USING HYDROSTRATIGRAPHIC CORRELATION OF SOIL CORES TO UNDERSTAND EPHEMERAL HYDROLOGY OF RIDGETOP WETLANDS

Wetlands on ridgetops of the Appalachian mountains have been found to interact with perched groundwater although little is known about the role groundwater plays in their ephemeral hydrology. In this research, we used a percussion drilling technique to create a subsurface map that illustrates the geometry of the groundwater body and geologic factors that control these wetlands. Soil cores were taken along a transect of the wetland in order to track how the soil layers changed throughout the subsurface. Soil layers were determined both from examining the cores as well as counting how many hammer blows it took to advance the drill through the ground. The cores exhibited gradual boundaries between a permeable soil layer, an impermeable clay layer, and finally a heavily weathered shale. When correlated, maps of these wetlands show that the weathered shale overlays a bedrock depression that creates a subsurface trap for groundwater. Groundwater accumulates in the more permeable silt loam that developed above the clay and connects to surface water. We interpret the gradual boundary between silt loam, clay, and weathered shale to indicate a residual soil, which is significant because it means these ridgetop systems originate through local geology as opposed to other proposed sources. By comparison, other explored ridges have a thin soil layer derived from sandstone, lacking the impermeable layer to collect groundwater and create wetlands. Mapping water levels onto the cross-sections showed that groundwater bodies routinely overflow the subsurface depression and flow towards lowlands, depending on antecedent soil conditions.