Geohydrology and water quality of the Durham Center Area, Durham, Connecticut
Contaminated ground water is widespread and persistent beneath the Durham Center area in the town of Durham, Conn. Most of the contaminants are organic halides, usually trichloroethene, 1,1,1-trichloroethane, and tetrachloroethene. Less extensive chemical contamination of surface water, soil, and glacial sediments also has been detected. Two manufacturing companies, located at the northern and southern ends of this largely residential area, are believed to be the principal sources of the organic compounds detected in ground water. The contamination of water in the bedrock, the primary source of drinking water throughout the area, is the major environmental concern. Maximum concentrations of trichloroethene in three bed- rock wells range from 4,500 to about 5,500 mg/L (micrograms per liter). Concentrations of trichloroethene greater than 5 mg/L, the maximum contaminant level established for drinking water by the U.S Environmental Protection Agency, have regularly been detected in water samples from many other bedrock wells for at least 9 years. The geohydrology of the area is highly complex. Compact lodgment till that is up to 30 feet thick and probably fractured, overlies the bedrock. The bedrock is lithologically heterogeneous, and con- sists mostly of red fluvial sandstone, siltstone, and conglomerate; it is locally interbedded with black lacustrine shales and gray sandstones. Lithology and stratigraphy interpreted from borehole-geophysical logs at Durham Center are consistent with the Portland Formation subfacies described in earlier geologic studies. Beds strike nearly north-south and dip gently eastward. At least one high-angle normal fault transects the bedrock; it strikes northeast and dips northwest. Acoustic televiewer logs, measurements at out-crops, and azimuthal, square-array, resistivity data indicate a dominance of northeast-striking fractures that dip steeply northwest and southeast. Less prevalent strike directions are north to east-north-east. The till and sedimentary bedrock are dual-porosity, dual-permeability media. The hydraulic conductivity of the bulk mass of till is believed to be on the order of tenths of a foot per day to about 2.5 feet per day, with a total porosity of about 25 percent and an estimated average fracture porosity of less than 1 percent. The reported transmissivities of the bedrock range from less than 1 to about 17,000 feet squared per day and storativity is generally about 10-4, but the accuracy of these values is uncertain. The intergranular porosity of the sandstone units is estimated to average 5 percent, and estimates of fracture porosity from square-array, resistivity soundings at two sites were 1.1 and 2.7 percent. The bedrock has characteristics of both a single aquifer and a multi-unit, artesian or leaky aquifer system. A local ground-water-flow system that includes the upper part of the bedrock is unconfined. A large- scale flow system in deeper parts of the bedrock has transported organic compounds across topographic drainage divides. Borehole-geophysical logs and head measurements indicate that the natural ground- water-flow system in the bedrock has been altered by drilled wells that connect fractures and by with- drawals from wells. A conceptual model of the movement and fate of organic contaminants suggests that (1) nonaqueous phase organic halides are retained near their source; (2) flow is primarily through fractures in the till and through fractures and bedding-plane openings in the sedimentary rocks; (3) retardation of contaminants occurs primarily by diffusion from fractures into the aquifer matrix; and (4) transport directions of dissolved organic halides are controlled by a combination of natural hydraulic gradients, hydraulic gradients produced by the cyclical pumping of wells, and by the strike directions of bedrock faults, fractures, and bedding planes.