The Effects of Mean Sea Level and Tidal Amplitude On Ground Water Discharge and Pore-Water Salinity Distribution Along A Forest/Marsh Boundary, North Inlet, SC

Advective groundwater flow in salt marshes is an important mechanism through which nutrients are exported to adjacent coastal waters. Groundwater flow also influences the distribution of pore-water salinity in the subsurface marsh, which affects botanical zonation, nutrient transport, and primary productivity. Recent idealized marsh island simulations have suggested that increases in tidal amplitude result in increased groundwater discharge, and that the elevation of MWL relative to the marsh platform is inversely related to groundwater discharge. These simulations were only representative of marsh islands (as opposed to forest-marsh boundaries) and relied on simulated tides and MWL position. The results were not verified by observational data. This study utilized tidal records and hydraulic head records to calculate and compare groundwater discharge along two marsh environments: 1) a fringing marsh boundary that was influenced by a large freshwater lens and 2) a marsh island that was less significantly influenced by a freshwater lens. Electrical resistivity surveys were conducted to image seasonal pore-water salinity distribution. The discharge trends were then correlated to trends in MWL and subsurface salinity. Our results indicated that discharge from the mid and low-lying marsh to Crabhaul Creek was dependent on the postion of MWL. Observations showed increases in groundwater discharge during periods of low MWL and decreases during during periods of high MWL. In the high marsh, the occurrence and magnitude of discharge or recharge was dependent on precipitation, tidal amplitude and MWL. The electrical resistivity surveys depicted two distinct salinity zones, neither of which displayed any correlation to MWL or tidal amplitude. The first was a shallow tidally influenced zone containing saline to brackish pore-water which remained relatively constant throughout the year. The second was a deeper freshwater flow zone that displayed variable values of resistivity. These results demonstrated the importance of MWL and tidal amplitude on discharge and hence nutrient export from salt marshes. It can be inferred that long-term sea level rise (at a rate that exceeds sediment accretion) will significantly decrease nutrient export from salt marshes