Abstract: Siting a Groundwater Supply in Glaciated Proterozoic Y Rocks of the Reservoir Fault Zone, New Jersey Highlands

Ralph E. Costa

Groundwater in the New Jersey Highlands is becoming an increasingly valuable resource as the demand for drinking water increases and the availability of open space decreases. At a municipality in New Jersey, the potential contamination of two overburden municipal groundwater supply wells has triggered the need for an alternate water supply. A test well program was initiated to provide a new groundwater supply comparable to the existing supply in quantity and quality.

In this study, it was determined that because the local glacial overburden consists of less than 50 feet of discontinuous till, it is not a potential source for municipal groundwater supply. In the past, test well programs and regional studies have failed to locate successful municipal groundwater supplies within the local Proterozoic Y pyroxene syenite and granite bedrock. Historically, greater than 75 % of public supply wells advanced in the Proterozoic Y rocks of the New Jersey Highlands have yielded less than 100 gallons per minute.

The available published geologic data and fracture trace analysis were used to site a test well location. The northeast-trending (locally N30E) trace of the near vertical post-Grenville Reservoir Fault was targeted. No existing water wells are known to be located within this zone. A shallow seismic reflection survey was designed to characterize the subsurface geologic structure between 50 and 500 feet below the surface. Data acquisition was conducted along two seismic lines oriented nearly perpendicular to the strike of the Reservoir Fault. The 30-fold seismic profiles indicate the location of many buried, narrow, steeply-dipping faults with 45 to 65 feet of displacement. A depth to structure map of the test well site reveals the presence of a fault-bounded, open, and northeast-striki g syncline and a comply horst and graben brittle fault system. These strongly indicated a potential water supply may be present.

The fracture trace analysis and shallow seismic reflection survey have proven successful for the location of buried water-bearing faults within the crystalline bedrock of the New Jersey Highlands. Although the study is not completed, two test wells have intersected faults at the surface locations and corresponding depths indicated by the remote sensing techniques. It is anticipated that the combined yield of two to three of the newly-sited bedrock supply wells will be equal to or greater than the combined yield of the two existing overburden municipal supply wells. In addition, the structural control on the flow in the bedrock aquifer will be used in the delineation of the Well Head Protection Area.

AAPG Search and Discovery Article #90954©1995 AAPG Eastern Section, Schenectady, New York