CHEMICAL INDICATORS OF GROUNDWATER-STREAM WATER INTERACTIONS IN A FORESTED, WETLAND-DOMINATED WATERSHED

The dynamics of groundwater recharge have been well-documented in a variety of landscapes and soil types yet there is limited information in lowland watersheds. We studied groundwater recharge and its relationship to streamflow in a third-order, 7000-ha forested watershed in the Atlantic coastal plain of the U.S. Our objective was to delineate the different sources of water moving into the riparian corridor and stream channel of Turkey Creek, a small ephemeral blackwater stream in a rural area near Charleston, South Carolina. Our methods included collecting stream water and discrete-depth groundwater samples from piezometers installed in transects orthogonal to and along the stream channel. Water levels and water chemistry showed clear temporal signals due to seasonal climate trends, daily evapotranspiration forcing, as well as individual storm events. Concentrations of natural chemical tracers (Na, Ca, SO4, Cl, etc.) were indirectly related to fluctuating water table levels in the stream. End member mixing analysis of the chemical tracer data from stream, groundwater, and precipitation samples indicated that precipitation and shallow groundwater from the upper meter of streambed sediment played a significant role in stream discharge. However, seasonal variation in precipitation patterns affected the relative distribution of precipitation and the shallow groundwater. As expected, during drought periods, groundwater was a more significant contributor to streamflow than during relatively wet periods. Deeper groundwater seemed to play a relatively minor role to streamflow in this watershed. Ultimately, the aim of this study is to develop a geochemically constrained groundwater-surface water model for lowland watersheds for this and similar regions that are under increasing threat from burgeoning population and associated land development.