Using Stable Water Isotopes and Dissolved Organic Matter to Delineate Groundwater-Surface Water Interactions in Low-Gradient Watersheds

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Nickolas, Lydia Beck
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The southeastern coast is among the fastest growing communities in the United States. From 2010-2011, Charleston, South Carolina experienced a 2.6% increase in population growth, ranking it as the 8th fastest growing metropolitan area in the US. The resulting urbanization and increased land use has led to significant alteration of coastal watersheds and degradation of associated fresh and estuarine water body health. Compounding this problem are the many effects of global change-related extreme weather patterns - e.g., large floods caused by moderate storms - that could have grave consequences for the ecology and people that live here. The overarching goal of our research project is to understand how land-use change and climate change related precipitation in coastal environments affects coastal watersheds. Understanding the sources and residence time of water in these watersheds is crucial to assessing the future impacts of global change. The main objective of this study was to delineate sources of water to streams in low-gradient coastal watersheds using stable isotopes of water and dissolved organic matter (DOM). Precipitation, surface water, and groundwater samples were collected from two sites and analyzed for δ18O and δ2H using a stable water isotope analyzer. End-member mixing analysis was performed to estimate the source contributions to Turkey Creek, a third order stream (draining WS-80 and UTC), which serves as a model for streams found in low-gradient watersheds of the southeastern coast. DOM in these water samples was analyzed using a fluorescence spectrophotometer (excited at a wavelength of 355 nm) and a total organic carbon analyzer. Isotopic analysis results indicate that δ18O signatures of precipitation (representing the most depleted source) experience some seasonal variability (depending on storm origin and timing), surface water (relatively more enriched source) is typically depleted in 18O during the cooler months, and groundwater δ18O values follow seasonal trends of depletion in the cold months and enrichment in the warm months, though the individual signature is largely related to depth. End-member mixing analysis suggests that 40.8% of the contribution to Turkey Creek originates from groundwater, while 59.2% originates from precipitation. DOM analysis demonstrated variable differences between each site, but the methodology proved somewhat flawed. These results suggest complex dynamics in these watersheds and serve to reinforce the previous conceptual model, which indicates a predominance of precipitation input to stream flow.
Stable water isotopes, DOM, coastal watersheds