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Ms.Vicki McCartney,SJCEHD August 29,2008 Work Plan for Bench-Scale Test and Injection Well Installation Project No.2046-3250-01 Parkinson's I-5 ARCO Service Station,Stockton,California Page 5 MTBE has historically been reported in all five of the water table monitoring wells, in each of the on-site grab water samples (GP-1 through GP-5), and in some of the depth-discrete grab groundwater samples collected from borings CPT-3 and CPT-4 located northeast of the site. Despite the widespread nature of the MTBE impact to groundwater, concentrations of MTBE in all five wells have significantly and steadily decreased over the historic monitoring period, as shown by a plot of MTBE over the entire 8-year monitoring period for wells MW-1 through MW-5 (Figure 6). Currently (July 2008), only wells MW-2 and MW-5 contain MTBE concentrations exceeding the secondary MCL for MTBE (370 and 100 [tg/L reported in wells (rrte" MW-2 and MW-5, respectively). The MTBE plume appears to be decreasing/shrinking over time and appears to be adequately characterized at this time. Over time, concentrations of TBA have also significantly decreased over the historic monitoring period (Figure 7), with maximum concentrations reported in most wells during the 2002/2003 timeframe. Currently, detectable TBA concentrations of 400�and 150 µg/L are reported in only wells MW-1 and MW-4, respectively. 'The TBA plume also appears to be decreasing/shrinking over time and is adequately characterized at this time. The vertical extent of groundwater impact beneath the site has been adequately characterized by the installation of well MW-6. No detectable concentrations of any COCs have been reported in samples from this well during the historic 2-year monitoring period. r PROJECT APPROACH In October 2004, Shaw submitted a Corrective Action Plan (CAP), dated October 26, 2004, recommending in-situ groundwater remediation using 03 injection. In a letter dated May 27, 2005, SJCEHD concurred with Shaw's recommendation to implement 03 injection at the site. An 03 injection system has been in operation at the adjacent former BP site since May 2004. As explained in the previous section, impact to groundwater at the site has decreased significantly since the time the CAP was prepared and approved by SJCEHD. Stratus has evaluated the geologic and hydrogeologic conditions, as well as the nature and current extent of impact to groundwater at the site, and agrees that the previously proposed and approved 03 injection remedial approach is likely a technically feasible and cost-effective choice for the site. At SJCEHD's direction, prior to implementation of the approved CAP and installation/operation of an 03 injection system at the site, Stratus proposes to conduct a bench-scale (laboratory) test for 03 using soil and groundwater collected from the site property. The objectives of bench- scale testing are to determine whether 03 and combined 03 and H2O2 (hydrogen peroxide) can reduce petroleum hydrocarbon and/or fuel-oxygenate concentrations in groundwater, to evaluate if naturally occurring metals in soil (such as As, Cr, Br, Pb, and Cu) may leach into and degrade groundwater, to evaluate if the changes in groundwater geochemistry due to 03 injection will cause valence state conversions of selected metals, and to estimate the 03 demand of soil and water at the site. In addition, if adverse effects on groundwater quality are observed during the K:\Parkinsons Hammer I-5 Arco\Work Plans\Work Plan for Ozone Bench Scale Test.doc