The Hackensack River basin, in the northern part of the New Jersey-New York metropolitan area, includes some of the most highly urbanized areas in the United States as well as a largely undeveloped 23.4 square mile area of tidal marsh referred to as the Hackensack Meadows. Bedrock in the Hackensack River basin, consisting of the Newark Group of Triassic age, is composed of diabase dikes and sills and gently westward dipping sandstone, conglomerate, and shale. The Brunswick Formation of the Newark Group is the only important bedrock aquifer in the basin. Water occurs in this aquifer in joints and fractures. The zone of most abundant and largest water-bearing joints and fractures occurs generally within 200 feet of land surface in lowland areas of major streams and within 400 to 500 feet of land surface in upland areas. Reported yields of industrial and public-supply wells tapping the Brunswick are as much as 600 gpm (gallons per minute): the median yield is 100 gpm. The formation is anisotropic; the greatest permeability and thus the movement of water in response to pumping are parallel to the strike of bedding. Therefore, wells in well fields alined perpendicular to strike have minimum interference. The Newark Group is overlain by unconsolidated deposits of till, varved silt and clay, alluvium, and sand and gravel of Quaternary age. Sand and gravel aquifers consist of (1) deltaic deposits formed at the mouths of streams that entered ancient Lake Hackensack in the western part of the basin and (2) valley-fill deposits along the eastern side of the basin. These aquifers locally yield large quantities of water (greater than 300 gpm) to wells. The chemical quality of water in the Brunswick Formation is generally good, and the water is relatively low in dissolved mineral matter in the upper area of the Hackensack River basin. In the lower area of the basin, water in the Brunswick is highly mineralized: specific conductance ranges from 579 to 3,480 micromhos per centimeter at 25 °C; chloride content ranges from 19 to 755 mg/L (milligrams per liter); and sufate content ranges from 87 to 966 mg/L. Chemical quality in both the Brunswick Formation and the unconsolidated deposits in the lower area is affected by induced recharge of poor quality surface water from the Hackensack River and Newark Bay. Water quality in these surface water bodies is influenced by tidal flooding and by the disposal of an average of 57 mgd (million gallons per day) of sewage and industrial wastes in the Hackensack Meadows. Future development of ground-water supplies in the upper area of the basin is restricted, because such development would decrease surface-water supplies which are almost entirely utilized for water supply. Additional development of ground water in the lower area of the basin is limited by the small amount of ground water in the basin and by the intrusion of highly mineralized surface water into the aquifers.