Biogeochemistry and natural attenuation of nitrate in groundwater at an explosives test facility

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, O2-depleted region of the bedrock aquifer under study (Tnbs2): (a) both NO3− and dissolved O2(DO) concentrations in groundwater decrease dramatically as groundwater flows from unconfined to confined aquifer conditions, (b) stable isotope signatures (i.e., δ15N and δ18O) of groundwater NO3− indicate a trend of isotopic enrichment that is characteristic of denitrification, and (c) dissolved N2 gas, the product of denitrification, was highly elevated in NO3−-depleted groundwater in the confined region of the Tnbs2 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 Tnbs2 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 SO2−4 as groundwater flows from aerobic, unconfined to anoxic, confined aquifer conditions. Nitrate sources were investigated by experimentally determining the δ15N and δ18O 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 Tnbs2 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 δ15N (ca. −7.4‰) and δ18O (ca. −25.7‰) values.