HR: 0800h
AN: B31A-0972    [Abstracts]
TI: Molecular and Geochemical Evidence of in situ Denitrification at a Dairy Field Site in the Central Valley of California
AU: Esser, B K
EM: bkesser@llnl.gov
AF: Lawrence Livermore National Lab, 7000 East Ave., Livermore, CA 94550 United States
AU: * Letain, T E
EM: letain2@llnl.gov
AF: Lawrence Livermore National Lab, 7000 East Ave., Livermore, CA 94550 United States
AU: Singleton, M J
EM: singleton20@llnl.gov
AF: Lawrence Livermore National Lab, 7000 East Ave., Livermore, CA 94550 United States
AU: Beller, H R
EM: beller2@llnl.gov
AF: Lawrence Livermore National Lab, 7000 East Ave., Livermore, CA 94550 United States
AU: Kane, S R
EM: kane11@llnl.gov
AF: Lawrence Livermore National Lab, 7000 East Ave., Livermore, CA 94550 United States
AU: Balser, L M
EM: balser1@llnl.gov
AF: Lawrence Livermore National Lab, 7000 East Ave., Livermore, CA 94550 United States
AU: Moran, J E
EM: moran10@llnl.gov
AF: Lawrence Livermore National Lab, 7000 East Ave., Livermore, CA 94550 United States
AB: Rising nitrate concentrations in California groundwater threaten an already strained water supply. Under certain conditions, however, intrinsic microbial denitrification can mitigate this problem. We present results from a field study at a central California dairy that document saturated-zone denitrification using a combination of molecular and geochemical methods. Geochemical measurements to assess denitrification included nitrate concentration, dissolved oxygen (DO) concentration, dissolved excess N₂, and stable isotope composition of nitrate. Sharp decreases in nitrate concentrations with depth corresponded to sharp decreases in DO concentrations and decreasing redox potential. Nitrate in groundwater from this study had δ¹⁵N values (5 to 60 ‰) and δ¹⁸O values (-4 to 25 ‰) that plotted with a δ¹⁸O/δ¹⁵N slope of 0.5, consistent with denitrification. Dissolved N₂ was found at concentrations well above Ar-normalized concentrations predicted for atmospheric N₂, consistent with reduction of nitrate to N₂. in situ denitrification was further documented by increased populations of denitrifying bacteria in zones with geochemical signatures of denitrification. Real-time, quantitative, Polymerase Chain Reaction (qPCR) analysis was used to determine denitrifying bacterial cell populations present in aquifer sediment samples by measuring the abundance of genes encoding nitrite reductase, a central enzyme involved in denitrification. Real-time qPCR primers and probes allowing for universal detection of both the nirS (Fe-containing nitrite reductase) and nirK (Cu-containing nitrite reductase) genes in environmental samples were designed based on multiple alignments of over 30 nirS and nirK gene sequences available in GenBank. Trends in total eubacterial populations were also monitored by real-time qPCR analysis. Although geochemical measurements alone can sometimes convincingly indicate denitrification, the real-time qPCR analysis used in this study provides additional valuable information, namely: populations of denitrifying bacteria in an area (which is useful for reactive-transport modeling) and localization of activity to a specific well (in contrast to geochemical indicators, which may record upgradient denitrification).
DE: 0404 Anoxic and hypoxic environments (4802, 4834)
DE: 0418 Bioremediation
DE: 0469 Nitrogen cycling
DE: 0471 Oxidation/reduction reactions (4851)
DE: 1831 Groundwater quality
SC: Biogeosciences [B]
MN: Fall Meeting 2005