SIMULATING NITRATE REMOVAL MECHANISMS IN AN AGRICULTURAL CATCHMENT WITH CONTRASTING NITRATE BASE-FLOW CONCENTRATIONS IN SUBCATCHMENT STREAMS

Groundwater flow paths and the legacy nitrates that they transport through the ecosystem create a lag time between changes in land surface management practices and water quality recovery in coastal tributaries and tidal waters. In addition to delaying surface water quality recovery, the lag time, in combination with a variety of confounding environmental and land use factors, makes it difficult to quantify the catchment-scale benefits of agricultural best management practices (BMPs). This difficulty has motivated a program of small watershed studies by the USGS and USDA whose purpose is to improve catchment-scale understanding of the timing and magnitude of the impact on surface water quality due to agricultural BMPs. In this study we resolve the key components of the nitrogen budget for the Upper Chester (Maryland, USA) catchment, one of the targeted small watersheds upstream of Chesapeake Bay. While subsurface nitrate transport and catchment removal processes have been widely investigated, there have been few fully distributed, three-dimensional modeling studies of nitrate transport and removal in catchments with nitrogen removal rates that are as highly spatially-variable as removal rates in our study site. We use a numerical groundwater simulation to link re-constructed land surface loadings to time-variable stream responses in two subcatchments that have similar land use histories but highly disparate nitrate export signatures. We estimate the impact of soil denitrification and in-stream nitrogen removal as well as the potential influence of retarded nitrate transport, and we show that in spite of spatial and temporal uncertainty in loading, multiple calibration scenarios agree that in-stream nitrate removal efficiencies vary significantly between the two sub-catchments, with one stream removing 60-70% of incoming nitrogen loads and the contrasting stream removing only 15-30%. We further demonstrate the use of nitrate simulations as a form of model validation for a groundwater simulation calibrated against an extensive set of atmospheric-derived environmental tracer data.