Effect of freshets on the flux of groundwater nitrate through streambed sediments

The passage of a flood wave over sandy stream sediments can cause changes in the discharge of groundwater to the stream that range from a transient reduction in the discharge rate to a reversal of flow and temporary storage of stream water in the stream sediments. These phenomena were examined in laboratory studies with an intact core and by numerical modeling to assess the effect of both physical and biological processes on the nitrate flux from nitrate-contaminated groundwater to a stream. The balance between the rates of groundwater flow through organic-rich sediments and of denitrification ultimately determines the efflux of nitrate to the stream. The transient impact on nitrate loading suggested by results from the numerical model indicates that high flood stage and low-sediment hydraulic diffusivity can result in excess nitrate loading to surface water following storm events, whereas large-sediment hydraulic diffusivity, regardless of the size of stream stage changes, facilitates greater removal of NO3- by denitrification by allowing for a longer residence time of water in the subsurface.