SURFACE WATER-GROUNDWATER EXCHANGE DURING EXPERIMENTAL OVERBANK FLOODS IN A SIMULATED RESTORED FLOODPLAIN

Restoring hydrologic connectivity between the channel and floodplain is a common practice in stream and river restoration. Floodplain hydrology and hydrogeology impact biogeochemical processing and potential nutrient removal, yet rigorous field evaluations of surface and groundwater flows during overbank floods are rare. We conducted five sets of experimental floods to mimic floodplain reconnection. Experimental floods entailed pumping stream water onto an existing floodplain swale, and were conducted throughout the year to capture seasonal variation. Each set of experimental floods entailed two replicate floods occurring on successive days to test the effect of varying antecedent moisture conditions. Water levels and specific conductivity were measured in surface water, shallow soils, and deep soils, along with surface flow into and out of the floodplain. Average residence time in the floodplain during the flood experiments decreased as antecedent soil moisture increased. Hydrologic flow mechanisms were spatially heterogeneous in surface water, in groundwater, as well as in exchange between the two. Surface water exhibited ponding/storage in some areas with rapid flow in others. Immediate propagation of hydrostatic pressure into deep soils was suggested at some locations. Preferential groundwater flow was suggested in locations where the pressure and electrical conductivity signals propagated too fast for Darcy flow. Preferential flow was particularly obvious where the pressure signal bypassed an intermediate depth but was observed at a deeper depth. Darcy flow was suggested at locations where the flood pressure signal propagated more slowly, in combination with changes in electrical conductivity. Finally, other areas exhibited no transmission of pressure or conductivity signals, indicating a complete lack of groundwater flow. Antecedent moisture conditions had a pronounced effect on connection between the floodplain surface and deep soils while seasonal variation appeared to affect surface water hydraulics. Understanding the variety of exchange mechanisms and their spatial variability will help understand the observed variability of floodplain impacts on water quality, and ultimately improve the effectiveness of floodplain restoration in reducing excess nutrient in river basins.