Influence of stream-groundwater interactions in the streambed sediments on NO3- flux to a low-relief coastal stream

Water-saturated, organic-rich sediments immediately surrounding a stream channel can provide a protective buffer between streams and adjacent land-based activities by removing plant nutrients from shallow groundwater flowing through them, but the hydrological factors that influence the effectiveness of nitrate removal are not well characterized. A two-dimensional, fully distributed, variably saturated flow and transport model was evaluated for its success in using mechanisms of biological reaction in streambed sediments to quantify nitrate flux into the stream under base flow conditions. The model was used to interpret the observed hydrological dynamics during storms at Cobb Mill Creek, Virginia. During base flow conditions, relatively deep groundwater flow paths carrying water containing high nitrate concentrations discharged through the streambed sediments, and high denitrification rates were observed along with a substantial reduction in the nitrate concentration. During storm events, reduced discharge of groundwater in the face of a diminished hydraulic gradient during passage of a flood wave led to longer residence times for water in the biologically active sediments underlying the stream channel, thus providing an opportunity for enhanced denitrification to further reduce nitrate loads to the stream. We conclude that in cases of low-relief streams with substantial hillslopes adjacent to the stream combined with transmissive sediments, storm events can actually contribute to enhanced removal of nitrate locally (at the hillslope scale) as opposed to a simple lowering of concentration due to dilution.