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Geohydrology of Southeastern Pennsylvania

November 1, 2002

Rapid population growth in southeastern Pennsylvania has increased the demand for ground water. In an effort to address the increased ground-water needs, a ground-water investigation in a 5,200-square-mile area of southeastern Pennsylvania was initiated. Information on the geohydrologic system of the area and the water-bearing capabilities of 51 geohydrologic units in six physiographic provinces or sections (Coastal Plain, Piedmont Upland, Piedmont Lowland, Gettysburg-Newark Lowland, South Mountain, and Reading Prong) has been summarized. Also included are statistical summaries by geohydrologic unit for well construction and discharge data (according to water use), as well as inorganic and radiochemical ground-water-quality data.

Characteristics of the ground-water-flow system in the study area, as well as aquifer hydrologic properties, are related to geology, but can be modified by human development. Ground-water flow in the Coastal Plain Physiographic Province, is through intergranular or primary openings under either unconfined or confined aquifer conditions. Historically, ground-water flowed toward the Delaware and Schuylkill Rivers, but the original flow paths and water quality have been altered significantly by urbanization. In igneous and metamorphic rocks (Piedmont Upland, South Mountain, and Reading Prong), ground-water flows through a network of interconnected secondary openings (fractures, joints, cleavage planes). Ground water in the carbonate rocks (Piedmont Lowland) also flows through a network of secondary openings, but these openings have been enlarged by solution. In the Triassic sedimentary rocks (Gettysburg-Newark Lowland), thin tabular aquifers are separated by much thicker, strata-bound aquitards. The fractured Triassic bedrock forms a very complex, anisotropic, and heterogeneous aquifer with horizontal permeability much greater than vertical permeability.

In general, ground-water flow in southeastern Pennsylvania takes place within local flow systems that discharge within days or weeks to adjacent stream valleys or surface-water bodies. Intermediate (South Mountain) and regional (Gettysburg-Newark Lowland) scale systems, however, in which residence times have been measured in months or years discharge to major streams or rivers that are located in different physiographic provinces or sections or tens of miles distant.

Well depths, casing lengths, reported yields, and specific capacities can vary significantly by geohydrologic unit, use of well, and topographic setting. Wells drilled in the Weverton and Loudon Formations, undivided, and the Montalto Quartzite Member (South Mountain) have median well and casing lengths of 374 and 130 feet, respectively, significantly greater than in almost every other geohydrologic unit in the study area. Wells drilled in the Peach Bottom Slate and Cardiff Conglomerate, undivided (Piedmont Upland) are typically shallow, with a median well depth of 90 feet. Wells in the Marburg Schist (Piedmont Upland) have the lowest median casing length—5.5 feet. Wells in the Stonehenge Formation (Piedmont Lowland), the most productive unit in the study area, have a median reported yield of 200 gallons per minute and a median specific capacity of 27 gallons per minute per foot. The Cocalico Formation (Piedmont Lowland) is the least productive unit with a median reported well yield of 2.5 gallons per minute and a median specific capacity of 0.01 gallons per minute per foot. In general, high-demand wells are significantly deeper, use significantly more casing, and have significantly greater yields than domestic wells drilled in the same unit. Commonly, wells drilled in valleys will have greater reported yields and specific capacities than wells drilled in the same unit on slopes or hilltops.

Except where adversely affected by human activities, the quality of ground water in southeastern Pennsylvania is suitable for most purposes. Yet several water-quality criteria are exceeded in many wells throughout the area. Water from 51 percent of 2,075 wells sampled had a pH higher or lower than the range specified in the U.S. Environmental Protection Agency (USEPA) secondary maximum contaminant level (SMCL). Of water samples analyzed, about 1 percent of 1,623 wells contained concentrations of chloride and 27 percent of 1,624 wells sampled contained concentrations of iron that exceeded the USEPA SMCL. Twenty-seven percent of 1,397 wells sampled contained water with manganese concentrations greater than the USEPA SMCL. Sulfate concentrations in the water of 3 percent of 1,699 wells sampled and total dissolved solids in the water from 10 percent of 1,590 wells sampled exceeded the USEPA SMCL. Concentrations of cadmium, chromium, cyanide, mercury, nickel, radium-226, selenium, and zinc in the water exceeded the USEPA maximum contaminant level (MCL) in less than 5 percent of the 183 to 620 wells sampled. Nine percent of 625 wells sampled contained water with lead concentrations that exceeded the USEPA MCL. Radon concentrations in the water of 92 percent of the 285 wells sampled exceeded the proposed USEPA MCL. Radium-228 in the water of 10 percent of the 240 wells sampled and nitrate in the water of 13 percent of 1,413 wells sampled exceeded the USEPA MCL. Gross-alpha activity in the water was measured only in the Chickies and Harpers Formations of the Piedmont Upland, with 23 percent of the 168 wells sampled exceeding the USEPA MCL.