The Impact Of Upland Development And Marsh Width On Groundwater Composition In Estuarine Tidal Creeks In The Southeastern Coastal United States

Coastal upland development has been shown to negatively impact surface water quality in tidal creeks along the southeastern coastal United States, but the impact of development on groundwater quality is not well understood. Increases in impervious cover associated with development has the potential to increase groundwater contamination as well as reduce fresh rainwater infiltration into the subsurface, which may decrease discharge of fresh groundwater to the estuary. We hypothesized that groundwater nutrient concentrations and salinity ranges would be higher in developed watersheds than in undeveloped watersheds. Groundwater discharging from coastal uplands often travels through salt marshes before discharging to tidal creeks. Salt marshes export nutrients to tidal creeks, and significant mixing and transformation can occur during transport through the salty, highly-reducing sediments of a salt marsh. We hypothesized that the mixing and reactions in salt marshes may obscure the impacts of development on groundwater composition discharging to the creeks. To test these hypotheses, we sampled groundwater in the upland area and below the creek bank of 15 tidal creeks located within developed and undeveloped watersheds (measured by percent impervious cover) that exhibited a range of marsh widths. Sampling took place over two years, with Year 1 sampling occurring at all 15 creeks during the summer, and with Year 2 sampling occurring at a subset of 6 creeks (chosen based on accessibility) revisited for summer and winter sampling. Samples were analyzed for salinity, dissolved organic carbon, nitrogen and phosphorus concentrations. Overall, significantly higher concentrations of nutrients were found in developed watersheds and lower concentrations of dissolved organic carbon were found in undeveloped watersheds. Concentrations of these constituents in groundwater sampled below the creek bank during Year 2 were often orders of magnitude higher than in groundwater sampled along the upland. No significant relationship was observed between land-use and salinity range. Significant relationships between marsh width and nutrient concentrations emerged at some individual creeks during Year 2 summer sampling. Seasonal differences in creek bank groundwater composition were observed. These differences may be related to lower mean sea levels during the winter season, during which time salt marshes may experience less tidally driven groundwater mixing in the sub-marsh aquifer. Results from this study will be used to improve best management practices of salt marsh tidal creeks along the southeastern coastal United States