HYPORHEIC ZONE HYDROLOGY AND NITROGEN DYNAMICS IN RELATION TO THE STREAMBED TOPOGRAPHY OF A N-RICH STREAM

The influence of riffle-pool units on hyporheic zone hydrology and nit rogen dynamics was investigated in Brougham Creek, a N-rich agricultur al stream in Ontario, Canada. Subsurface hydraulic gradients, differen ces in background stream and groundwater concentrations of conservativ e ions, and the movement of a bromide tracer indicated the downwelling of stream water at the head of riffles and upwelling in riffle-pool t ransitions under base flow conditions. Channel water also flowed later ally into the floodplain at the upstream end of riffles and followed a subsurface concentric flow path for distances of up to 20 m before re turning to the stream at the transition from riffles to pools. Differe nces in observed vs predicted concentrations based on background chlor ide patterns indicated that the hyporheic zone was a sink for nitrate and a source for ammonium. The removal of nitrate in the streambed was confirmed by the loss of nitrate in relation to co-injected bromide i n areas of downwelling stream water in two riffles. Average stream wat er nitrate-N concentrations of 1.0 mg/L were often depleted to <0.005 mg/L near the sediment-water interface. Consequently, an extensive vol ume of the hyporheic zone in the streambed and floodplain had a large unused potential for nitrate removal. Conceptual models based mainly o n studies of streams with low nutrient concentrations have emphasized the extent of surface-subsurface exchanges and water residence times i n the hyporheic zone as important controls on stream nutrient retentio n. In contrast, we suggest that nitrate retention in N-rich streams is influenced more by the size of surface water storage zones which incr ease the residence time of channel water in contact with the major sit es of rapid nitrate depletion adjacent to the sediment-water interface .