The geohydrology of the Gallup's Quarry area in Plainfield, Connecticut was characterized by the U.S. Geological Survey, in cooperation with the U.S. Environmental Protection Agency, to provide a preliminary framework for future remedial efforts. Gallup's Quarry, an inactive sand and gravel pit, was the site of unregulated disposal of an unknown volume of chemical wastes from at least the summer of 1977 until January 1978. Existing information collected for the Connecticut Department of Environmental Protection during 1978-82 showed that ground water beneath Gallup's Quarry and adjacent land to the northwest was contaminated by organic and inorganic compounds. There is also some evidence for contamination of Mill Brook, which is located north and northwest of the disposal areas.

Geologic mapping and subsurface data show that unconsolidated surficial materials up to 90 feet thick overlie fractured crystalline bedrock in most of the Gallup's Quarry area. The surficial materials consist primarily of stratified drift and till. Texture changes vertically and laterally within the stratified drift; grain size ranges from very coarse to fine. Till blankets the bedrock surface beneath the stratified drift and is a few feet to as much as 25 feet thick. Bedrock is exposed at land surface in a hill in the southeastern part of the quarry and slopes to depths of up to 90 feet beneath the area west and north of the disposal sites. The bedrock is a dark, fine-grained, fractured and jointed blastomylonite and hornblende gneiss of the Quinebaug Formation. It is likely that a west- northwest-trending fault is present in the bedrock beneath Gallup's Quarry; this fault, if present, may provide a preferential pathway for ground-water flow and contaminant transport.

The principal horizontal direction of ground-water flow and movement of dissolved contaminants in the stratified drift was to the northwest of the waste-disposal areas toward Mill Brook in 1978. Estimates of average annual recharge based on regional analyses for 1978-91 are 30 inches in areas where stratified drift are exposed and 9.6 inches in areas where till and crystalline bedrock are exposed. The hydraulic conductivity of the coarse-grained stratified drift, identified in earlier U.S. Geological Survey studies as part of an aquifer capable of yielding large quantities of water, may be several hundred feet per day. The hydraulic conductivity of the till, based on regional information, is likely 0.04 to 24 feet per day.

More detailed geohydrologic information is required to develop effective remedial programs at Gallup's Quarry, particularly in the areas north and west of the waste-disposal areas. Detailed subsurface geologic mapping and definition of head distribution and other hydraulic properties of each geohydrologic unit would indicate directions and rates of ground-water flow. Most importantly, the present extent of contamination of water and sediments throughout the area will have to be determined.