A Three-Dimensional, Finite-Difference, Steady-State groundwater Flow Model to Evaluate Hydrogeologic Data Collected from Pine Run Basin, Bucks County, Pennsylvania

Abstract

A three-dimensional finite-difference groundwater flow model was used to evaluate hydrogeologic data for 9.9 square miles of Pine Run Basin, Bucks County, Pennsylvania. The steady-state model was calibrated using February 1988 field data including groundwater, stream, and reservoir elevations, groundwater recharge, baseflow measurements, stream seepage analyses, and aquifer tests. Pine Run Basin is underlain by Triassic continental rocks, chiefly arkosic sandstones, siltstones, and shales of the heterogeneous Stockton Formation. The Stockton Aquifer was modeled using three layers which parallel the basin's topography and represent a shallow unconfined aquifer and two underlying leaky-­confined aquifers. The hydraulic conductivity of the Stockton Aquifer is anisotropic with its primary axis parallel to the northeast striking beds. The model utilized a three-dimensional anisotropy ratio of 1.0: 0.5: 0.1 representing hydraulic conductivities parallel to strike, up and down dip, and perpendicular to bedding. The anisotropy ratios coincide well with the stratigraphic setting of a middle member comprised of braided stream channel deposits of arkosic sandstones paralleling the strike of the formation, and an upper member consisting of red shale overbank deposits fining upward into lake-bed deposits. Hydraulic conductivities of 10, 5, 1 and 0.8, 0.4, 0.08 ft/d were used for the middle and upper members, respectively. Model results indicate the unconfined aquifer contributes the majority of baseflow to Pine Run. Groundwater divides coincide with the surface water divides except for the dip slope boundary where underflow may occur. The model was most sensitive to changes in hydraulic conductivity and areal recharge.