HR: 15:20h
AN: H52C-07    [PDF]
TI: Reactive Transport of Nitrate in Northern California Groundwater basins: An Integrated Characterization and Modeling Approach
AU: * Esser, B K
AF: Lawrence Livermore National Laboratory, Chemical Biology & Nuclear Sciences Division, Livermore, CA 94551-0808 United States
AU: Moran, J E
AF: Lawrence Livermore National Laboratory, Chemical Biology & Nuclear Sciences Division, Livermore, CA 94551-0808 United States
AU: Hudson, G B
AF: Lawrence Livermore National Laboratory, Chemical Biology & Nuclear Sciences Division, Livermore, CA 94551-0808 United States
AU: Carle, S F
AF: Lawrence Livermore National Laboratory, Environmental Sciences Division, Livermore, CA 94551-0808 United States
AU: McNab, W
AF: Lawrence Livermore National Laboratory, Environmental Restoration Division, Livermore, CA 94551-0808 United States
AU: Tompson, A F
AF: Lawrence Livermore National Laboratory, Environmental Sciences Division, Livermore, CA 94551-0808 United States
AU: Moore, K
AF: University of Arizona, Department of Hydrology, Tucson, AZ 85721 United States
AU: Beller, H
AF: Lawrence Livermore National Laboratory, Environmental Restoration Division, Livermore, CA 94551-0808 United States
AU: Kane, S
AF: Lawrence Livermore National Laboratory, Environmental Restoration Division, Livermore, CA 94551-0808 United States
AU: Eaton, G
AF: Lawrence Livermore National Laboratory, Chemical Biology & Nuclear Sciences Division, Livermore, CA 94551-0808 United States
AB: More than 1/3 of active public drinking water supply wells in California produce water with nitrate-N levels indicative of anthropogenic inputs ($>$ 4 mg/L). Understanding how the distribution of nitrate in California groundwater basins will evolve is vital to water supply and infrastructure planning. To address this need, we are studying the basin-scale reactive transport of nitrate in the Livermore and Llagas basins of Northern California. Both basins have increasingly urban populations heavily reliant on groundwater. A distinct nitrate "plume" exists in the Livermore Basin (Alameda County) whereas pervasive nitrate contamination exists in shallow groundwaters of the Llagas Basin (Santa Clara County). The sources and timing of nitrate contamination in these basins are not definitively known; septic systems, irrigated agriculture and livestock operations exist or have existed in both areas. The role of denitrification in controlling nitrate distribution is also unknown; dissolved oxygen levels are sufficiently low in portions of each basin as to indicate the potential for denitrification. We have collected water from 60 wells, and are determining both groundwater age (by the $^{3}$H/$^{3}$He method) and the extent of denitrification (by the excess N$_{2}$ method). Excess nitrogen is being determined by both membrane-inlet and noble gas mass spectrometry, using Ar and Ne content to account for atmospheric N$_{2}$. We are also analyzing for stable istotopes of nitrate and water, nitrate co-contaminants, and general water quality parameters. Preliminary analysis of archival water district data from both basins suggests positive correlations of nitrate with Ca$^{+2}$, Mg$^{+2}$ and bicarbonate and negative correlation with pH. In the Llagas Basin, a negative correlation also exists between nitrate and temperature. Flow path-oriented reactive transport modeling is being explored as a tool to aid in the identification of both the sources of nitrate and evidence for denitrification in both basins. Geostatistical realizations of Llagas Basin lithology provide a framework for addressing complexity in nitrate flow paths and spatial variation of denitrifying conditions. Particle transport simulations help calibrate the flow field to groundwater age data. Changes in groundwater geochemistry along the flow paths are used to constrain inverse geochemical models that employ redox reactions, carbonate mineral equilibria, and ion exchange mechanisms to attribute nitrate source types, and, in conjunction with excess N$_{2}$ data, quantify nitrate losses via denitrification.
DE: 1040 Isotopic composition/chemistry
DE: 1045 Low-temperature geochemistry
DE: 1831 Groundwater quality
DE: 1832 Groundwater transport
DE: 1869 Stochastic processes
SC: Hydrology [H]
MN: 2003 Fall Meeting