THREE-DIMENSIONAL MODELING OF SUBSURFACE NITROGEN TRANSPORT AT THE WATERSHED SCALE

Groundwater is an important pathway of nitrogen transport to surface water systems. Watershed-scale mathematical models commonly use empirical transfer functions to link sources of nitrogen contamination to surface water, oversimplifying groundwater nitrogen transport and transformation. We applied a new three-dimensional multispecies reactive numerical model, SF Monod, to evaluate subsurface nitrogen transport to a stream network. The simulation domain was a 0.23 km² suburban watershed in the Maryland Piedmont served entirely by septic systems. SF Monod is a particle-tracking code coupled with biodegradation and inorganic reactions that accounts for spatially variable denitrification rates and biomass metabolism, where the finite difference code ParFlow is used to calculate the groundwater flow field used as input. The particle-tracking approach can be used for watershed-scale simulations because relatively low spatial resolution is needed for the finite-difference flow model while still meeting numerical stability requirements. Results for this example application show that higher nitrate concentrations and denitrification rates were found near the streams and septic tanks than in deep groundwater. Near the ground surface, the unsaturated zone had a lower denitrification rate than the saturated zone. Due to long groundwater travel distances and times, denitrification was nearly complete for flowpaths entering deep groundwater, resulting in low (< 1 mg/L) nitrate-N concentrations in that zone. Calculated first-order denitrification rates ranged from 0 to 6 x 10^-4 1/d. Because of the biomass growth and dispersion of the septic effluent, overall denitrification rates increased over time. However, the model predicted that nitrate flux to the stream channel will continue to increase over time because the nitrate flux did not reach steady state by the end of 160 years of simulation time. Multispecies reactive transport results were also compared with a case where nitrate was considered as a conservative tracer. The conservative tracer simulation predicted nitrate concentrations well at locations where nitrate concentrations were low and groundwater travel times were short.