A MULTIDISCIPLINARY STUDY OF CONTROLS ON GROUND-WATER FLOW IN A KARST AQUIFER IN THE ORDOVICIAN BEEKMANTOWN GROUP, BERKS COUNTY, PA

A multidisciplinary approach has been employed that includes detailed fracture mapping, water table monitoring, ground-water chemistry and lithogeochemistry to study factors that affect cave formation and ground-water flow through Ordovician carbonates proximal to an aggregate quarry in Ontelaunee Township, Berks County, Pennsylvania.

Bedding in the quarry is oriented S38W-60NW and sedimentary structures demonstrate the rocks in the pit are not isoclinally folded. Clay- and boulder-filled dissolution conduits up to 5m wide exist parallel to bedding and are exposed in the south wall of the quarry; dissolution features are also prominent within a 10m wide, S80E-68SW fracture zone near the southwestern corner. Dissolution is apparent in bedding-parallel carbonate veins, whereas veins perpendicular to bedding are less weathered.

Ten synchronized pressure transducers with data loggers installed in a 1-km² monitoring well field continuously recorded water table fluctuations and temperature during a storm event. By identifying water table responses to storms, together with rock fracture characteristics mapped in the quarry, prediction of dissolution channel formation in the aquifer may be possible.

Ground-water temperature in each well remained unchanged during the storm, with values ranging from 18ºC (temperature of adjacent streams) to 12ºC. The 12ºC end member may represent longer-term resident ground water and wells with intermediate temperatures reflect mixing of infiltrating surface waters and cold resident ground water.

Pressure transducer data suggest different sections of the well field respond to storm-induced perturbations in one of three distinct patterns. (1) Wells characterized by a symmetrical fluctuation coincident with the rise and fall of the regional ground-water table are interpreted to represent zones in the aquifer with unrestricted flow. (2) Wells with a rapid initial water level increase followed by a gradual return to base levels are interpreted to indicate more restricted flow. (3) Wells characterized by an initial spike superimposed on pattern (1) may have experienced initial channel blockage, e.g. clay subsidence, followed by a clearing of blockage allowing water levels to return rapidly to regional water table levels.