SURFACE-WATER/GROUNDWATER INTERACTION IN A FLOODPLAIN ENVIRONMENT IN THE PIEDMONT PHYSIOGRAPHIC PROVINCE, GEORGIA, USA

Stream-groundwater interaction in the Piedmont physiographic province of southeastern North America is complex and not well documented. Major streams commonly occupy floodplains composed of heterogeneous alluvium underlain by saprolite and/or fractured crystalline bedrock. Understanding and quantifying details of interaction in this complex setting is problematic. This research investigates the hypothesis that there is a close and dynamic link between groundwater and surface water in the Piedmont, but excepting baseflow to streams, pressure effects dominate surface-water/groundwater interaction. Consequently, bank storage during rising stream stage involves minimal infiltration of stream water. Furthermore, we hypothesize that widely observed, rapid hydraulic head fluctuations in bedrock, saprolite and alluvium commonly result from pressure pulses caused by stream stage fluctuations. At a study site in the Georgia Piedmont river stage and hydraulic head in adjacent alluvium, saprolite, and bedrock respond quickly to rainfall events. River stage and hydraulic head in adjacent sand-dominated alluvium rise as much as two meters within 24 to 36 hours of storm onset. River stage and hydraulic head decline slightly more slowly to pre-storm levels after passage of storm runoff. Numerical modeling suggests that river stage is the primary control of rapid groundwater hydraulic head fluctuations in alluvium, saprolite and bedrock. However, complex floodplain stratigraphy and topography result in a complex groundwater response. Groundwater response appears to be mediated by elastic storativity, even in sand-dominated, unconfined strata immediately adjacent to the river channel. An elastic response is also consistent with hydrochemical and isotopic data from the site, which detect no infiltration of river water even when hydraulic gradient is temporarily directed from the river into adjacent alluvium.