USING WATER ISOTOPES AND DISSOLVED ORGANIC MATTER TO DELINEATE GROUNDWATER-SURFACE WATER INTERACTIONS IN LOW-GRADIENT WATERSHEDS

Rates of urbanization and climate change have been rapidly increasing and have a strong potential to affect sensitive coastal watersheds. In the Charleston, South Carolina, USA, urban land use has increased over 250% between 1973 and 1994 and is predicted to increase by another 200% by 2030, which holds implications for the water budget and water quality of this region. Understanding the sources and residence time of water in these lowland watersheds is crucial to assessing current and future impacts of urbanization and climate change. The aim of this study was to profile and delineate sources of water that contribute to stream flow and groundwater in low-gradient coastal watersheds using stable isotopes of water and dissolved organic matter (DOM) as natural chemical tracers. We hypothesized that (i) isotopic analysis would help reveal relative inputs to low-gradient streams from precipitation, soil water, riparian water, and shallow streambed water as well as the unique residence time in each location and (ii) DOM content and chemical composition would also indicate specific sources of water to the streams as a function of soil infiltration volume and rates. In this study, precipitation, surface water, and groundwater samples from water table wells and piezometers were collected from a forested low-gradient watershed near Charleston and were analyzed for stable isotopes using a bench-top water isotope analyzer. DOM was measured using ultraviolet-visible spectroscopy and total organic carbon content. Results indicate a high degree of organic enrichment in surface waters and lower DOM content in deeper groundwater, suggesting removal of DOM by infiltrating groundwater. Preliminary isotopic analysis has indicated isotopically-depleted precipitation as the primary source of most surface water at our test site and a significant level of fractionation and/or mixing in deeper wells. We will also present results of a corollary hypothesis that soil porewater and groundwater will likely exhibit a larger extent of fractionation based on increased residence time.