HR: 0800h
AN: H11D-1300    [Abstracts]
TI: Modeling of Reactive Transport of Nitrate in a Heterogeneous Alluvial Fan Aquifer, San Joaquin Valley, California
AU: * Green, C T
EM: ctgreen@usgs.gov
AF: US Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025 United States
AU: Phillips, S P
EM: sphillip@usgs.gov
AF: US Geological Survey, 6000 J Street, Placer Hall, Sacramento, CA 95819 United States
AB: Fate of nitrate in an alluvial fan aquifer in the San Joaquin Valley, California, was investigated with combined laboratory analyses, field measurements, geostatistics, and flow and reactive transport modeling. In the summer of 2003, groundwater wells and lysimeters were installed along a 1-km transect extending upgradient from the Merced River through an unfarmed riparian zone, a corn field, and an orchard. Groundwater levels have been monitored continuously. Saturated and unsaturated pore waters were analyzed quarterly for nutrients, anions, and cations. Sediment core samples from above and below the water table were analyzed for organic matter, nutrients, inorganic chemistry, and potential denitrification using denitrification enzyme assays (DEA's) based on the acetylene block technique. Curve fitting of DEA's provided core-scale estimates of microbial populations and growth coefficients. DEA biomass was similar to values obtained with the most probable number technique. Growth coefficients were found to be relatively uniform across the site, while biomass varied by several orders of magnitude. Age dates estimated from Chlorofluorocarbon (CFC) and Sulfur Hexafluoride (SF₆), together with analyses of nitrogen species and excess nitrogen gas, provided approximate aquifer-scale, zero-order denitrification rates. The field and laboratory measurements served as input for geostatistical realizations of sediment properties and simulations of reactive transport of nitrate in the saturated zone. Analyses of cores, drillers' logs, and previous interpretations of the local geology were used to generate transition probability models of hydrofacies distributions within Holocene alluvium and pre-Holocene fans, and maps of the boundaries between these stratigraphic sequences. Multiple 3-D realizations were created and ranked based on lateral and vertical bulk-flow properties. For realizations representing a range of geological conditions, 3-D flow was computed with boundary conditions interpolated from a regional model. A random walk particle tracking algorithm was applied to the flow fields to simulate reactive transport of nitrate. Numerical results highlight important issues in the relationships between aquifer-scale estimates of denitrification and core-scale estimates of kinetic parameters in heterogeneous systems.
DE: 0469 Nitrogen cycling
DE: 1829 Groundwater hydrology
DE: 1831 Groundwater quality
DE: 1832 Groundwater transport
DE: 1847 Modeling
SC: Hydrology [H]
MN: Fall Meeting 2005