APPLICATION OF GEOCHEMICAL END-MEMBER MIXING ANALYSIS TO DELINEATE WATER SOURCES IN A LOWLAND WATERSHED

Water flux in lowland watersheds is influenced by streams, groundwater, soil type, topography, precipitation, evapotranspiration, and runoff. However, the relative influence of each of these factors is usually hard to ascertain based on hydrology alone. The overall goal of this research is to understand watershed dynamics in response to storm events as a function of soil type, depth to water table, and topography in the 7000-hectare Turkey Creek Watershed located in the Francis Marion National Forest near Charleston, SC. The soils in this watershed range from moderately well drained to poorly drained and are influenced by seasonally shallow water tables. This watershed is also characterized by very flat slopes with an overall elevation range of 4-14 m above sea level. We installed a series of water table wells, piezometers, and stream level gauges in the streambed, at the margin areas adjacent to the riparian zone, and in the upland areas. Preliminary chemical analysis of water samples from the wells, the streams, and rainfall indicate that of the major ions analyzed, Na, Ca, Cl, and SO4 dominated the system. However, a substantial charge imbalance existed within the deeper groundwater samples compared with shallow groundwater and surface water samples, possibly due to dissolved carbonates which were not included in the preliminary chemical analyses. The Santee limestone aquifer, a porous fossiliferous limestone, is believed to geochemically influence the groundwater in the watershed and hence may contribute significantly to the groundwater geochemistry. This analysis will be included in the ongoing research to decipher the mixing relationships with various sources of water to the lowland watershed. Each of these sources is referred to as a specific end-member and is expected to have unique water chemistry. We will present results of the end-member mixing analysis to discern the relative contribution of the sources of streamflow water.