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a S E C 0 R Ms. Lori Duncan ARCO Station 4493 Site Closure Report October 31,2006 Page 5 there will be less GRO present, but the remaining amount will also dissipate more slowly due to ` lower partitioning coefficients and solubility. Site data indicates that this decrease is taking place, and that further plume growth is unlikely due to the evidence that GRO concentrations are already decreasing. Examination of the geology of the plume centerline indicates that there is a further barrier to contaminant migration in the form of a thick clay lens extending east from approximately well MW- 9M past the MW-11 cluster. This lens of low-permeakiility material tends to slow plume migration ' considerably and is likely a reason why the plume did not extend farther initially. Thus, it is extremely unlikely that the current plume will grow any larger; rather,the existing plume will continue to shrink due to natural attenuation until standards areE achieved. SENSITIVE RECEPTORS Low concentrations of residual and dissolved petroleum hydrocarbons are situated beneath an open ground surface. Normal surface air movement is likely to dilute and remove impacted soil vapors from the site before concentrations reach risk levels. The area surrounding the site utilizes a municipal water,source supplied by California Water Service Company(CWSC). SECOR contacted CWSC on May 5,2006,to verify that municipal water supply wells were not situated in the immediate site vicinity. According to the CWSC web site,"Cal Water's Stockton District was formed in 1926 with the purchase of the water system from Pacific Gas and Electric Company. To meet the needs of our 42,0001 customers, we use a combination of local groundwater and water purchased from the Stockton East Water District,which is imported from the Stanislaus and Calaveras Rivers. The Stockton system pumps 28 million gallons of water per day and includes 56 wells, 17 booster pumps, 12 storage tanks,and one imported water connection.Cal Water proactively maintains and upgrades its facilities to ensure a reliable, high-quality supply' (http://www.ralwater.com/). According to the sensitive receptor survey conducted in 2004,eightwells are located within a 2,000- foot radius of the site. The records included information about one municipal well, one "air conditioner"well, one irrigation well, and five wells of unknown use. The closest identified well is located approximately 1,000 feet southeast of the site and is located in the basement of the City of Stockton Police Department. This well is screened between 170 feet and 215 feet deep. Copies of these reports, and a figure illustrating approximate well locations, are provided in Attachment G. During preparation of the sensitive receptor survey in 2004, URS contacted City of Stockton Municipal Utility District(CSMUD)to ascertain whether they operated any wells in the vicinity of the ARCO site. According to Mr. Larry Deimler with CSMUD, there are no city wells operating within 2,000-feet of the site. I URS contacted the Stockton office of CWSC in 2004 toE determine whether they operated any wells in the vicinity of the ARCO site. According to Mr. Eric Mar with CWS there are no active wells with 2,000 feet of the site, and groundwater in the vicinity is not usable due to impacted water quality. On December 6,2004, URS performed a 500-foot reconnaissance of the site to identify andlorverify the location of wells, water bodies, and schools. No mEunicipal, domestic, or irrigations wells were I