TIDES, GROUNDWATER FLOW AND SALT POND DEVELOPMENT AT SLAUGHTER BEACH SALT MARSH, DELAWARE

Salt marshes are subject to various environmental forces that result in salt marsh inundation and degradation. Seasonal and decadal monitoring of salt marsh at Slaughter Beach, DE documented long-term and short-term variations in number and sizes of salt ponds. Over 400 salt ponds ranging in size between 0.5 m² to 0.11 km² were identified on 5.5 km² salt marsh platform. The purpose of this study is to quantify hydrologic conditions and measure groundwater discharge of a salt marsh, particularly the impact of tidal forces on groundwater fluctuation. Four wells with nests of mini-piezometers with ONSET Pressure Transducers were installed along a transect crossing the largest salt pond (0.11 km²) in the study area. Nests of ONSET Pressure Transducers installed at depths of 1 m, 3 m, and 6 m recorded groundwater hydraulic head at five-minute intervals during 3 full tidal cycles (~90 days). High resolution aerial imagery of the studied ponds was collected at peak high tide and low tide using drone.Changes in groundwater elevation were correlated with tidal data recorded by the USGS stream gauge in Cedar Creek. The tidal method for Diffusivity was used for determining aquifer characteristics.Our results document the presence of 2 aquifers; deep (3m) and shallow (1m). Relationship between groundwater elevation and tidal fluctuations is strong in the deep aquifer and weak in the shallow aquifer. Analysis of drone imagery reveal no changes in the shape or size of the pond during 1 tidal cycle. We assume that tides has little impact on the short-term changes of the salt pond morphology. Groundwater elevation decreases in proportion to distance from Cedar Creek and also decreases with depth. We suggest that the deep aquifer is confined. Calculated values of Diffusivity (ranging from 361 m²/day to 1127 m²/day) and Hydraulic Conductivity (0.25 m/day to 1.61 m/day) will be used to calculate ground water discharge and establish boundary conditions for both aquifers in the future. This short-term study has established a baseline for further hydrogeologic investigations that will help to assess salt marsh vulnerability to various scenarios of sea level rise and anthropogenic impact.