Detecting transmissive bedrock fracture zones under cover of glacial formations using residential water-well production data

Tracing fractures under glacial drift commonly involves costly and often unfeasible (in populated areas) geophysical methods or outcrop surveys, often far from the area of interest. A hypothesis is tested, that the specific capacity data for wells penetrating through glacial drift into a bedrock aquifer display two statistical populations: assuming uniform well construction, the wells with high specific capacity penetrate transmissive fracture zones, while those with low specific capacity encounter non-fractured rock characterized by primary porosity. The hypothesis was tested on 617 wells drilled into the Pennsylvanian Sharon Sandstone, Geauga County, Ohio (USA). Hydraulic conductivity, calculated using the Cooper and Jacob (1946) approximation to Theis’ non-equilibrium radial flow equation, followed quasi-log-normal distribution (geometric mean 9.88 × 10−6 m/s). The lower values presumably correspond to primary porosity, and higher values correspond to bedrock fracture zones. The higher hydraulic conductivity followed two distinct orientations (N34°E, N44°W), corresponding with the regional fracture pattern of the Allegheny Plateau. A variogram showed that the wells within a kilometer of each other correlate and that wells penetrating the thicker glacial blanket have lower hydraulic conductivity and larger drawdown.