Glacial deposits, as much as 360 feet thick, underlie the study area. The upper 29 to 118 feet, a sand and gravel unit, is the aquifer tapped for water by all wells in the area. This unit is underlain by impermeable clay that is at least 100 feet thick.
Ground-water flow is northeastward at an estimated rate of 3 to 6 feet per day. Hydraulic conductivities in the aquifer range from 85 to 150 feet per day; 120 feet per day provided the best match of field data in a ground-water flow model. The depth to water ranged from 1 to 20 feet.
Chemical anlayses indicate that ground water is contaminated with organic chemicals from near the Hangar/Administration building at the U.S. Coast Guard Air Station to East Bay, about 4,300 feet northeast. The plume, which follows ground-water flow lines, ranges from 180 to 400 feet wide. In the upper reach of the plume, hydrocarbons less dense than water occur at the surface of the water table; they move downward in the aquifer as they move toward East Bay. Maximum concentrations of the major organic compounds include: benzene, 3,390 micrograms per liter; toluene, 55,500 micrograms per liter; xylene, 3,900 micrograms per liter; tetrachloroethylene, 3,410 micrograms per liter; and bis (2-ethyl hexyl) phthalate, 2,100 micrograms per liter. Soils are generally free of these hydrocarbons; however, in the vicinity of past drum storage, aircraft maintenance operations, and fuel storage and dispensing, as much as 1,100 micrograms per kilogram of tetrachloroethylene and 1,500 micrograms per kilogram of bis (2-ethyl hexyl) phthalate were detected. At a few locations higher molecular weight hydrocarbons, characteristic of petroleum distillates, were found.
