Hydrogeologic and Flow Controls on Solute Transport in Streams

In gaining streams, water chemistry will be a mixture of groundwater, often with relatively high salinity, and surface runoff with lower salinity. When surface runoff generates a flow event, this stream water salinity will usually decline to a smaller value and then increase back to the original one. The variation in stream water salinity is dependent on the permeability of the streambed which controls the degree of bank storage and subsequent return flow. Prior studies have reported the old saline stream water is often pushed ahead by the pulse of new flow event water to cause a lag effect between the chemograph and the hydrograph. Exploring how flow patterns and streambed permeability influence the variation in the water salinity and the lag effect may assist us in better understanding the complex stream-aquifer interaction and better performance of hydrograph separation. We simulate water flow and solute transport in a stream interacting with the adjacent groundwater system using commonly used HydroGeoSphere – a coupled surface water groundwater simulator. Different flow events (duration and amplitude) are carried out in the stream with various streambed permeability values. Preliminary results show that when either flow amplitude or duration is small, the minimum concentration observed during the flow event does not vary significantly at the downstream boundary irrespective of streambed permeability. If flow duration is smaller than a critical value (approximately 0.5 d in this case) with fixed amplitude, the minimum concentration at the downstream boundary decreases with the increase in the K value of the streambed due to bank storage. Interestingly, the minimum concentration increases back again with further increase in K because flow intensity is greatly reduced at the upstream through stronger bank storage which results in little stream flow downstream.