The 187-square mile study area is in the Triassic-Jurassic Newark Basin. Most of the area is underlain by sedimentary rocks of Upper Triassic age (74 percent) and intrusive diabase of Jurassic age (12 percent) and includes two southwest-northeast trending valleys underlain by carbonate and crystalline rock.

Ground water in the sedimentary rocks of Triassic age moves through a network of interconnecting secondary openings fractures, bedding planes, and joints. The ground-water system consists of beds with a relatively high transmissivity separated by beds with a relatively low transmissivity that form a leaky, multiaquifer system. Ground water is unconfined in the shallower part of the aquifer and confined or semiconfined in the deeper part of the aquifer. Most deep wells are open to several water-bearing zones and are multiaquifer wells.

The frequency of occurrence of water-bearing zones decreases with depth. Sixty-five percent of water-bearing zones for all hydrogeologic units are within 200 feet of land surface, and 85 percent are within 300 feet of land surface. On the basis of the median specific capacity of nondomestic wells, carbonate rocks, the Brunswick Group, and the Stockton Formation are the most productive hydrogeologic units. Carbonate rocks and the Stockton Formation have the highest median nondomestic well yields (156 and 120 gallons per minute, respectively) among the hydrogeologic units. Thirty-four percent of domestic wells drilled in diabase, 30 percent in the Lockatong Formation, and 21 percent in carbonate rock yield less than 5 gallons per minute.

Average water budgets for the Cooks, Tinicum, Paunnacussing, and Mill Creek Basins weighted by drainage area were calculated for 1991-92. Average annual precipitation was 41.7 in. (inches); average annual evapotranspiration (ET) and other losses were 26.2 in. or 63 percent of precipitation; average annual streamflow was 15.9 in., or 38 percent of the average precipitation; and the average annual change in ground-water storage was a decrease of 0.3 in., or less than 1 percent of the average annual precipitation. Average estimated recharge for 1991-92 weighted by drainage area was 10.1 in. [0.485 (Mgal/d)/mi² (million gallons per day per square mile)]; this is equal to a recharge rate of 758 gallons per day per acre.

Water budgets for the Tohickon Creek Basin were calculated for 1968-91 (prior to regulation of the stream by Lake Nockamixon). The average annual precipitation was 47.2 in.; average annual ET and other losses were 24.3 in., or 51 percent of the average annual precipitation; and annual streamflow was 22.6 in., or 48 percent of the average annual precipitation.

Streamflow hydrographs for 1991-92 for Cooks, Tinicum, Paunnacussing, and Mill Creeks were separated into baseflow and surface-runoff components. Average annual ground-water discharge to streams weighted by drainage area was 8.4 in. [0.403 (Mgal/d)/mi²], which was 20 percent of the average annual precipitation and 53 percent of the average annual streamflow. Average annual surface runoff weighted by drainage area was 7.4 in., which was 18 percent of the average annual precipitation and 47 percent of the average annual streamflow. Annual base flow for 1936-71 for Tohickon Creek ranged from 2.5 in. [0.12 (Mgal/d)/mi²] in 1965 to 8.4 in. [0.40 (Mgal/d)/mi²] in 1945. The median base flow was 5.3 in. [0.25 (Mgal/d)/mi²].

Water from wells in the crystalline rocks has the lowest median pH (5.8), the lowest median specific conductance (139 microsiemens per centimeter), the lowest median alkalinity [16 mg/L (milligrams per liter) as CaCOg], and the highest dissolved oxygen concentration (9.0 mg/L) of the hydrogeologic units. Water from wells in carbonate rocks has the highest median pH (7.8) and the highest median alkalinity (195 mg/L as CaCO3) of the hydrogeologic units. Water from wells in the Lockatong Formation has the highest median specific conductance (428 microsiemens per centimeter) and the lowest dissolved oxygen concentration (0.8 mg/L) of the hydrogeologic units. Water from wells in crystalline rocks contains the lowest concentrations of total dissolved solids (TDS) of the hydrogeologic units. Water from the Lockatong Formation contains the highest concentration of TDS of the hydrogeologic units. Water from only 1 of 83 wells sampled exceeded the U.S. Environmental Protection Agency (USEPA) secondary maximum contaminant level (SMCL) for TDS; the well is in the Lockatong Formation. Five of 86 samples (6 percent) and 6 of 75 samples (8 percent) exceed the USEPA SMCL for iron and manganese, respectively. Nitrate is the most prevalent nitrogen species in ground water. The median nitrate concentration for all hydrogeologic units is 2.3 mg/L. Of 71 water samples from wells, no concentrations of nitrate exceed the USEPA maximum contaminant level. The median dissolved radon-222 activity was highest for water samples from wells in crystalline rock [3,600 pCi/L (picocuries per liter)] and lowest for water samples from wells in the Lockatong Formation (340 pCi/L) and diabase (350 pCi/L). Water samples for analysis for volatile organic compounds (VOC's) were collected from 34 wells in areas where the potential existed for the presence of VOC's in ground water. VOC's were detected in 23 percent of the 34 wells sampled. The most commonly detected compound was trichloroethylene (13 percent of sampled wells).