Evaluation of Sources and Fate of Ground Water Nitrate at a Semi-arid Catchment in the Central California Coast Ranges using Stable Isotopes UCRL-POST-218111 Vic Madrid 1 , ([email protected]), H.R. Beller 1 , B. Esser 2 , G.B. Hudson 2 , M. Singleton 2 , W. McNab 1 , and S. Wankel 3 , Environmental Restoration Division 1 Lawrence Livermore National Laboratory, Chemical Biology and Nuclear Science Division 2 Lawrence Livermore National Laboratory, P.O. Box 808, Livermore, CA, USA 94551-0808, Isotope Tracers of Hydrological and Biogeochemical Processes 3 , U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025 science for a changing world Livermore Livermore Tracy Tracy Alameda County San Joaquin County San Francisco Site 300 Site 300 NORTH Alameda County Contra Costa County San Francisco Contra Costa County San Joaquin County Scale: Miles 0 5 10 HE Process Area OU HE Process Area OU NORTH Site 300 boundary 0 2,000 4,000 Scale : Feet Tnbs 2 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 NO 3 - /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 (Tnbs 2 ): (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., δ 15 N and δ 18 O) 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 Tnbs 2 aquifer. Long-term nitrate concentrations were relatively high and constant in recharge-area monitoring wells (typically 70 to 100 mg NO 3 - /L) and relatively low and constant in the downgradient, confined region (typically <0.1 to 3 mg NO 3 - /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 Tnbs 2 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 δ 34 S of sulfate as ground water flows from aerobic, unconfined to anoxic, confined aquifer conditions. Nitrate sources were investigated by experimentally determining the δ 15 N and δ 18 O 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 Tnbs 2 ground water. In particular, nitrate and nitrite resulting from RDX photolysis reflected a dramatically depleted δ 15 N 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 NO 3 - in the confined, O 2 -depleted region of the Tnbs 2 aquifer: (a) both NO 3 - and DO concentrations in ground water decrease significantly as ground water flows from unconfined to confined aquifer conditions; (b) stable isotope signatures (i.e., δ 15 N and δ 18 O) of ground water NO 3 - indicate a trend of isotopic enrichment that is characteristic of denitrification; and (c) dissolved N 2 gas was highly elevated in NO 3 - depleted ground water in the confined region of the aquifer. 2. Long-term NO 3 - concentrations are relatively high and constant in recharge-area monitoring wells (typically 70-100 mg NO 3 - /L) and relatively low and constant in the downgradient confined region (typically < 0.1 - 3 mg NO 3 - /L), suggesting a balance between rates of NO 3 - loading and removal by denitrification. 3. Dissolved organic carbon concentrations are insufficient to meet the electron donor requirements for heterotrophic denitrification of the NO 3 - 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 SO 4 -2 concentration resulting from oxidation of pyrite is supported by the data. 5. Nitrate sources were investigated by experimentally determining the δ 15 N and δ 18 O signatures of NO 3 - from three potential anthropogenic sources: Ba(NO 3 ) 2 (mock explosive), HNO 3 (nitric acid), and photolysis of the explosive RDX. The isotopic signatures of these potential NO 3 - sources were markedly different than those of NO 3 - in Tnbs 2 ground water samples, indicating that other sources (e.g, septic discharges and natural soil) must contribute significantly to the NO 3 - loading. 6. Nitrate in Site 300 ground water, including background ground water samples, exhibits a relatively narrow range of δ 15 N and δ 18 O signatures largely consistent with those found in natural soil. Isotopic composition is commonly expressed in terms of the δ unit, which is defined for N and O stable isotopes as follows: δ 15 N or δ 18 O (‰) [(R sample - R standard / R standard ] x 1000 where R = ( 15 N/ 14 N) for δ 15 N, and R = ( 18 O/ 16 O) for δ 18 O Measurement of excess nitrogen gas (N 2 ) in ground water N 2 (from denitrification) = N 2 measured - N 2 equilibrium - (N 2 /Ar) air X (Ar measured - Ar equilibrium ) Relative changes in concentrations of major cation and anion species between average upgradient and downgradient ground water compositions in the Tnbs 2 aquifer 3D Conceptual Model Background High Explosive Process Area nitrate isoconcentration contour map for the Tnbs 2 aquifer Cross section showing distribution of nitrate in Tnbs 2 aquifer Abstract Site 300 ground water plumes Tnbs 2 type log Summary Chemolithotrophic denitrification with pyrite as the electron donor 14NO 3 - + 5FeS 2 (pyrite) + 4H + 7N 2 + 10SO 4 2- + 5Fe 2+ + 2H 2 O Measurement of δ 15 N and δ 18 O in NO 3 - Slope = 0.5045 (Tnbs 2 only) Tnbs 2 aquifer Background wells RDX combustion TNT combustion Barium nitrate Nitric acid -5 0 5 10 15 20 25 30 35 40 45 50 -5 0 5 10 15 20 25 30 δ 15 N (NO 3 - ) δ 18 O (NO 3 - ) NORTH Pit 6 Operable Unit High Explosives (HE) Burn Pit Site Boundary HE Process Area Operable Unit Building 834 Operable Unit Building 854 Operable Unit Site 300 Operable Unit; Building 833 Release Site Site 300 Operable Unit; Building 801 Release Site Site 300 Operable Unit; Building 851 Release Site Nitrate Perchlorate 0 2,000 Scale : feet Legend Volatile organic compounds High explosive compounds Depleted uranium Tritium General Services Area Operable Unit Building 850/Pits 3 & 5 Operable Unit Building 832 Canyon Operable Unit W-815-1918 Lithology groups Well details Sandstone Siltstone Claystone Conglomerate Screened interval Sand pack 280 60 50 100 NGAM 50 100 60 0 RI Tps Tnsc 1 Tnbs 2 Tnbs 1 Qt Qal Tps Tnsc 1 Tnbs 2 Tnbs 1 Qt Qal [<0.01] [87] [74] [30] [10] [<0.01] [15] [4] [<0.01] [58] [97] [90] [<0.01] 125 0 0 125 Scale : feet 2:1 vertical exaggeration W-809-02 [87] Monitor well ID Ground water elevation Screened interval Legend Sand pack Saturated water-bearing zone NO 3 - concentration, mg/L 900 850 800 750 700 650 600 550 500 Elevation (ft above MSL) NW SE A A' Unconfined Confined 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-04 W-815-2111 W-6G W-6K W-6L 120 100 80 60 Excess N 2 & nitrate equivilant (mg/L) as NO 3 - 40 20 0 6 5 4 3 2 1 0 δ 34 S (SO 4 -2 ) W-815-04 W-818-01 W-818-06 W-815-2111 W-809-02 W-809-03 W-35C-04 W-818-07 W-6G 25 20 15 10 5 0 δ 15 N (NO 3 - ) 8 7 6 5 4 3 2 1 0 Dissolved oxygen (mg/L) Unconfined Confined 0 10 20 30 40 50 60 70 80 90 100 0 5 10 15 20 25 δ 15 N (NO 3 - ) NO 3 - (mg/L) 0 20 -20 40 60 80 -40 -60 -80 100 -100 Relative change % NO 3 HCO 3 Cl K Mg Ca SO 4 Na 0 5 10 15 20 25 0 5 10 15 20 25 δ 15 N (NO 3 - ) δ 18 O (NO 3 - ) Tnbs 2 EWFA Background wells B832 Cyn B854 B834 GSA δ 15 N vs δ 18 O of NO 3 (all Site 300) δ 15 N vs δ 18 O of NO 3 - NO 3 - concentration vs δ 15 N (NO 3 - ) Potential anthropogenic sources Upgradient Downgradient SPRING5 SPRING4 SPRING14 SPRING3 Lagoon 819 Lagoon 814 Lagoon 817 Lagoon 807-B Lagoon 806/807 Lagoon 807-A Corral Hollow Creek Extent of Saturation Unconfined Confined W-818-08 GALLO1 G W-6G W-818-07 W-818-06 W-818-01 W-809-02 W-809-03 W-808-01 W-815-04 W-35C-04 W-815-2111 A' A Tnbs 2 aquifer Geochemical trends along Tnbs 2 ground water flow path Excess nitrogen by Membrane Inlet Mass Spectrometry (MIMS) Ground water flow NO 3 - isooncentration contour (mg/L) Legend Leach line Former rinsewater lagoon Line of section A' A Septic Tank Stream (ephemeral) Site 300 boundary Guard well Ground water extraction well Monitor well Water-supply well (pumping) Spring G 90 90 90 90 45 45 10 10 45 90 45 90 90 90 90 NO 3 - isoconcentration contour (mg/L) 45 0 600 300 Feet Excess N 2 DO δ 15 N δ 34 S W-6L W-6K 90 90 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.