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Site History and Previous Investigations
<br />Former Chevron Service Station No. 94054
<br />and Materials was used for the risk assessment. The groundwater data collected from DP-1, DP-2,
<br />DP-3, DP-4, DP-5 and DP-6 was used to evaluate the human health risk for Residential Area 1 and
<br />the groundwater data collected from MW-1, MW-2, MW-3, MW-4 and MW-8 over the most
<br />recent four quarters, at the time, was used to evaluate the human health risk for Residential Area
<br />2 and the site. A tier I assessment of the data indicated that the levels of petroleum hydrocarbon
<br />chemicals detected in shallow groundwater did not pose a significant human health risk to
<br />residents of Residential Area 1 and levels of benzene detected in shallow groundwater
<br />exceeded the health based screening levels in the data used to evaluate Residential Area 2
<br />and the site. A tier II assessment of the data, for Residential Area 2 and the site, indicated that
<br />the levels of petroleum hydrocarbon chemicals detected in shallow groundwater did not pose a
<br />significant human health risk to residents of Residential Area 2 or potential future residents of the
<br />site (SECOR International Incorporated, 2002). The human health risk assessment concluded that
<br />petroleum hydrocarbon levels associated with the site do not pose a significant health risk to
<br />residents of the surrounding area or potential future residents of the site. The assessment
<br />recommended maintaining the operation of the DVE system until it became ineffective in
<br />removing hydrocarbon mass at the site and at that time proposing closure for the site (SECOR
<br />International Incorporated, 2002).
<br />In July 2002, Terra Vac supervised the installation of three additional sparge wells (SP-7, SP-8 and
<br />SP-9) to augment the DVE system. SP-7 was installed within the former gasoline UST pit, SP-8 was
<br />installed approximately 15 feet north of MW-4 and SP-9 was installed approximately 15 feet south
<br />of MW-4 (Terra Vac Corporation, 2002). The greatest concentrations of petroleum hydrocarbon
<br />constituents detected in the soil samples collected during the associated sparge well
<br />installations were in the soil sample collected from SP-7 at 15.5 feet bgs. TPH-GRO and BTEX were
<br />detected at concentrations of 4,500, 10, 14, 110 and 560 mg/kg, respectively, in the soil sample
<br />collected from SP-7 at 15.5. feet bgs.
<br />In January 2003, Terra Vac submitted the Fourth Quarter 2002 Operations Report for the DVE
<br />system. In the report Terra Vac states that hydrocarbon removal has become asymptotic and
<br />that operation of the system is no longer a cost effective mechanism for remediation. Through
<br />December 3, 2002, the system had removed an approximate 2,176 pounds of hydrocarbons.
<br />The report states that the system had been shut down due to winter rains, however, does not
<br />state the system had been taken offline completely (Terra Vac Corporation, 2003). A report
<br />documenting the system decommissioning is not available. The remediation wells were
<br />destroyed during the fourth quarter of 2003 (SECOR International Incorporated, 20040).
<br />In February and March 2004, SECOR supervised the advancement of ten cone penetrometer
<br />test (CPT) borings (CPT-1, CPT-2, CPT-3, CPT-4, CPT-5, CPT-5B, CPT-6, CPT-7, CPT-8 and CPT-9) and
<br />28 direct push soil borings at 14 locations (DP-1, DP-2, DP-3, DP-4, DP-5, DP-6, DP-7, DP-8, DP-9,
<br />DP-10, DP-11, DP-12, DP-13 and DP-14). One direct push boring was installed at each location to
<br />collect soil samples (DPS) and one direct push boring was installed at each location to collect
<br />groundwater samples (DPW). The direct push borings advanced during 2004 are separate from
<br />the direct push borings advanced in 2002, even though the locations DP-1, DP-2, DP-3, DP-4, DP-
<br />5 and DP-6 have the same location IDs. CPT-1, CPT-2, CPT-3, CPT-4, CPT-5, CPT-6, CPT-7, CPT-8
<br />and CPT-9 were advanced in a grid like pattern from west to east and north to south, starting
<br />with CPT-1 in the northwest corner and ending with CPT-9 in the southeast corner. CPT-5B was
<br />installed directly next to CPT-5. The DP-1, DP-2, DP-3, DP-4, DP-5, DP-6, DP-7, DP-8 and DP-9
<br />borings were advanced next to the CPT boring with the corresponding number. The DP-10, DP-
<br />11 and DP-12 borings were advanced in the former gasoline UST pit. The DP-13 and DP-14
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