Chemical Indicators of Groundwater-Stream Water Interactions in a Forested, Wetland-Dominated Watershed within the U.S. Atlantic Coastal Plain

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 stream flow in a third-order, 7000-ha forested watershed in the Atlantic coastal plain of the U.S. The 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. Time series data for water level and water chemistry showed clear signals due to seasonal climate trends, individual storm events, and daily evapotranspiration forcing.. Concentrations of natural chemical tracers (Na+, Ca2+, SO42-, Cl-, HCO3- and water quality indicators (pH, temperature, specific conductance) were 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 dry 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, associated land development, and resulting changes to the hydrologic cycle in these systems.