could not support heterotrophic denitrification, (b) common occurrence of disseminated pyrite in the aquifer, and (c) trend of increasing sulfate concentrations and decreasing δ34S of sulfate as ground water flows from aerobic, unconfined to anoxic, confined aquifer conditions. Nitrate sources were investigated by experimentally determining the δ15N and δ18O signatures of nitrate from five potential anthropogenic sources: barium nitrate (mock explosive), nitric acid, RDX photolysis, RDX combustion, and trinitrotoluene (TNT) combustion. The isotopic signatures of these potential nitrate sources were markedly different than those of nitrate in Tnbs2 ground water. In particular, nitrate and nitrite resulting from RDX photolysis reflected a dramatically depleted δ15N value (ca. -7.4 ‰). Our results suggest that other sources (e.g., natural soil and septic releases) contribute significantly to the nitrate loading at Site 300.

Site 300 is located 13.5 highway miles southeast of Livermore and 8.5 miles southwest of Tracy.

1. Several independent lines of evidence strongly suggest that microbial denitrification is naturally attenuating NO3- in the confined,

O2-depleted region of the Tnbs2 aquifer: (a) both NO3

- and DO concentrations in ground water decrease significantly as ground water flows from unconfined to confined aquifer conditions; (b) stable isotope signatures (i.e., δ15N and δ18O) of ground water NO3

- indicate a trend of isotopic enrichment that is characteristic of denitrification; and (c) dissolved N2 gas was highly elevated in NO3

- depleted ground water in the confined region of the aquifer.

2. Long-term NO3- concentrations are relatively high and constant in recharge-area monitoring wells (typically 70-100 mg NO3

-/L) and relatively low and constant in the downgradient confined region (typically < 0.1 - 3 mg NO3

-/L), suggesting a balance between rates of NO3

- loading and removal by denitrification.

3. Dissolved organic carbon concentrations are insufficient to meet the electron donor requirements for heterotrophic denitrification of the NO3

- loading. However, the observed biogeochemical trends could be explained by autotrophic denitrification with pyrite, a mineral commonly found in the aquifers solid matrix, as the electron donor.

4. Inverse modeling using PHREEQC and accounting for mass and charge balances demonstrated that the differences in geochemical indicators between the oxic upgradient and the anoxic downgradient ground water can be explained by thermodynamically-plausible geochemical processes associated with autotrophic denitrification, water-rock interactions, and dilution. In particular, the expected increase in SO4

-2 concentration resulting from oxidation of pyrite is supported by the data.

5. Nitrate sources were investigated by experimentally determining the δ15N and δ18O signatures of NO3- from three potential

anthropogenic sources: Ba(NO3)2 (mock explosive), HNO3 (nitric acid), and photolysis of the explosive RDX. The isotopic signatures of these potential NO3

- sources were markedly different than those of NO3- in Tnbs2 ground water samples, indicating that other

sources (e.g, septic discharges and natural soil) must contribute significantly to the NO3- loading.

6. Nitrate in Site 300 ground water, including background ground water samples, exhibits a relatively narrow range of δ15N and δ18O signatures largely consistent with those found in natural soil.

Isotopic composition is commonly expressed in terms ofthe δ unit, which is defined for N and O stable isotopes as follows:

δ15N or δ18O (‰) [(Rsample - Rstandard / Rstandard] x 1000 where R = (15N/14N) for δ15N, and R = (18O/16O) for δ18O

Measurement of excess nitrogen gas (N2) in ground water

N2 (from denitrification) = N2 measured - N2 equilibrium - (N2/Ar)air X (Armeasured - Arequilibrium)

Relative changes in concentrations of major cation and anion species between averageupgradient and downgradient ground water compositions in the Tnbs2 aquifer

3D Conceptual Model

Background

High Explosive Process Area nitrateisoconcentration contour map for the Tnbs2 aquifer

Cross section showing distribution of nitrate in Tnbs2 aquifer

Abstract

Site 300ground water

plumes

Tnbs2 type log

Summary

Chemolithotrophic denitrification with pyrite as the electron donor

14NO3- + 5FeS2 (pyrite) + 4H+ 7N2 + 10SO4

2- + 5Fe2+ + 2H2O

Measurement of δ15N and δ18O in NO3-

Slope = 0.5045 (Tnbs2 only)

Tnbs2 aquifer

Background wells

RDX combustion

TNT combustion

Barium nitrate

Nitric acid

-5

05

101520253035404550

-5 0 5 10 15 20 25 30δ15N (NO3

-)

δ18O

(NO

3- )

NO

RTH

Pit 6 Operable Unit

High Explosives(HE) Burn Pit

Site

Bou

ndar

y

HE Process AreaOperable Unit

Building 834Operable Unit

Building 854Operable Unit

Site 300Operable Unit;Building 833Release Site

Site 300Operable Unit;Building 801Release Site

Site 300Operable Unit;

Building 851Release Site

NitratePerchlorate

0 2,000Scale : feet

LegendVolatile organiccompounds

High explosivecompounds

Depleted uraniumTritium

General Services Area Operable Unit

Building 850/Pits 3 & 5 Operable Unit

Building 832 CanyonOperable Unit

W-815-1918

Lithology groups

Well details

SandstoneSiltstoneClaystoneConglomerate

ScreenedintervalSand pack

28060

50

100

NGAM

50

100

600 RI

Tps

Tnsc1

Tnbs2

Tnbs1

Qt

Qal

Tps

Tnsc1

Tnbs2

Tnbs1

Qt

Qal

[<0.01][87]

[74]

[30]

[10]

[<0.01]

[15]

[4][<0.01]

[58][97]

[90]

[<0.01]125

00 125

Scale : feet2:1 vertical

exaggeration

W-809-02

[87]

Monitor well ID

Ground water elevation

Screened interval

Legend

Sand pack

Saturated water-bearing zoneNO3

- concentration, mg/L

900

850

800

750700

650

600

550

500

Elev

atio

n (ft

abo

ve M

SL)

NW SEA A'

Unc

onfin

edC

onfin

ed

W-808-01

W-809-02

W-818-08

W-809-03 W-815-04

W-818-01 W-818-06,W-818-07

W-35C-04W-815-2111W-6G

W-6KW-6L

120

100

80

60

Exc

ess

N2

& n

itra

teeq

uivi

lant

(mg

/L) a

s N

O3-

40

20

0

65

4

3

2

1

0

δ34 S

(SO

4-2)

W-8

15-0

4

W-8

18-0

1

W-8

18-0

6

W-8

15-2

111

W-8

09-0

2

W-8

09-0

3

W-3

5C-0

4

W-8

18-0

7

W-6

G

25

20

15

10

5

0

δ15 N

(NO

3- )

876543210

Dis

solv

edo

xyg

en (m

g/L

)

Unc

onfin

edC

onfin

ed

0102030405060708090

100

0 5 10 15 20 25δ15N (NO3

-)

NO

3- (m

g/L

)

0

20

-20

40

60

80

-40

-60

-80

100

-100

Rel

ativ

e ch

ang

e %

NO3HCO3ClKMgCa

SO4Na

0

5

10

15

20

25

0 5 10 15 20 25δ15N (NO3

-)

δ18O

(NO

3- )

Tnbs2

EWFA

Background wells

B832 Cyn

B854

B834

GSA

δ15N vs δ18O of NO3 (all Site 300)

δ15N vs δ18O of NO3-NO3

- concentration vs δ15N (NO3-)

Potential anthropogenicsources

Upgradient Downgradient

SPRING5

SPRING4

SPRING14

SPRING3

Lagoon 819 Disposal Lagoon 833

Lagoon814

Lagoon 817

Lagoon 807-B

Lagoon 806/807

Disposal Lagoon 830

Lagoon 807-A

Corral Hollow Creek

Extent ofSaturation

Unconfined

Confined

W-818-08

GALLO1G

W-6G

W-818-07

W-818-06

W-818-01

W-809-02W-809-03

W-808-01

W-815-04

W-35C-04W-815-2111 A'

A

Tnbs2aquifer

Geochemical trends along Tnbs2 ground water flow path

Excess nitrogen by MembraneInlet Mass Spectrometry (MIMS)

Groundwater flow

NO3- isooncentration

contour (mg/L)

Legend

Leach lineFormer rinsewater lagoon

Line of sectionA'A

Septic Tank

Stream (ephemeral)Site 300 boundary

Guard wellGround water extractionwellMonitor wellWater-supply well(pumping)Spring

G

9090

9090

4545

1010

45

90

45

90

9090

90

NO3- isoconcentration

contour (mg/L)45

0 600300

Feet

Excess N2

DO

δ15N

δ34S

W-6LW-6K

9090

This work was performed under the auspices of the U.S. Department of Energy by the University of California,Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48.