Application of Geochemical End-Member Mixing Analysis to Delineate Water Sources in a Lowland Watershed

Water flux in lowland watersheds are influenced by streams, groundwater, soil type, topography, precipitation, evapotranspiration, etc. 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 lowland watersheds in response to storm events as a function of soil type, depth to water table, and topography. We used an end-member mixing technique involving geochemical data to delineate the influence of various sources of water in the Turkey Creek Watershed, which is 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 high water tables. This watershed is also characterized by very flat slopes and overall elevation of 4-14 m above sea level. We installed a series of water table wells and piezometers in the stream bed, at the margin areas adjacent to the riparian zone, and in the upland areas away from the riparian zone. Water samples from the wells, the streams, and rainfall were collected on a regular basis and were analyzed for major cations and anions. Preliminary rainfall data indicate that major ion concentrations were substantially lower than those measured in the well and stream water samples. Additionally, some well water contained higher levels of Ca ²⁺, Na ⁺, SO ₄ ^2-, and Cl ^- compared with other wells possibly due to influence of a deeper groundwater system of higher salinity. We hypothesize that storm events produce flushing of shallow groundwater to the streams, but the deeper groundwater system is not connected to the stream. Using these data we are developing a geochemically constrained water budget model that could be applicable for such lowland watersheds to complement pure hydrological models.