STRATIGRAPHIC AND STRUCTURAL CONTROLS ON BEDROCK-AQUIFER HYDRAULICS

Recent hydrogeologic investigations at bedrock sites in the Appalachian Plateau, Valley and Ridge, and low Taconics provinces reveal a range of distinctive stratigraphic and structural controls on aquifer hydraulics. Site-investigation methods include core analysis and outcrop mapping, gamma logging and borehole-wall imaging, single- and cross-borehole flowmetering, and discrete-zone hydraulic testing and monitoring.

At the Appalachian Plateau site in western New York, upper and lower aquifers in the shallow dipping carbonate bedrock are separated by confining beds of massive limestone. Hydraulically conductive zones commonly are along enlarged bedding partings in solution-prone beds. The major flow zone is at the base of a gypsiferous bed in the lower aquifer. Vertical hydraulic-head differences between the upper and lower aquifers in this recharge area typically are more than 20 feet. Blind thrust faults and other structures result in localized disruptions in aquifer- and confining-bed continuity.

At the Valley and Ridge sites in central Pennsylvania, many hydraulically conductive zones are associated with lithologic changes in the clastic- and carbonate-bedrock sequences. Bedding partings and cleavage, which is orthogonal to the dipping beds, impart a strong hydraulic anisotropy along bedding strike.

In the slightly metamorphosed shale at the low Taconics site in eastern New York, the major hydraulically conductive zones are sub-parallel shear zones, which consist of closely spaced fractures that display a wide range of orientations. Less densely spaced fractures provide hydraulic connection between the dipping shear zones, which have vertical head differences of less than 0.2 feet in this discharge area.