HYDROGEOLOGIC SIMULATION OF GREAT MARSH, CHESTER COUNTY, PENNSYLVANIA USING GIS AND MODFLOW 6 TO UNDERSTAND THE WETLAND'S PROLONGED EXISTENCE

Great Marsh is a 1,011 ha wetland located in the Piedmont Upland Section of Chester County, Pennsylvania. Great Marsh is significant for its preservation of 16.4 ka tundra pollen, documenting climate change from a Pleistocene periglacial tundra environment into an upland deciduous wetland through the Holocene. Understanding the hydrogeologic setting of Great Marsh is fundamental for interpreting hydrologic conditions of the geologic past and predicting the hydraulic influence of future climatic conditions.

A groundwater flow model was constructed using MODFLOW 6 to simulate the Great Marsh Watershed. The watershed has an area of 2,220 ha, drained by Marsh Creek. Model topography was established using a LiDAR digital elevation model (DEM) with 10 m spacing. Geologic units include graphitic felsic gneiss and quartz monzonite felsic gneiss of the Mesoproterozoic Baltimore Gneiss Formation, saprolite, periglacial colluvium, and Pleistocene and Holocene alluvium. A precipitation recharge rate of 0.302 m/yr was used in the model, determined from regional hydrograph baseflow separation. Model discharge was simulated using the MODFLOW river package based on stream and wetland locations draped over the DEM using GIS. For the purposes of this investigation, the model simulation was performed under quasi steady-state conditions.

Model-simulated hydraulic heads closely matched the locations of mapped wetlands. The model confirmed previous studies that concluded the wetlands resulted from a lithologic transition from fractured graphitic gneiss to weathering-resistant quartz monzonite felsic gneiss near the base of the watershed. Fractures, faults, and lithologic discontinuities indicated by lineation analysis using aerial image photogrammetry and LiDAR hillshade models may explain the location of springs and preferential groundwater discharge locations within the watershed. Furthermore, we conclude that the structural geometry of the Great Marsh Watershed produces a climate-resilient wetland that is relatively insensitive to changes in recharge, which suggests that future climate change is unlikely to significantly alter the hydrologic conditions of Great Marsh.