Statewide Characterization of Bedrock Fractures in Connecticut for Hydrogeologic Purposes

The State of Connecticut relies on wells bored into fractured bedrock for most of its domestic drinking-water supplies, and there is concern about how best to characterize the source areas for these wells and protect them from contamination. Roughly three-quarters of the State is underlain by metamorphosed sedimentary and igneous rocks in the form of schists and gneisses; the remainder is underlain by clastic sedimentary rocks and basalt flows of the early Mesozoic Hartford and Pomperaug basins. Unconsolidated surficial deposits of variable thickness from the Pleistocene continental glaciation overlie most of the bedrock. Aquifer tests indicate that the surficial deposits constitute the most productive reservoir for ground water but that joint sets in bedrock constitute the most important pathways for ground-water flow to bedrock wells. Single near-vertical joint sets in bedrock may produce elliptical zones of contribution to wells having long axes that are parallel to the fracture trend. Bedrock geologic maps at 1:24,000 scale have been published for nearly all of the State, so lithologic units and formations have well-known distributions and foliation patterns, although fracture data are limited. If the fracturing habit of each formation is internally consistent with respect to foliation, however, then a limited number of observations of each formation should allow us to infer the approximate nature and orientations of the dominant bedrock fracture sets throughout the State from published geologic maps. We are currently analyzing representative outcrops of each of the major bedrock formations in the State to determine the relationship between the degree and nature of foliation and the orientation and intensity of associated fracture sets. Our preliminary observations indicate that the most fundamental difference is between layered and nonlayered rocks. Layered rocks, including schists, gneisses, and the early Mesozoic clastic sedimentary rocks, commonly have a dominant layer-parallel joint set, or parting, aligned along pre-existing rock fabric including foliation, compositional layering, or bedding. The degree of development of this parting can vary depending on such variables as lithology and the presence or absence of tectonic folds. In many of these rocks, a second set of near-vertical joints is developed orthogonal to the strike of foliation and parallel to dip direction. In both the layered and the nonlayered rock types, a third set of unroofing joints is developed subparallel to the land surface and decreases in intensity with depth; in areas with gently dipping foliation or layering, these unroofing joints develop along foliation parting. The early Mesozoic basalts, which are classified as nonlayered, have well-developed cooling joints. A hydrogeologic map of the State of Connecticut depicting these bedrock fracture characteristics and their dominant trends will be useful to State and local managers concerned with the protection of ground-water supplies.