CHARACTERIZING COASTAL MARSH GROUNDWATER HYDROLOGY WITH MULTICHANNEL ELECTRICAL RESISTIVITY TOMOGRAPHY

Subsurface waters are highly enriched in nutrients compared to surface waters in the coastal salt marsh environment and may, therefore, have a large effect on biogeochemical processes. There have been numerous studies examining marsh platform hydrology, however, there is a lack of information on the hyporheic zone within tidal creeks in coastal salt marsh environments. Hyporheic exchange has been studied in various non-tidal freshwater systems and verified as a significant pathway for solute transport in typical stream settings. The total volume of water cycled through salt marsh tidal creek hyporheic zones could prove to be a significant component of the overall marsh water budget, however, it is difficult to quantify this process. Electrical resistivity tomography (ERT) can be used to measure the changes and composition in porewater fluids, making it an ideal technique to visualize coastal groundwater dynamics. We present time series ERT data showing the development of a hyporheic zone in marsh sediments beneath a typical tidal creek at Waties Island, SC. The formation and dispersal of this zone depends upon tidal creek water level, suggesting very short residence times for pore water in near-creek (1 – 2 meters) sediments. Changes in vertical and horizontal cross-sectional dimensions of the zone through time suggest that shallow marsh sediments surrounding a tidal creek (within 1- 2 meters) are isotropic in terms of tidally pumped pore water infiltration and drainage. Salt buildup and flushing in shallow sediments surrounding tidal creeks is potentially a result of the spatial extent of the tidal hyporheic zone throughout the fortnightly cycle. Ongoing efforts to groundtruth ERT readings will hopefully lead to development of time series resistivity measurements as a tool to estimate pore water flow velocity.