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i c - Commingled Plume#35 S E C O R Chevron 9-4275/ARCO#2133 Site Conceptual Model September 1,2006 Page 3 In 2001, a CPT investigation, consisting of five borings (CPT -1 through CPT-5) along the eastern side of Plymouth Road, was conducted to assess soil stratigraphy and collect discrete groundwater samples (Figure 2). Groundwater samples were collected from the identified water-bearing zone or depth intervals showing distinct pore pressure. Three to four depth- discrete groundwater samples per boring were submitted for analysis. Groundwater samples were generally collected at depths of 10- to 12- feet below ground surface (bgs), 24 to 25 feet bgs, 35 to 40 feet bgs, and 68 to 78 feet bgs. TPHg was detected in two of the 17 groundwater samples at concentrations of 73 micrograms per liter(µg/L) (CPT-5 at 37 feet bgs) and 130 µg/L (CPT-3 at 40 feet bgs). Methyl tert butyl ether(MtBE)was detected in six of the 17 groundwater samples at concentrations ranging from 0.59 µg/L (CPT-4 at 10 feet bgs) to 2.6 µg/L (CPT-5 at 37 feet bgs). The site was demolished and six USTs were removed in June 2005. Two new USTs (30,000 and 15,000 gallons), dispenser islands, and a station building were constructed in June 2005. In preparation for site demolition, seven onsite groundwater monitoring wells (MW-1, MW-2, MW-3, MW-4, MW-5, MW-7, and MW-9) and ten SVE/AS wells (VW-1, GM-1, GM-2, GM-3, GM-4, GM-5, GM-6, GM-7, GM-8 and GM-9) were destroyed in April and May 2005. Replacement monitoring wells were installed in February 2006 following completion of the station rebuild, as proposed in SECOR's Work Plan for Well Destruction and UST Removal and the Work Plan for Soil and Groundwater Delineation and Addendum to Work Plan for Well Destruction and UST Removal (SECOR, 2005). Locations of the replacement wells are shown on Figure 2. Soil samples were collected during the excavation of the USTs and the results are discussed in Section 3. Groundwater monitoring has been performed at the Chevron site since March 1989. Tables 1 and 3 present the current and historical analytical data. Groundwater flow has generally been to the north-northeast at a hydraulic gradient of 0.003 foot per foot (fUft) (Table 5). The second quarter 2006 groundwater elevation and hydrocarbon concentrations are shown on Figures 5 and 6, respectively. Well construction details are presented in Table 6. Historical soil analytical data for the Chevron 9-4275 facility are presented in Table 7. 2.2.3. ARCO 2133 Facility Four USTs and associated product lines and dispensers are present at the site. A release from a piping leak was discovered in 1984. A release of approximately 2,000 gallons of gasoline was reported in May 1989, and an unknown quantity of fuel was released at the northern dispenser island in 1995. Since discovery of the piping leak in 1984, 21 groundwater monitoring wells (E-1 through E-12, BHD-1, BHD-2, PR-1, PR-2, and MW-1 through MW-5), 12 soil vapor extraction (SVE) wells (V-1 through V-12), and nine air sparge (AS)wells (S-1 through S-9) have been installed. A site plan of the ARCO facility is presented on Figure 4. In 1998, during the installation of a pay quick machine, a contractor at the site broke a fiberglass product line beneath the southern dispenser island. The broken line was repaired immediately and approximately three cubic yards of soil were excavated from beneath the broken product line. Soil samples collected after the excavation of the soil contained low levels of petroleum hydrocarbons. 94275-ARCO 2133 SCM 9-2006 3