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1 200 ft lower in altitude than the McKin Superfund Site (fig. 1). The flood plain along the river is largely forested with hardwood and softwood trees as well as a dense shrub layer in places. Some abandoned agricultural fields also are in or adjacent to the flood plain. Ground-water discharge points were apparent at several locations on the western bank of the river. The most prominent ground-water discharge point is a spring on the western bank of the river, known locally as Boiling Springs (fig. 2). Ground-water discharge is also evident in an area of sand boils located approximately 10 ft upstream from the confluence of the unnamed tributary with the Royal River. The McKin Superfund Site is a former waste collection, transfer, and disposal facility that operated from 1965 to 1978. The McKin Superfund Site and study area are in a rural setting of farmland, woodland, and single-family homes. Most homes in the area were supplied by individual wells completed in bedrock until contaminants were detected in ground water. Since 1978, residents have received their water from a public-supply system (U.S. Environmental Protection Agency, 1992). Mean annual flow of the Royal River at USGS streamflow-gaging station 01060000 at Yarmouth, Maine, for the period October 1, 1949 to September 30, 1996 was 273 ft3/s (Nielsen and others, 1997, p. 115). Streamflow near the study area typically is about 55 percent of streamflow at Yarmouth (John Sevee, Sevee & Maher Engineers, Inc., written commun., 1997). DATA COLLECTION AND ANALYSIS Study methods included the installation of vapor-diffusion samplers to determine discharge areas along the Royal River for the TCE plume in ground water, on-site analysis of vapors in the diffusion samplers, synthesis of available hydrologic and geologic information, ground-penetrating-radar surveys along the flood plain of the Royal River, and manual installation of auger holes through alluvial sediments. Construction and Analysis of VaporDiffusion Samplers Vapor-diffusion samplers were constructed using the method described by Vroblesky and Hyde (1997). The samplers consist of a 40-mL, uncapped air-filled glass bottle wrapped in two polyethylene bags held in place with cable ties. A duplicate sample for quality control was constructed by placing an additional polyethylene-wrapped glass bottle in the outer bag. The sampler was attached to a survey flag for marking and retrieval. When placed in water or saturated sediments containing volatile organic compounds (VOCs), organic vapors diffuse through the two layers of polyethylene and equilibrate with air in the bottle. The time required for equilibration is 24 hours or less in a controlled setting (Vroblesky and others, 1996; Vroblesky and Hyde, 1997). Additional time may be required after placement of the samplers to allow concentrations of VOCs in ground-water to re-equilibrate in disturbed materials; a period of 2 weeks is recommended (Don Vroblesky, U.S. Geological Survey, oral commun., 1997). After retrieving the sampler, the outer bag is removed to shed attached sediment, and a cap is immediately screwed onto the bottle over the inner bag. Vapor-diffusion samplers were installed at 156 locations (fig. 2) using one of two methods. Most were installed about 6 to 8 in. into the river-bottom sediments by penetrating the bottom sediments with a narrow-bladed shovel, forcing the shovel forward, and inserting the sampler in the gap that was formed between the shovel and sediment. The type of sediment each sampler was placed in was recorded. At about 20 percent of the locations, the river was too deep to use this method. In these locations, a driver method was used in which a steel pipe outfitted with an inner blocked pipe was driven into the sediment with a handheld slide hammer. The inner pipe was then removed and the sampler was inserted to the bottom of the outer pipe. Finally, the outer pipe was removed leaving behind the sampler, and sediments were allowed to collapse around the sampler. Samplers installed by the driver method were typically about 1 ft into the riverbottom sediments. Samplers were placed every 50 ft along the western bank of the river (fig. 2) starting about 800 ft upstream from the mapped edge of the TCE plume (fig. 1) (Sevee & Maher Engineers, Inc., 1993). The spacing was reduced to 25 ft from the area of Boiling Springs to the Maine Central Railroad trestle (fig. 2). 2 Distribution of Trichloroethylene and Geologic Controls on Contaminant Pathways near the Royal River, McKin Superfund Site Area, Gray, Maine Figure 1. Location of McKin Superfund Site, study area, extent of trichloroethylene in ground water, and potentiometric surface contours for the surficial aquifer, Gray Maine. 120 43 53'30" o