The aquifers underlying the 134.6-square-mile city of Philadelphia are divided by the Fall Line into the unconsolidated aquifers (chiefly sand and gravel) of the Coastal Plain and the consolidated-rock aquifers (chiefly schist of the Wissahickon Formation) of the Piedmont. Ground water is present under confined and unconfined conditions. The principal units of the confined-aquifer system are the lower and middle sands of the Potomac-Raritan-Magothy aquifer system. The lower sand unit is the most productive aquifer in Philadelphia. The median yield of wells screened in the lower sand unit is 275 gal/min (gallons per minute), and yields of some wells are as high as 1,350 gal/min. The median specific capacity is 16 (gal/min)/ft (gallons per minute per foot of drawdown). The principal units of the unconsolidated unconfined-aquifer system are the upper sand unit of the Potomac-Raritan-Magothy aquifer system and the informally named Trenton gravel. The median yield of wells tapping these two undifferentiated units is 90 gal/min, and yields of some wells are as high as 1,370 gal/min. The median specific capacity is 12 (gal/min)/ft. The consolidated unconfined-aquifer system consists mainly of the Wissahickon Formation. The median yield of nondomestic wells that tap the Wissahickon Formation is 45 gal/min, and yields are as high as 350 gal/min. The median specific capacity is 0.5 (gal/min)/ft. Urbanization has considerably modified the hydrologic cycle in Philadelphia. Impervious surfaces have reduced recharge areas and evapotranspiration and have increased direct runoff. Leakage from the water-distribution system, which is supplied from the Delaware and Schuylkill Rivers, was about 60 to 72 Mgal/d (million gallons per day) in 1980. Groundwater infiltration to sewers is estimated to be as much as 135 Mgal/d when the water table is high. The potentiometric surface of the lower sand unit has been lowered substantially by pumping. By 1954, cones of depression were more than 50 ft (feet) below sea level at the U.S. Naval Base and more than 70 ft below sea level along the Delaware River northeast of the naval base. As a result of withdrawals, declining heads in the lower sand unit caused water to flow downward from the overlying unconsolidated deposits and the water table to decline below sea level along the Delaware River. Beginning in the mid1960's, ground-water withdrawals from the lower sand unit decreased, and, by 1979, water levels had risen 25 ft at the U.S. Naval Base and 45 ft farther north along the Delaware River. As of 1985, water levels in the lower sand unit were controlled largely by pumping in nearby parts of New Jersey. Urbanization also has caused substantial degradation of the quality of ground water in Philadelphia. By 1945, the quality of water in the unconfined aquifer system began to deteriorate as contaminants present at the land surface migrated down- ward. Withdrawal of water from the deeper confined aquifers caused a head decline that resulted in downward movement of contaminated water from the overlying unconfined aquifer system. Consequently, water in the confined aquifers deteriorated progressively in chemical quality so it resembles water in the unconfined aquifer system. The concentration of dissolved solids in water samples collected during 1979-80 ranged from 90 to 4,480 mg/L (milligrams per liter). The average concentration of 778 mg/L was 45 percent higher than that of samples collected during 1945-58. Water from the unconfined unconsolidated aquifers generally had the highest dissolved-solids concentration. The concentration of dissolved iron in water samples collected during 197980 ranged from 0 to 220 mg/L and exceeded 0.30 mg/L in 71 percent of the samples. The average concentration of 17 mg/L was nearly 30 percent higher than that of samples collected during 1945-58. Many wells have been abandoned because of elevated iron concentrations. The concentration of dissolved manganese in water