Effect of mixing on oxygen reduction, denitrification, and isotopic fractionation in a heterogeneous aquifer in an agricultural setting.

Large fluxes of nitrate in agricultural settings threaten water quality worldwide. In groundwater, the transport and fate of anthropogenic nitrate can be influenced by denitrification, which occurs under anaerobic conditions. It is difficult to characterize denitrification because (1) waters from multiple flow paths commingle in the aquifer due to dispersion and (2) a sample typically includes water from multiple lithologies. For a site near Merced, California, geostatistical simulations, flow modeling, and backward random walk particle tracking were applied to simulate the influence of mixing on apparent reaction rates, redox boundaries, and fractionation of stable isotopes of nitrogen. Results show that dispersion has a strong effect on the distribution of redox indicators. When reactions are rapid, mixing can create the appearance of gradual redox transitions that mask the actual reaction rates and the degree of isotopic fractionation.