CHEMICAL INDICATORS OF GROUNDWATER-STREAM WATER INTERACTIONS IN A FORESTED, LOWLAND WATERSHED WITHIN THE U.S. ATLANTIC COASTAL PLAIN

There is limited information on the dynamics of groundwater recharge 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 United States. 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 the collection of precipitation, stream water and discrete-depth groundwater samples from water table wells and piezometers installed in transects orthogonal to and along the stream channel. Time series water level and water chemistry data showed clear signals due to seasonal climate trends, individual storm events, and daily evapotranspiration forcing. Concentrations of natural chemical tracers (Na⁺, Ca²⁺, Si, Cl^-, inorganic carbon), as well as water quality indicators (pH, temperature, specific conductance) correlated to fluctuating water table levels in the stream. Preliminary end-member mixing analysis of chemical 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 influence of precipitation and the shallow groundwater on stream flow. 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 stream flow in this watershed. Further work using principal components analysis (PCA) to analyze source and stream data suggested that this model of source contribution to stream flow for Turkey Creek may only be valid during certain hydrologic conditions in the watershed. Ultimately, this work will aid in the development of 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-use change, and resulting changes to the water budget.