BEDROCK FRACTURES CONTROL GROUNDWATER FLOW AND WETLAND INITIATION IN METAMORPHIC TERRANE AT GREAT MARSH, SOUTHEASTERN PENNSYLVANIA

Great Marsh, an extensive (3 km²) remnant of pre-colonial valley bottom wetlands of southeastern Pennsylvania, is a rare, critical freshwater ecosystem with diverse plant and animal habitats that has existed continuously for ~11,200 years. As one of Pennsylvania’s few remaining examples of a relatively unaltered pre-historic valley bottom wetland ecosystem, understanding the origins of this landscape is highly relevant to modern wetland and floodplain restoration. One hypothesis is that the secondary porosity of highly fractured bedrock underlying and surrounding the marsh yields a high groundwater flow rate. The bedrock is Precambrian gneiss, which generally is not thought to yield high rates of groundwater flow; however, prominent springs do exist within the marsh. Therefore, we investigate bedrock composition, fracture spacing and orientation, and groundwater springs in the Great Marsh area to evaluate the origin and occurrence of groundwater that has supported a marsh throughout the Holocene. Bedrock composition was determined from field mapping and analysis of thin sections with a petrographic microscope. Bedrock fractures within an 80-km radius of the marsh were mapped from satellite imagery and aerial photographs. Fracture orientations (strike and dip) were determined in the field with a Brunton compass at multiple locations along road cuts. We installed ten HOBO temperature loggers in July 2024 to collect temperature data continuously at 15-minute intervals at springs and small streams throughout the marsh. We augment these data with nearby air temperature monitoring stations and continuous water temperature from Mayfly sensors installed and operated by the Great Marsh Institute; some of these sensors have been operating for several years. Groundwater springs at the marsh have consistently low (~11 °C), near-constant temperatures throughout the year. In contrast, water temperatures in small streams and shallow ponds throughout the marsh demonstrate different thermal sensitivity to fluctuating air temperatures, suggesting that some have greater hyporheic exchange with shallow groundwater. Our observation is that the water that supports the marsh ecosystem is associated with springs emitted along prominent bedrock fractures.