HR: 14:50h
AN: H52C-05    [PDF]
TI: Investigating Natural Attenuation and Sources of Nitrate in Groundwater Using Stable Isotopes and Other Techniques
AU: * Beller, H R
EM: beller2@llnl.gov
AF: Lawrence Livermore National Laboratory, 7000 East Ave., L-542, Livermore, CA 94551
AU: Madrid, V
EM: madrid2@llnl.gov
AF: Lawrence Livermore National Laboratory, 7000 East Ave., L-542, Livermore, CA 94551
AU: Hudson, G B
EM: hudson5@llnl.gov
AF: Lawrence Livermore National Laboratory, 7000 East Ave., L-542, Livermore, CA 94551
AU: McNab, W W
EM: mcnab1@llnl.gov
AF: Lawrence Livermore National Laboratory, 7000 East Ave., L-542, Livermore, CA 94551
AU: Carlsen, T M
EM: carlsen1@llnl.gov
AF: Lawrence Livermore National Laboratory, 7000 East Ave., L-542, Livermore, CA 94551
AB: We conducted an interdisciplinary study to characterize the distribution and fate of nitrate 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 nitrate concentrations ranging from $<$0.5 to $>$200 mg NO$_{3}$$^{-}$/L. Several lines of evidence strongly suggest that denitrification is naturally attenuating nitrate in the confined, oxygen-depleted region of the bedrock aquifer under study (Tnbs$_{2}$): (a) both nitrate and dissolved oxygen (DO) concentrations in groundwater decrease dramatically as groundwater flows from unconfined to confined aquifer conditions, (b) stable isotope signatures (i.e., $\delta$$^{15}$N and $\delta$$^{18}$O) of groundwater nitrate indicate a trend of isotopic enrichment that is characteristic of denitrification, and (c) dissolved nitrogen gas, the product of denitrification, was highly elevated in nitrate-depleted groundwater in the confined region of the Tnbs$_{2}$ aquifer, as determined by membrane-inlet mass spectrometry. Long-term nitrate concentrations were relatively high and constant in recharge-area monitoring wells and relatively low and constant in the downgradient confined region, suggesting a balance between rates of nitrate loading and removal by denitrification. Chemolithoautotrophic denitrification with pyrite as the electron donor is plausible in the Tnbs$_{2}$ aquifer, based on the low dissolved organic carbon concentrations that could not support heterotrophic denitrification, the common occurrence of disseminated pyrite in the aquifer, and the trend of increasing sulfate as groundwater flows from aerobic, unconfined to anoxic, confined aquifer conditions. Nitrate sources were investigated by experimentally determining the $\delta$$^{15}$N and $\delta$$^{18}$O signatures of nitrate from three potential anthropogenic sources of nitrate at Site 300: barium nitrate (mock explosive), nitric acid, and photolysis of the explosive RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine). The isotopic signatures of these potential nitrate sources were markedly different than those of nitrate in Tnbs$_{2}$ groundwater samples, suggesting that other sources must contribute significantly to the nitrate loading at Site 300.
DE: 0400 Biogeosciences
DE: 1030 Geochemical cycles (0330)
DE: 1040 Isotopic composition/chemistry
DE: 1831 Groundwater quality
SC: Hydrology [H]
MN: 2003 Fall Meeting