Large quantities of ground water are used by the Philadelphia Naval Base and many industries in south Philadelphia, as well as by municipalities near Philadelphia in both Pennsylvania and New Jersey.The areal contact between unconsolidated sand, gravel, and clay strata of the Atlantic Coastal Plain and consolidated rocks of the Piedmont province extends in a northeast-southwest line through central Philadelphia. This line of contact, the so-called Fall Line, sharply separates the area of highly productive sand and gravel aquifers of the Coastal Plain from the less productive bedrock aquifers of the Piedmont Plateau.The daily withdrawal of ground water from sand and gravel in the Philadelphia area, including Bristol, Pa., and Trenton, N. J., is approximately 110 million gallons. Wells around Philadelphia yield an additional 13 million gallons a day from consolidated rocks of the Piedmont province. The Raritan formation of Cretaceous age is the principal aquifer for wells in south Philadelphia and the adjacent Camden area. The large quantity of water pumped and the limited extent of this aquifer in the area provide a potentially serious interstate water problem. Ground-water levels in the strategically important Naval Base area have not declined seriously as yet, but increasing withdrawal planned in adjacent New Jersey is expected to lower water levels in south Philadelphia.The Delaware and Schuylkill Rivers are believed to be the chief sources of recharge to the Raritan formation, even though heavy pumping on either side of the Delaware River may be expected to affect water levels on the other side. Recharge appears to be adequate generally for present rates of withdrawal, but further studies are needed for an evaluation of the safe maximum yield of the area.The chemical quality of ground water pumped from the Raritan formation is satisfactory for most purposes, but periodic analyses indicate that the concentration of dissolved solids is increasing. This deterioration of the chemical quality probably is due to the character of the water and bottom sediments of the Delaware and Schuylkill Rivers. Shallow aquifers in south Philadelphia contain large amounts of gases generated by the decay of river silts and city refuse occupying low areas.The States of Pennsylvania and New Jersey, together with the municipalities concerned, are coordinating their efforts for a practical solution to the ground-water problems of the Philadelphia area.