USING END-MEMBER MIXING ANALYSIS TO UNDERSTAND STORM-EVENT STREAMFLOW IN LOWLAND WATERSHEDS OF COASTAL SOUTH CAROLINA

Land use and land cover changes have been impacting the hydrologic cycle of coastal plain watersheds of the southeastern United States. To better understand effects of land use change on these watersheds, it is critical to understand natural groundwater and surface-water interactions. End-member mixing analysis (EMMA) is a hydrogeochemical, multiple-tracer approach for assessing the source contributions to stream flow. EMMA assumes that stream chemistry changes rapidly, but end-member chemistry remains relatively constant. This assumption is suspect for some end-members, particularly given certain coastal plain characteristics, such as shallow water table and minimal hydraulic gradient. The objectives of this study were: 1) to assess the end-member chemical conditions necessary for applying EMMA in the coastal plain, and 2) to apply EMMA to two first-order, forested lowland watersheds in coastal South Carolina.

Watershed-80 (WS-80), in the US Forest Service’s Santee Experimental Forest near Charleston, SC, drains 200 hectares dominated by clayey soils. Upper Debidue Creek (UDC), near Clemson University’s Baruch Institute in Georgetown, SC, drains 162 hectares of mostly sandy soils. Samples from water-table wells, piezometers, lysimeters, and rain gauges at these two sites were analyzed for major cation and anion concentrations, and results were used to construct EMMA models for each site.

Preliminary findings suggested that deep groundwater was less susceptible to chemical fluctuation than shallow and riparian groundwaters, which are subject to effects from storm events and seasonal changes. Rainwater ion chemistry varied widely, in response to meteorological factors. These findings support the use of seasonal and/or event-specific tracer concentrations for end-member chemical characterization. The EMMA models will be used to compare groundwater and stream flow response to storm events at WS-80 and UDC. We hypothesize that the clayey soils at WS-80 will cause faster runoff response to storm events, as indicated by decreased groundwater and increased precipitation contributions to streamflow, than at UDC. In a time of rapid land-use change, this study contributes to a growing understanding of groundwater and surface-water interactions in minimally disturbed, forested, lowland watersheds.