HYDROLOGICAL AND BIOGEOCHEMICAL DYNAMICS OF TWO URBAN RIPARIAN CORRIDORS IN GREENVILLE, SOUTH CAROLINA

Riparian zones are important biogeochemical and hydrological hotspots for nutrient cycling and groundwater-surface water interactions. Urbanization can severely alter the geomorphology and hydrology of stream systems, which largely influence riparian processes. In this study, subsurface hydrology and surface water-groundwater mixing were observed in two urban riparian corridors in Greenville, South Carolina in an effort to characterize and better understand urban riparian hydrological and biogeochemical dynamics. Changes in water table levels and surface/subsurface chemistry were recorded and observed along riparian transects from June through December 2010 in two headwater streams in the Rocky Creek watershed of the Enoree River Basin. Transect wells were equipped with water level recorders for continuous stage monitoring, and water samples of surface water, hyporheic water, groundwater, soil moisture, and throughfall were routinely collected for basic chemical analysis. The well data revealed a strong evapotranspirative diurnal fluctuation in the water table throughout the summer, a diminishing lateral hydraulic gradient away from the stream during prolonged dry periods, and temporary reversals in hydraulic gradient during storm events that changed the stream from a gaining to a losing reach. The water chemistry along each transect varied with distance from the stream, but the transect chemistry was consistent and the spatial trends relatively robust over the period of record despite the dynamic hydrologic regime. At site RC27A, groundwater nitrate concentrations were more dilute than the stream and hyporheic samples, and contained little sulfate and dissolved organic carbon (DOC). Groundwater samples at RC33, by contrast, revealed unexpectedly elevated concentrations of nitrate, sulfate, chloride, and DOC, presumably due to a nearby leaky sewer line. In spite of the differences in groundwater nutrient loading, both sites exhibited low to undetectable concentrations of phosphate and ammonium. Results of this study highlight the spatial and temporal chemical and hydrologic variability of urban riparian systems and provide insights into the physical and chemical dynamics across the groundwater – surface water interface.