BOREHOLE GEOPHYSICAL CHARACTERIZATION OF A FAULTED KARST AQUIFER SYSTEM IN ROCKINGHAM COUNTY, VIRGINIA

The poly-deformed karst aquifer systems of western Virginia are increasingly relied upon to meet the water demands of an expanding population, yet relatively little detail is known about the construction of most supply wells and few examples demonstrate which geologic factors actually govern the movement of groundwater in the region. Borehole geophysical logging can improve our understanding of the hidden structural and hydrologic complexities within Virginia’s fractured rock aquifer systems and directly affect mapping, modeling and resource management decisions.

Oriented optical televiewer imagery, and other wireline methods, allowed for the characterization of a previously unknown major fault system and a zoned karst aquifer underneath the Town of Dayton in southern Rockingham County. A 140 meter (460 ft) deep well installed adjacent to a major spring encountered fault breccia along its entire uncased length and for a time connected two water-bearing zones under different hydraulic heads. The “static” water level measured near the surface concealed an ambient groundwater flow from shallow to deep portions of the well and aggressive development of the well had no measureable short-term effect on the spring. A downward ambient flow was also measured in a 214 meter (702 ft) deep well installed farther from the spring, moving groundwater from shallow depths to receiving features as deep as 180 meters (590 ft). This well encountered uniformly dipping bedrock that was pervasively cross-cut by E-W and NW-SE striking near-vertical fractures and faults. The same family of steep fractures and faults was found to cut across the fault breccia in the 140 meter well and a secondary breccia was observed partially filling portions of the main water bearing fracture-conduit even though there appeared to be no fault offset along the fracture plane.

Groundwater movement in the area appears to be heavily influenced by the E-W striking, near vertical fracture- and fault-sets that post-date the presumably Alleghenian thrust related fault breccia. More work is needed to further define the fault system underlying Dayton and to determine if the downward groundwater gradients observed in the supply wells are natural or induced by nearby pumping.