ANISOTROPIC GROUNDWATER FLOW CONDITIONS IN PENNSYLVANIA’S CUMBERLAND VALLEY CARBONATES

The influence of bedrock structure on groundwater flow in Pennsylvania’s Cumberland Valley was studied in order to better understand groundwater flow patterns and behavior. Bedrock in the Valley consists of highly deformed Ordovician carbonates that strike NE/SW, parallel to the Valley axis. Caves, springs, and a lack of surface drainage exemplify the karst landscape and aquifer. Previous studies show a hydraulic gradient from the Blue Ridge to the Conodoguinet Creek, the Valley’s main drain, which flows down the valley axis northeast to the Susquehanna River. However, recent dye tracing in the Valley has revealed groundwater flow nearly parallel to the bedrock strike and to hydraulic head contours (equipotential lines), demonstrating the highly anisotropic conditions in the Valley.

In an attempt to forecast groundwater flow routes, a graphic model was prepared by comparing the mean hydraulic conductivity from existing pump test data literature to the velocity of dye tracer tests; the comparison resulted in a 100:1 ratio. Adobe Illustrator was used to compress the hydraulic head contour map to the 1:100 ratio; groundwater flow lines were then drawn perpendicular to compressed equipotential lines. Before the map was de-compressed to its original form the flow lines and contours were grouped. The resulting map shows groundwater flow vectors crossing equipotential lines at an acute angle, in a NE direction. More local patterns demonstrate a strike oriented preferential flow and preferential flow along joint and fracture traces. Lithology changes, strike and dip angles, and hydraulic pressure as a result of lithology and structure, were found to be the most influential controls on groundwater behavior in the Valley. The validity of the model was tested by comparing recent dye trace vectors to the model results.