DETECTION OF TRANSMISSIVE BEDROCK FRACTURED ZONES UNDER COVER OF GLACIAL FORMATIONS USING RESIDENTIAL WATER WELL PRODUCTION DATA

We tested feasibility of identifying location and trends of transmissive bedrock fractured zones under the blanket of glacial till by mapping hydraulic conductivitiy estimated from residential well production data. Well Logs and Drilling Reports typically include rudimentary lithological column and well production test data: static water level, duration and rate of pumping or bailing, and water level at the end of the production test. We used data from the tests in 617 wells of Sharon Sandstone aquifer in Geauga County, Ohio to estimate hydraulic conductivity using Cooper and Jacob’s (1946) and Jacob’s (1950) approximation for Theis’ (1935) non-equilibrium radial flow equation. As the wells were neither cased nor screened and the production test rates were low (5-20 gpm, or 1.1-4.5 m³/hr), the well loss can be assumed negligible. The resulting hydraulic conductivity values followed quasi-log normal distribution with geometric mean of 9.88x10^-6 m/s. The hydraulic conductivity values were mapped and grouped into two populations: the low values presumably corresponding to primary porosity zones and high values assumed corresponding to the fractured zones. The mapped patterns of the higher hydraulic conductivity values followed two distinct orientations: N34⁰E and N44⁰W. The trends of hydraulic conductivity correlated fairly well with the regional fracture pattern of Allegheny Plateau Province, N21⁰E and N57⁰W reported earlier for Southwestern Pennsylvania and Northwestern West Virginia with equal amount of separation between the joint systems (Engelder, 1985; Evans, 1994 and Ver Steeg, 1944). N34⁰E also follows the alignment of glacial advance and retreat as a result of the stress loading and subsequent unloading during glaciation. The results imply that the residential well-test data can be used for detecting regional transmissive fractures under the cover of glacial drift.

References

Engelder, T (1985) Loading Paths to Joint Propagation During a Tectonic Cycle:an Example from the Appalachian Plateau, USA. J. of Struct. Geol. 7 (3/4) 459-476

Evans, M A (1994) Joints and Décollement Zones in Middle Devonian Shales: Evidence for Multiple Deformation Events in the Central Appalachian Plateau. Geol.Soc. of Am. Bull. v. 106, 447-460

Ver Steeg, K (1944) Some Structural Features of Ohio. J.of Geol. v. 52, 131-138