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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, ([email protected]), 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.
