Groundwater-Surface Water Interactions in a Lowland Watershed: Evaluation of Source Contribution
Garrett, Catherine Guinn
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The U.S. southeastern, lower coastal plain is a region undergoing rapid urbanization as a result of expansive population growth. Land use change has been shown to affect watershed hydrology by dramatically altering stream flow and ultimately, impairing water quality and ecologic health. However, because very few long-term studies have focused on groundwater-surface water interactions in lowland watersheds, it is difficult to establish what the impact of development might be in the coastal plain region. The objectives of this study were: (1) to use end-member mixing analysis (EMMA) to identify the hydrologic processes that influence coastal, lowland watersheds as well as; (2) validate a conceptual model of source contribution to stream flow. Hydrological monitoring and water sample collection from groundwater, soil water, precipitation, and stream sites was conducted over a two-year period at a watershed located in the Francis Marion National Forest near Charleston, SC (Upper Turkey Creek [UTC] and Watershed 80 [WS-80]). All water samples collected were analyzed for major anions and cations to identify potential natural tracers present. Stream flow at UTC and WS-80 was ephemeral and highly dependent on evapotranspiration rates and rainfall amount and intensity. EMMA at UTC using Cl--, Mg2+, Ca2+, and Si as tracer concentrations helped identify hyporheic groundwater, shallow groundwater, soil water, and precipitation as potential sources to stream flow. Hydrograph separation with EMMA for a series of storm events suggested that source contribution to stream flow varied as a function of antecedent moisture condition; stream flow during dry conditions was dominated by both soil water (up to 65%) and precipitation (up to 66%) contributions whereas stream flow during wet conditions was dominated by precipitation (as much as 70%). The results of this study helped refine a conceptual model that can be used to examine potential impacts of development on important hydrologic processes (groundwater recharge, interflow, runoff, etc.) that influence stream flow in these lowland watersheds.