CHANGES IN HYPORHEIC VOLUME AS A FUNCTION OF AQUIFER VARIABILITY

Nitrate contamination of aquifers and surface water bodies from agricultural fields remains a challenge for managing ecosystem quality. Utilizing the natural attenuation capacity of the hyporheic zone is one strategy that can reduce the concentration of nitrate, but the complex physical dynamics of groundwater surface water interaction provide a barrier to effective quantification. One issue is predicting the changes in hyporheic zone volume and geometry as a function of changing hydrologic conditions in the surrounding basin. To address this issue, a third order reach along Elton Creek near Freedom, New York and the surrounding aquifer was observed via tracer tests, groundwater wells, hyporheic zone wells, and stream gaging from May-September of 2011-2013. A steady decline in the water table head over the summer led to an overall expansion of the hyporheic zone on the month scale. Storm events provided additional expansion of the hyporheic zone as the surface flood wave approached, but the hyporheic zone quickly contracted afterwards due to a recharge pulse in the aquifer. Along the stream reach, tracer tests revealed the hyporheic zone expanded and contracted asymmetrically as a function of streambed topography and aquifer conditions. Riffle-pool reaches were observed to have larger short term storage zones as compared to planar beds, but later in the season drops in the water table led to large expansions in short term storage in planar beds and very small expansions in riffle-pool beds. With these data a representative numerical model was developed using the Navier-Stokes and Brinkman-Forchheimer equations. Expansion and contraction of the hyporheic zone was tested as a function of different groundwater and surface water hydrograph shapes over a six-month period with pool-riffle and planar bed morphologies.