ARCHIVED REPORTS_XR0005707 CASE 2

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URS Mr Mike Infurna Page 6 San Joaquin County Environmental Health Department ARCO 6020 September 29, 2003 Mass Remaining in Groundwater The area of impacted groundwater is assumed to be approximately 8,000 square feet and the TPHg/MtBE plume is assumed to be an average depth of 36 feet bgs Assuming an average depth to water of 21 feet,the plume thickness is 15 feet Assuming a porosity of 0 3,the volume of impacted groundwater is 36,000 cubic feet or 269,000 gallons Assuming an average TPHg concentration of 100 parts per billion (ppb) and an average MtBE concentration of 200 ppb(by volume)yields a volume of 0 0269 gallons of TPHg and 0 0538 gallons of MtBE Multiplying these volumes by the respective densities,there are an estimated 0 20 pounds of TPHg and 0 74 pounds of MtBE dissolved in groundwater EXPOSURE EVALUATION An exposure evaluation was prepared for this site to identify potential receptors that could be exposed to the existing soil and groundwater impacts at the site and the pathways through which exposure could occur The exposure evaluation flowchart(Figure 7)for the site was prepared using the following assumptions • The site will remain paved, preventing exposure of on-site occupational workers to soil, • Impacted soil vapors are not present below the station building, ' • Residential areas are not located in the vicinity of the site and will not be impacted by volatile organic compounds (VOCs) in soil vapors near the dispenser islands and UST complex, • There are no groundwater supply wells to the vicinity that could potentially be impacted by fuel- related compounds from the site, and • The site will not be developed into a residential property while impacted soil and groundwater remain With these assumptions, the risks to human health and the environment posed by the site involve potential exposure of on-site construction workers to impacted shallow soil and soil vapors The decreasing trend in groundwater concentrations in MW-6 suggests that the source of the groundwater impacts has been removed and that hydrocarbon concentrations in shallow soil are low CONCLUSIONS AND RECOMMENDATIONS Soil and groundwater beneath the UST complex have been impacted by petroleum hydrocarbons Petroleum hydrocarbons and MtBE have been reported in groundwater samples from MW-6, which is located immediately downgradient(north)of the UST complex Newly installed groundwater monitoring wells MW- 11, MW-12, and MW-13D have defined the vertical and lateral extent of these impacts Analyses of contaminant concentration trends in MW-6 since monitoring of this well began in February 2001 indicate that • K IWprocessll5 BP ARM0602012003 Assessment16020 Assessment&SCM 2003 doc