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= k Mr Harlin Knoll R 10 N ' ISSan Joaquin County Environmental Health Division 20 February 2001 Page 5 contain detectable concentrations of TPHg, BTEX, and MTBE Analytical results from this and the three previous rounds of groundwater sampling are summarized in Table 1 Details of the groundwater sampling procedures are included in Attachment 3 Total Residual Mass Calculations The volume and mass of impacted soil and groundwater were calculated to determine p the amount of residual hydrocarbon as TPHg The approach included overestimating Ithe area of impact and then calculating volumes of soil and groundwater in the vicinity of well MW-2, the only location with measurable TPHg concentrations Analytical data used included TPHg concentrations from Orion's soil samples collected during the installation of MW-2 and the results from Orion's recent groundwater sampling of MW- 2 The calculations detai 4 October 2000 report showed a reported total residual mass estimated t 026 pounds This residual mass would not significantly change using the above parameters for these groundwater sampling results r Summary of the Site Monitoring Program • As directed by the SJCEHD, BFS has complied with the agency request of 4 January 2001 to collect an additional round of groundwater sampling at the site TPHg decreased from 440 µg/1 from the previous groundwater sampling results in MW-2 to 170 µg/I for this round MTBE changed from 7 7 µg/I from the previous groundwater sampling results in MW-1 to 42 µg/1 for this round Analytical data from this effort compared to previous investigation results show the concentration of hydrocarbon constituents in the soils at the site are generally below regulatory goals or standards Groundwater monitoring wells were placed at locations agreed to by the agency to determine the extent of soil and groundwater impacts An estimate of contaminant mass in soil and groundwater is calculated at approximately 1/4 pound of gasoline constituents remaining at the site As presented above, soil and groundwater impacts from previous site operations are limited in extent and below regulatory objectives For the constituents found, degradation by natural attenuation from the bacterial action in moist, shallow sods is likely The site data were evaluated against the CWRCB guidance document, Guidelines for Investigation and Cleanup of MTBE and Other Ether-based Oxygenates, dated 27 March 2000 The sae's MTBE concentrations (below 50 pg/I in groundwater) and its distance I (greater than 2,000 feet) from a drinking water well indicate that a low risk site classification should be assigned Therefore, BFS believes that the site assessment objectives have been met and the site should be formally approved as closed Following written acceptance of site closure, BFS will close the monitoring wells and . restore the surfaces around the well boxes to their previous condition S\BFS\STOCKTON\LTR\LTR007 DOC 2/20/01