SALINITY AND GROUNDWATER FLOW BENEATH A FRINGING SALT MARSH: ELECTRICAL RESISTIVITY TOMOGRAPHY

We investigated the subsurface hydraulic conditions and spatial salinity variations in a tidal salt marsh fringing an upland forest at North Inlet, SC. Forested uplands can discharge large volumes of fresh groundwater into neighboring salt marsh systems. The volume of freshwater being introduced from the mainland is dependent on tidal cycles, precipitation events, and evapotranspiration. We used electrical resistivity tomography (ERT) surveys, in-situ salinity sampling, and hydraulic head data to understand the interactions of the fresh upland groundwater and the saline porewater of the salt marsh. The study site consists of a transect that extends 108 meters from a forested upland, across a low-lying salt marsh, and terminates at a tributary named Crabhaul Creek. The ERT data was corroborated with salinity samples from the groundwater monitoring wells at the site. Pressure transducers were installed in well nests located in the upland, the mid-marsh, and the creekbank to gain insight into the direction of groundwater flow. Saline porewater dominates the upper two meters of the marsh surface and is a result of the downward percolation of seawater during tidal inundation. Generally, a freshwater plume (~ 5 m in vertical length) is migrating horizontally from the upland, along a stratigraphic confining clay layer (at ~6 m vertical depth), and is discharged into Crabhaul Creek. An upward hydraulic flow anomaly was discovered in the mid-marsh, approximately 36 to 51 meters from the beginning of the transect, and suggests that the underlying brackish water is discharging to the surface during low tide. These preliminary results point to a complex relationship between tidal fluctuation and upland groundwater discharge. These results will be used to constrain numerical models of the marsh in order to simulate submarine groundwater discharge.