Field data and ground water modeling in a layered fractured aquifer

Surface refraction seismic surveys were conducted within a 7.3 km(2) east-central Pennsylvania watershed to characterize the geometry of its shallow layered and fractured bedrock, These data were augmented with observations from 29 rock cores, Hydraulic conductivity (K) of the bedrock was determined by interval packer testing within the boreholes. Finally, the MODFLOW ground water model was applied to the layered aquifer geometry in a three-dimensional format to determine watershed-scale aquifer parameters by model calibration to observed watershed data. Seismic testing showed that the aquifer could be characterized by four distinct layers, each with its own characteristic seismic velocity Hydraulic conductivity determined by field testing decreased with depth, ranging from greater than 1.0 m day(-1) in the shallow, highly fractured layer to approximately 0.01 m day(-1) in the regional aquifer, Its distribution with depth reflected the increase in seismic velocities and decrease in extent of fracturing observed in the cores. Aquifer geometry determined from seismic testing and rock cores was used to define model layer geometry, and K values from model calibration compared favorably to those determined by field testing. The ground water model was also used in calibration mode to determine specific yields (S-y) of the characteristic fracture layers, These were all low, ranging from 0.005 in the highly fractured layer to 0.0001 in the regional aquifer. When aquifer geometry determined from detailed seismic surveys is used as the basis for watershed-scale ground water modeling in these shallow layered and fractured systems, hydraulic parameters derived by field testing compare well to those determined by model calibration.