Vadose Zone Transport of Nitrate in the Sierra Pelona Groundwater Basin, Agua Dulce, California

Using data from previous studies about nitrate contamination in Agua Dulce, CA and the numerical modeling program HYDRUS-1D, nitrate transport in the vadose zone associated with septic system discharges was evaluated. Thirty-six modeling scenarios were assessed and the results analyzed. With soil borings less than 6 m deep, steady state nitrate transport within the system is reached within 200 days, while steady state is not reached with the deeper boring within 500 days. The modeling results show that the nitrate concentration plume is less than the maximum contaminant level (MCL) of 10 mg/L within 1.3 m below the septic leachfield in all scenarios. The analysis of the fully-mixed tank scenario clearly shows that nitrate concentrations in groundwater are of greater concern during drought conditions. These findings help explain the pattern of nitrate concentrations observed in the aquifer system in this region.