Nitrate Reaction Rates and Stable Isotope Fractionation In An Alluvial Fan Aquifer, San Joaquin Valley, CA

As a part of the U.S. Geological Survey National Water Quality Assessment Program's investigation of agricultural contamination, the influence of geological heterogeneity on groundwater samples' inferred ages, attenuation rates of O2 and NO3-, and fractionations of stable isotopes of N and O in NO3-, was evaluated using forward and inverse modeling and field observations. The study site included a 1-km transect of multi-level well nests, installed in an alluvial fan aquifer near Merced, California. From 200 geostatistical realizations conditioned on cores and geophysical logs of the aquifer's heterogeneity, six were selected to represent a range of possible geological conditions. Flow and reactive transport were modeled using MODFLOW and RWHet, respectively. Flow model hydraulic parameters were assigned on the basis of the distribution of hydrofacies in the geostatistical realizations. Model results were fitted to measured groundwater head and atmospheric age tracer concentrations using PEST. Also fitted were estimates of source concentration of NO3- and source stable isotope ratios, denitrification progress, denitrification rate, and isotopic fractionation factors, all derived from measured solute concentrations and stable isotope ratios. Results show that mixing of groundwaters due to geological heterogeneity reduces the magnitude of inferred reaction rates, and isotopic fractionations. The influence of heterogeneity was relatively strong. Model results varied widely depending on the particular realizations of the heterogeneity and the estimated value of the dispersivity. In general, the estimated values of dispersivity were as influential as first order rate coefficients on the modeled trends of denitrification over time, and were more influential than the fractionation factor on the modeled trends in isotopic enrichment during denitrification. Due to the strong influence of heterogeneity, the influence of mixing and dispersion should be considered in rigorous analyses of ages, rates, and stable isotope fractionations.