A study was conducted to define the hydrogeology and describe the ground-water flow in the Pohatcong Valley in Warren County, N.J. near the Pohatcong Valley Ground Water Contamination Site. The area is underlain by glacial till and alluvial sediments and weathered and competent carbonate bedrock. The northwest and southeast valley boundaries are regional-scale thrust faults and ridges underlain by crystalline rocks. The unconsolidated sediments and weathered bedrock form a minor surficial aquifer. The carbonate rocks form a highly transmissive fractured-rock aquifer with well yields commonly as high as 500 gallons per minute. Ground-water recharge and flow in the crystalline-rock aquifer bordering the valley is minor compared to flow in the carbonate-rock aquifer, and little ground water flows into the carbonate-rock aquifer directly from the crystalline-rock aquifer. The thrust faults separating the carbonate and crystalline rocks may further impede flow between the two rock types. Interpretations of water-level and transmissivity data collected during 2000 to 2003 indicate that the carbonate formations generally can be considered to be one aquifer. The transmissivity of the carbonate-rock aquifer was estimated from the results of four aquifer tests conducted with two public supply wells. The transmissivity estimated from aquifer tests at a well located in Washington Borough is about 8,600 square feet per day. An aquifer test at a well located near the southwest border of Washington Borough was conducted to estimate transmissivity and the direction and magnitude of anisotropy. The estimated direction of maximum horizontal transmissivity near the second well is about 58? east of north and the magnitude is 7,600 square feet per day. The minimum horizontal transmissivity is 3,500 square feet per day and the ratio of anisotropy (maximum transmissivity to minimum transmissivity) is 2.2 to 1. Stream base-flow data indicate that Pohatcong Creek steadily gains flow, but most of the gain is from tributaries originating in the crystalline rock areas (valley walls). Therefore, it is concluded there are no major heterogeneities (such as karst springs) in ground-water discharge to surface water. During periods of low ground-water levels, it is likely that, within the study area, Pohatcong Creek gains no flow from the carbonate-rock aquifer and may even lose flow to the surficial aquifer (which then recharges the carbonate-rock aquifer). There are few sites in the Pohatcong Valley with large-scale (greater than 10 million gallons per year) ground- or surface-water withdrawals. The only substantial withdrawals in the valley are from two public supply wells and from two industrial facilities. Average annual withdrawals during 1997-2002 at these four locations totaled 298 million gallons per year. About 95 percent of the water withdrawn by the large industrial user (108 million gallons per year) is re-injected into the aquifer. In some locations throughout the valley, water levels in the shallow surficial deposits were substantially higher than those in underlying carbonate-rock aquifer. Water levels in the deep part of the surficial aquifer and underlying carbonate-rock aquifer were similar, although the gradients were often (but not always) downward. Furthermore, data collected during aquifer tests at a public supply well in Washington Borough and a public-supply well west of Washington Borough show that the deep part of the surficial aquifer is hydraulically well connected to the underlying carbonate-rock aquifer at these two locations. The shallow surficial deposits, however, are not well connected to the deep surficial deposits and carbonate rock at these two locations. The overall ground-water-flow pattern in the valley appears to be that precipitation recharges the surficial aquifer and is discharged from the surficial aquifer to the underlying bedrock aquifer and the Pohatcong Creek and its tri