Background
The site is located at 12 Swanton Street in Winchester, MA (UTM coordinates 4702910 mN, 324875 mE (Figure 1). The site is currently occupied by an automotive repair and used car sales facility. The site formerly dispensed gasoline and diesel fuel. The property consists of a 1,806 square foot building on a 0.31-acre lot (1) (Figure 2). The site is entirely asphalt paved except for landscaped islands located in the northeast and southwest of the property and a smaller landscaped island located in the northwest of the site. The site building is connected to municipal water and sanitary sewer. Nearby residents are also on the municipal water and sanitary system (2). The site is located at an elevation of approximately 49 feet above Mean Sea Level (based upon the National Geodetic Vertical Datum of 1929). The topography is relatively flat with a mild grade from east to west. Regionally, the topography to the east rises sharply in elevation culminating in the Middlesex Fells Reservation located approximately 1,224 feet east of the site. The area to the west slopes gently to the Aberjona River approximately 2,021 feet west of the site (Figure 1).
In May 1999, six (6) underground storage tanks (USTs) were removed from the site under a permit issued by the Winchester Fire Department. The USTs consisted of three (3) gasoline USTs (4,000-gallon, 3,000-gallon, and 2,000-gallon), one (1) 3,000-gallon diesel UST, one (1) 500-gallon waste oil UST and one (1) 500-gallon heating oil UST.
On July 8, 1999, the MADEP Northeast Regional Office was notified of a 72-hour reportable condition at the site when a soil headspace reading exceeding 100 parts per million (ppm) was obtained from soil samples collected from within 10 feet of an underground storage tank (UST) outer wall. Approximately 20 cubic yards of soil were stockpiled when the six (6) USTs were removed from the site in May 1999. The four (4) gasoline USTs, the dispensing island, and the single 250-gallon waste oil UST were located in front of the site building (Figure 2). The 500- gallon heating oil UST was located at the rear of the building. The MADEP issued a Notice of Responsibility (NOR) dated November 19, 1999 to Bossi Realty Trust for a gasoline release associated with the UST system.
Regional Geology and Hydrogeology
Based on the Webb Engineering and REMSERV, Inc. observations, the site geology from ground surface ranges from coarse to fine sand to silty fine sand with some to little silt, some to little gravel, and little to trace clay. The Webb site investigation characterized the site soils as silty fine sand and gravel fill to depths of approximately six (6) to eight (8) feet overlying dense glacial till (6). REMSERV, Inc. observed a layer of tan medium to fine sand with little silt and little coarse sand from five (5) to 10 feet in B101. The same soil type exhibited little gravel and trace clay at a depth of eight (8) to 10 feet in this boring. Soils below 13 feet in all REMSERV, Inc. borings consisted of brown to black coarse to fine sand and silty sand with little to trace clay and some to trace gravel.
The site is located in the Mystic River Drainage Basin (8). Three (3) water supply reservoirs that service the Town of Winchester within a mile east of the site are topographically and hydrogelogically upgradient of the site. Storm water from the site is drained through catch basins located on Swanton Street, which discharge to the Aberjona River located west of the site.