Impacts of Coastal Development on Groundwater Composition in Tidal Creek Basins

Alicia Wilson, University of South Carolina

21 February 2020

Salt marshes serve as zones of groundwater mixing and reaction between freshwater uplands and estuaries. We asked whether nutrient enrichment from upland development can be transmitted through salt marshes, or whether salt marshes can buffer estuarine waters from coastal development. Groundwater was sampled from fifteen tidal creek basins in South Carolina to test for differences associated with development and marsh width. Groundwater samples from near creekbanks and below freshwater uplands were analyzed for salinity, dissolved organic and inorganic nitrogen and phosphorus, and dissolved organic carbon. In aggregate, including creekbank and upland samples, groundwater concentrations of total dissolved phosphorus were significantly higher in developed watersheds than in undeveloped watersheds. Spatial differences in nitrogen were complex, depending on nitrogen species and sampling season. In fresh groundwater below the uplands, concentrations of dissolved organic carbon were lower in developed watersheds than in undeveloped watersheds, and upland concentrations were lower in the winter than in the summer. These results are consistent with the hypothesis that development  can affect groundwater compositions in coastal systems and therefore may affect coastal nutrient and carbon fluxes. However, results also revealed significant linear correlations between marsh width, salinity, and nutrient concentrations in some marshes. This suggests that salt marshes can act as buffers for development, and specifically suggests that the buffering capacity of salt marshes increases with width. Narrow or trenched salt marshes are far less likely to be effective buffers.