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An interdisciplinary study was conducted to characterize the distribution and fate of NO3- in groundwater at Lawrence Livermore National Laboratory (LLNL) Site 300, a high-explosives test facility in the semi-arid Altamont Hills of California. Site 300 groundwater contains NO3- concentrations ranging from <0.5 to >200 mg NO3-/L. Several lines of evidence strongly suggest that denitrification is naturally attenuating NO3- in the confined, O-2-depleted region of the bedrock aquifer under study (Tnbs(2)): (a) both NO3- and dissolved O-2(DO) concentrations in groundwater decrease dramatically as groundwater flows from unconfined to confined aquifer conditions, (b) stable isotope signatures (i.e., delta(15)N and delta(18)O) of groundwater NO3- indicate a trend of isotopic enrichment that is characteristic of denitrification, and (c) dissolved N-2 gas, the product of denitrification, was highly elevated in NO3--depleted groundwater in the confined region of the Tnbs(2) aquifer. Long-term NO3- concentrations were 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. Chemolithoautotrophic denitrification with pyrite as the electron donor is plausible in the Tnbs(2) aquifer, based on the low dissolved organic C concentrations (< 1.5 mg/L) that could not support heterotrophic denitrification, the common occurrence of disseminated pyrite in the aquifer, and the trend of increasing SO42- as groundwater flows from aerobic, 4 unconfined to anoxic, confined aquifer conditions. Nitrate sources were investigated by experimentally determining the delta(15)N and delta(18)O signatures of NO3- from three potential anthropogenic sources of NO3- at Site 300: Ba(NO3)(2) (mock explosive), HNO3, and photolysis of the explosive RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine). The isotopic signatures of these potential NO3- sources were markedly different than those of NO3- in Tnbs(2) groundwater samples, suggesting that other sources must contribute significantly to the NO3- loading at Site 300. In particular, NO3- and NO2- resulting from RDX photolysis reflected dramatically depleted delta(15)N (ca. -7.4parts per thousand) and delta(18)O (ca. -25.7parts per thousand) values. (C) 2004 Elsevier Ltd. All rights reserved.