HYDROGEOLOGIC MAPPING OF PLEISTOCENE- AND HOLOCENE-AGE DEPOSITS IN SOUTHEASTERN PENNSYLVANIA: ASCERTAINING THE GROUNDWATER FLOW REGIME FOR AN IMPACTED 100-YEAR OLD PETROLEUM REFINERY

Intensive lithological sampling using split spoon, Geoprobe, and Rotosonic techniques, grain size analyses, and 14C dating were performed between 1996-2005 to determine the age and spatial distribution of lithologically similar Pleistocene- and Holocene-age deposits underlying a heavily industrialized and impacted 100-year old petroleum refinery in Trainer/Marcus Hook, Delaware County, Pennsylvania, in the Atlantic Coastal Plain Province. Numerous 14C dates in excess of >37 ka indicated that the subcropping terraces at the site should be considered part of the Sangamon-age Cape May Formation. The presence of lower terrace deposits was revealed by two 14C dates of 31,380 years B.P. (+4,530/-2,880) and 29,330±600 years B.P., obtained directly adjacent to a prominent paleochannel that was deeply entrenched through the Cape May Formation terrace during an extended period of incision during the late Pleistocene. Glacial-age tributary alluvium was found as deep as –45 ft/-13.72 m msl in basal paleochannel fill; these are the only deposits at the site that may be contemporaneous with the late Wisconsin-age (approximately 15-20 ka) Trenton Gravel. The deposition of these permeable Pleistocene-age deposits was followed by off-lap deposition of low permeability Holocene-age alluvial silt, clayey silt, silty clay, and peat that began approximately Cal B.P. 5,920 to 5,620 (5,030±60 years B.P. conventional age) and continued into the modern era until the site was developed. Highly detailed cross-sections and visualizations utilizing nearly 1,230 individual boring logs were produced to develop a precise “solids” model for use in realistically representing site-wide groundwater recharge and flow conditions. The most significant project discoveries included identifying a paleochannel that bisected the site into two separate hydrogeological regimes, mapping a complex industrial fill history, and resolution of a long-standing discrepancy in the regional stratigraphic nomenclature.