Discrete-Fracture Dual-Permeability Simulation of Heterogenous Drawdown in the Brunswick Group, North Penn 6 Superfund site, Landsdale, Pennsylvania

Abstract

Aquifer pumping tests conducted by the U.S. Geological Survey (USGS) at the John Evans Site (North Penn 6 Superfund Site, Lansdale, PA) produced anomalous water level changes in observations wells. In some observations wells, changes were not radially symmetric, and more distant wells had greater drawdown than proximal wells. Triassic sandstones and shales at the site are tilted, and contain a large number of fractures, which may affect groundwater movement. Therefore, the computer code FRAC3DVS, which allows simulation of fractures, was used to develop a conceptual model of these anomalies, which was generally consistent with hydrogeologic properties. Both steady state and transient models were constructed. Hydrologic properties and the number, location, orientation, and aperture of fractures were varied in order to simulate the results of the aquifer pumping test. The steady state modeling showed that the range of influence of a few connected fractures created deviations in the flow field on the scale of 10s of meters, similar to the site scale. Thus a simplified network of fractures was used to model the flow anomalies. Five transient simulations from the site-specific model are presented to demonstrate the calibration process and goodness of fit. The best fit, having a root mean square error of 0.07 m, was obtained using a model with three continuous dipping fracture sets with an aperture of 1.0E-4 m and a matrix hydraulic conductivity of 1.2E-9 m/s. The transient modeling showed that anomalies were produced when tilted fractures that only intersect some wells are used and when the contrast between the fracture and matrix hydraulic conductivity is high.