Abstract

Green, C. and Phillips, S., 2006, Modeling of Reactive Transport of Nitrate in a Heterogeneous Alluvial Fan Aquifer, San Joaquin Valley, California. National Water Quality Monitoring Council, 5^th National Monitoring Conference, May 7-11, 2006, San Jose, CA.

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. Groundwater wells and lysimeters were installed to monitor hydrology and chemistry along a 1-km transect that extends upgradient from the Merced River. Sediment core samples from above and below the water table were analyzed for organic matter, nutrients, inorganic chemistry, and microbial parameters. Curve fitting of denitrification enzyme assays (DEAs) provided estimates of microbial biomass and growth coefficients in sediment cores. DEA biomass was similar to values obtained with the most probable number technique and varied by orders of magnitude between cores. At the aquifer scale, estimates of groundwater age and excess nitrogen gas implied zero-order decay rates of 00.8 mg/L/yr, which suggests that the large volume of groundwater exceeding nitrate drinking-water standards will continue to expand under current agricultural practices and hydrogeologic conditions. To quantify the fate and transport of nitrate, the field and laboratory measurements served as input for geostatistical realizations of sediment properties and simulations of reactive transport. Analyses of sediment cores, well logs, and previous interpretations of the local geology were used to generate (1) transition probability models of hydrofacies distributions within Holocene alluvium and pre-Holocene fans, and (2) maps of the boundaries between the 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, flow was calculated with boundary conditions interpolated from a regional groundwater model. Reactive transport of nitrate was simulated for the saturated zone of the detailed 3-D domain. Numerical results highlight the importance of distinguishing between aquifer- and core-scale estimates of reaction rates and kinetic parameters in heterogeneous systems.