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Site Background
<br />Two underground storage tanks (USTs #6 and #7) were removed from the site prior to 1979.
<br />On April 25, 1991, Stockton Service Station Equipment Company removed a 520-gallon kerosene tank
<br />(UST #5) from the north wash and lube rack area, southwest of the existing dispenser island. A single soil
<br />sample collected beneath UST #5 reportedly contained no detectable Total Petroleum Hydrocarbons
<br />(TPH)-diesel (<10 ppm) or benzene, toluene, ethyl benzene and xylenes (BTEX, <0.005 ppm). San
<br />Joaquin County Public Health Services, Environmental Health Division (EHD) issued a No Further
<br />Action letter for UST #5 on October 18, 1995.
<br />On June 15, 1995, Fisch Environmental removed a 12,000-gallon gasoline tank (UST #3) from the area
<br />immediately west of the dispenser islands, and collected soil samples for laboratory analysis from beneath
<br />the former UST. On June 29, 1995, based on the soil sample results, Fisch Environmental over-excavated
<br />the tank basin to a depth of approximately 27 feet and collected additional soil samples. The over-
<br />excavation soil samples also contained elevated concentrations of TPH-gasoline, and BTEX.
<br />During November 1998, the county public works department replaced USTs #1 and #2 with upgraded
<br />systems. Advanced GeoEnvironmental collected soil samples from beneath the USTs and underground
<br />product piping.
<br />During March 1999, Tait Environmental Management (Tait) supervised the removing of UST #4, in-place
<br />abandoning of UST #8,and soil and/or groundwater sampling from two hand-auger soil borings (HA-1
<br />and HA-2), two truck-drilled soil borings (5-3 and SB-3), and two groundwater monitoring wells (MW-1
<br />and MW-2). The well construction and groundwater monitoring data are summarized in Table C, and the
<br />groundwater analytical results are summarized in Table A. Tait documented the project results in Site
<br />Assessment Report, dated May 11, 1999.
<br />On July 13, 2000, Tait completed a water well survey for the site vicinity. Tait identified nine water wells
<br />within 2,000 feet of the site, including the municipal irrigation well at Stribley Park.
<br />On February 7, 2002, Ramage Environmental published Monitoring Well Installation and Groundwater
<br />Monitoring Report, which documented the installation of one additional monitoring well (MW-3), which
<br />is shown on Figure 2. The well construction details and groundwater monitoring data are summarized in
<br />Table C, and the groundwater analytical results are included in Table A.
<br />On March 7, 2003, Ramage Environmental published Site Conceptual Model and Work Plan, Additional
<br />Site Investigation and Remedial Feasibility Testing. On July 14, 2003, EHD approved the work plan.
<br />Also on March 7, 2003, Ramage Environmental installed a passive free product skimmer in MW- 1, and
<br />initiated periodic free product removal site visits. Between March 7, 2003 and December 12, 2006, after
<br />54 site visits, Ramage Environmental removed approximately 221.45 ounces of accumulated free product
<br />from the skimmer, and apparently eliminated the free product from the groundwater surface.
<br />On December 1, 2003, to further define the plume of impacted groundwater, Ramage Environmental
<br />installed three additional on-site groundwater monitoring wells (MW-4, MW-5 and MW-6). The well
<br />locations are shown on Figure 2, Appendix A, groundwater monitoring data are included in Table A,
<br />Appendix B, and the well construction details are included in Table C, Appendix B. Ramage
<br />Environmental documented the results of the investigation in Monitoring Well Installation and
<br />Groundwater Monitoring Report, dated January 16, 2004.
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