The report area comprises 270 square miles, and includes most of the Towns of Babylon and Islip, and parts of the Towns of Huntington, Smithtown, and Brookhaven, in southwestern Suffolk County, New York. Almost all the water used in the area is obtained from wells screened in permeable zones of the ground-water reservoir which consists of unconsolidated deposits of gravel, sand, silt, and clay as much as 1,800 feet thick. The ground-water reservoir contains three principal aquifers. From the surface down these are (a) surficial deposits of sand and gravel of Pleistocene age, (b) sands of the Magothy (?) Formation of Cretaceous age, and (c) the Lloyd Sand Member of the Raritan Formation of Cretaceous age. At present only the upper two aquifers are tapped by wells. Natural replenishment of the ground-water reservoir in the area takes place entirely by infiltration of precipitation and averages about 215 mgd (million gallons per day). Average ground-water runoff to streams above tidewater is 114 mgd, and it is estimated that an additional 54 mgd is discharged into tidal reaches of streams. Ground-water evapotranspiration is computed to be about 10 mgd and submarine outflow from the area is estimated to be 18 mgd. The average streamflow of the area above tidewater is 120 mgd. Because of the permeable soils and low relief, direct runoff is only about 5 percent of the average streamflow. Streams are perennial along their middle and lower reaches and exhibit well-sustained low flows. Flooding rarely occurs although continued urbanization may result in minor flooding problems as additional storm sewers are constructed. Water in most of the area is generally of good quality; however, it may be contaminated locally. Some streams and parts of the water-table aquifer contain low concentrations of synthetic detergents and other dissolved constituents from domestic and industrial wastes. Salty water occurs in parts of the water-table aquifer in the area under and bordering Great South Bay and under the barrier beaches. Present information, however, indicates that submarine outflow in the artesian aquifers is sufficient to maintain the fresh water-salt water interface some distance seaward of the barrier beaches. Ground-water withdrawals in 1960 averaged 39 mgd, most of which was returned to the ground through cesspools, leaching beds, and recharge wells; pumpage did not appreciably affect the natural water balance of the groundwater reservoir. If withdrawals continue to be artificially recharged, pumpage can be increased at least fivefold before consumptive losses materially reduce ground-water levels. However, if the area were completely sewered in the future, an adequate supply of ground water for a substantially increased population could not be obtained without (a) reducing the amount of ground water in storage in the reservoir or (b) recharging treated-sewage effluent.