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 NO 3 − 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 NO 3 − concentrations ranging from 200 mg NO 3 −/L. Several lines of evidence strongly suggest that denitrification is naturally attenuating NO 3 − in the confined, O 2-depleted region of the bedrock aquifer under study (Tnbs 2): (a) both NO 3 − 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., δ 15N and δ 18O) of groundwater NO 3 − 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 NO 3 −-depleted groundwater in the confined region of the Tnbs 2 aquifer. Long-term NO 3 − concentrations were 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