Evaluation of Sources and Fate of Ground Water Nitrate at a Semi-arid Catchment in the Central California Coast Ranges using Stable Isotopes science for a changing world Vic Madrid1, (firstname.lastname@example.org), 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 UCRL-POST-218111 Abstract High Explosive Process Area nitrate Measurement of δ15N and δ18O in NO3- δ15N vs δ18O of NO3 (all Site 300)We are conducting an interdisciplinary study to characterize the potential sources, distribution, isoconcentration contour map for the Tnbs2 aquifer 25 Tnbs2 GSAand fate of nitrate in ground water at Lawrence Livermore National Laboratory (LLNL) Site 300, a B834 EWFAhigh-explosives test facility in the Altamont Hills of central California. This site has been used Legend 20 B832 Cynsince the 1950s to conduct a variety of experiments involving nitrogenous chemical explosives SPRING4 A A Line of section A Isotopic composition is commonly expressed in terms of δ18O (NO3-) Lagoon 807-B Disposal Lagoon 830 B854such as hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX). Site 300 ground water contains nitrate Former rinsewater lagoon the δ unit, which is defined for N and O stable isotopes as 15 Background wellsconcentrations ranging from <0.5 to >200 mg NO3-/L. Several lines of evidence strongly suggest W-808-01 Leach linethat denitrification is naturally attenuating nitrate in the confined, oxygen-depleted region of the Septic Tank follows: Stream (ephemeral) 10bedrock aquifer under study (Tnbs2): (a) both nitrate and dissolved oxygen (DO) concentrations Lagoon 807-Ain ground water decrease dramatically as ground water flows from unconfined to confined Lagoon 819 Disposal Lagoon Site 300 boundary 833 δ15Nor δ18O (‰) [(Rsample - Rstandard / Rstandard] x 1000aquifer conditions, (b) stable isotope signatures (i.e., δ15N and δ18O) of ground water nitrate Lagoon 806/807 G Guard well where R = (15N/14N) for δ15N, and R = (18O/16O) for δ18O 5 Ground water extractionindicate a trend of isotopic enrichment that is characteristic of denitrification, and (c) dissolved Gr well ou W-809-02 Monitor well 0nitrogen gas concentrations, the product of denitrification, were highly elevated in nitrate- wa nd W-809-03 Water-supply well 0 5 10 15 20 25depleted ground water in the confined region of the Tnbs2 aquifer. Long-term nitrate t er (pumping) 15N (NO -) flo Lagoon Spring δ 3concentrations were relatively high and constant in recharge-area monitoring wells (typically 70 w 814 90to 100 mg NO3-/L) and relatively low and constant in the downgradient, confined region (typically NO3- isooncentration 90 contour (mg/L) NO3- concentration vs δ15N (NO3-) δ15N vs δ18O of NO3-<0.1 to 3 mg NO3-/L), suggesting a balance between rates of nitrate loading and removal by Extent of W-818-01 Tnbs2 100 50 Potential anthropogenicdenitrification. Chemolithoautotrophic denitrification with pyrite as the electron donor is Saturation W-815-04plausible in the Tnbs2 aquifer, based on the: (a) low dissolved organic carbon concentrations SPRING3 aquifer 90 45 sources Tnbs2 aquifer(<1.5 mg/L) that could not support heterotrophic denitrification, (b) common occurrence of 90 80 40 Background wells Lagoon 817 SPRING5 W-818-08 RDX combustiondisseminated pyrite in the aquifer, and (c) trend of increasing sulfate concentrations and 90 70 35 δ18O (NO3-) NO3- (mg/L)decreasing δ34S of sulfate as ground water flows from aerobic, unconfined to anoxic, confined W-818-07 60 30 TNT combustionaquifer conditions. Nitrate sources were investigated by experimentally determining the δ15N 50 25 Barium nitrateand δ18O signatures of nitrate from five potential anthropogenic sources: barium nitrate (mock W-818-06 40 20 Nitric acidexplosive), nitric acid, RDX photolysis, RDX combustion, and trinitrotoluene (TNT) combustion. d 30 15The isotopic signatures of these potential nitrate sources were markedly different than those of 45 f ine W-6G c on ed 20 10nitrate in Tnbs2 ground water. In particular, nitrate and nitrite resulting from RDX photolysis Un nfin 10 W-6L 10 5 Slope = 0.5045 (Tnbs2 only)reflected a dramatically depleted δ15N value (ca. -7.4 ‰). Our results suggest that other sources SPRING14 Co W-6K 0 0(e.g., natural soil and septic releases) contribute significantly to the nitrate loading at Site 300. W-35C-04 0 5 10 15 20 25 -5 0 5 10 15 20 25 30 -5 W-815-2111 A δ15N (NO3-) δ15N (NO3-) G GALLO1 Geochemical trends along Tnbs2 ground water flow path Background equivilant (mg/L) as NO3- Measurement of excess nitrogen 120 Excess N2 & nitrate ek 100 Excess N2Site 300 is located 13.5 highway miles southeast of Livermore and 8.5 miles w Cre 0 300 600 llo gas (N2) in ground water 80 Hosouthwest of Tracy. Co rral Feet 60 40 20 N2 (from denitrification) = N2 measured - 0 N2 equilibrium - (N2/Ar)air X (Armeasured - Arequilibrium) NORTH Scale: Miles San 8 NORTH Joaquin0 5 10 Contra Costa County 7 DO oxygen (mg/L) 6 County Dissolved San Tracy 5 Cross section showing distribution of nitrate in Tnbs2 aquifer Francisco 4 Alameda 3 County 2 Livermore Site 300 A A 1 NW SE 0 25 900 W-808-01 Scale : Feet 20 δ15N Unconfined 0 2,000 4,000 δ15N (NO3-) 850 W-809-02 Confined 15 Elevation (ft above MSL) 800 W-809-03 W-815-04 10 750 W-818-06, 5 HE Process Area OU 700 W-818-01 W-818-08 W-818-07 0 W-35C-04 6 Tnbs2 type log 650 W-6G W-815-2111 5 δ34S 90 δ34S (SO4-2) 600  W-6K 4   Tps Site 300 Qt 90 W-6L [<0.01] 3 550 Tnbs2 W-815-1918 boundary  2 45 Qal 500   1 60 NGAM 280 0 RI 60 Lithology groups  0 Tnsc1 Unconfined W-809-02 W-809-03 W-815-04 W-818-01 W-6G W-818-06 W-818-07 W-815-2111 W-35C-04 Sandstone Legend   Confined Siltstone W-809-02 Monitor well ID 125 [<0.01] Claystone Ground water elevation Tnbs1 [<0.01] Excess nitrogen by Membrane Conglomerate 3D Conceptual Model Sand pack Screened interval [<0.01] Inlet Mass Spectrometry (MIMS) Well details Saturated water-bearing zone 0 0 125 Upgradient Downgradient Screened  NO3- concentration, mg/L Scale : feet interval NO3- isoconcentration 2:1 vertical 45 contour (mg/L) exaggeration 50 50 Sand pack Summary 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: Chemolithotrophic denitrification with pyrite as the electron donor (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 100 100 Tnbs2 14NO3- + 5FeS2 (pyrite) + 4H+ 7N2 + 10SO42- + 5Fe2+ + 2H2O that is characteristic of denitrification; and aquifer (c) dissolved N2 gas was highly elevated in NO3- depleted ground water in the confined region of the aquifer. 100 2. Long-term NO3- concentrations are relatively high and constant in recharge-area monitoring wells (typically 70-100 mg NO3-/L) 80 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. 60 Site 300 3. Dissolved organic carbon concentrations are insufficient to meet the electron donor requirements for heterotrophic 40 denitrification of the NO3- loading. However, the observed biogeochemical trends could be explained by autotrophic denitrification Relative change % Site Boundary NORTH ground water with pyrite, a mineral commonly found in the aquifers solid matrix, as the electron donor. plumes 20 Ca Mg K Cl HCO3 NO3 4. Inverse modeling using PHREEQC and accounting for mass and charge balances demonstrated that the differences in Site 300 Site 300 0 geochemical indicators between the oxic upgradient and the anoxic downgradient ground water can be explained by Na SO4 Operable Unit; Legend Operable Unit; Building 850/Pits 3 & 5 Building 801 Building 851 Operable Unit Release Site thermodynamically-plausible geochemical processes associated with autotrophic denitrification, water-rock interactions, and Volatile organic Release Site -20 compounds Building 854 Operable Unit dilution. In particular, the expected increase in SO4-2 concentration resulting from oxidation of pyrite is supported by the data. Tritium -40 5. Nitrate sources were investigated by experimentally determining the δ15N and δ18O signatures of NO3- from three potential Building 834 Depleted uranium Operable Unit High explosive -60 anthropogenic sources: Ba(NO3)2 (mock explosive), HNO3 (nitric acid), and photolysis of the explosive RDX. The isotopic signatures compounds High Explosives Building 832 Canyon Operable Unit Site 300 Operable Unit; of these potential NO3- sources were markedly different than those of NO3- in Tnbs2 ground water samples, indicating that other (HE) Burn Pit Building 833 Nitrate Release Site sources (e.g, septic discharges and natural soil) must contribute significantly to the NO3- loading. Perchlorate -80 Scale : feet Pit 6 Operable Unit 6. Nitrate in Site 300 ground water, including background ground water samples, exhibits a relatively narrow range of δ15N and δ18O 0 2,000 -100 signatures largely consistent with those found in natural soil. HE Process Area Operable Unit Relative changes in concentrations of major cation and anion species between average General Services Area Operable Unit upgradient and downgradient ground water compositions in the Tnbs2 aquifer 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.