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Groundwater Monitoring Well Installation and Ozone Microsparge System Installation and Startup Report— ConocoPhillips(76)Service Station No. 11195,Lathrop,California May 20,2003 7.0 CONCLUSIONS AND RECOMMENDATIONS This work was performed to facilitate the installation of an ozone microsparging remediation system to address hydrocarbon impacted groundwater at the site. The ozone microsparging system will be inspected on a monthly schedule to conduct O&M activities. The system operation will be enhanced and maintained as needed. Progress sampling will be conducted monthly on selected wells and will be coordinated with the regular scheduled quarterly monitoring and sampling program at the site. After the first six months of system operation(3`d quarter, 2003), a remedial system progress report will be issued. Future progress reports will be prepared on a semi-annual basis. The remedial system effectiveness will be evaluated after six months and recommendations if any, will be made at that time. The four new off-site monitoring wells were installed in an effort to delineate the downgradient extent of the dissolved hydrocarbon plume. Based on the analytical results of the groundwater samples collected from the new wells,the extent of the dissolved plume has been delineated. Three of the four new wells were reported to be free of dissolved hydrocarbon concentrations. MtBE was only detected in well MW-19, at a concentration of 200 ppb. GR recommends that the new monitoring wells be added to the quarterly groundwater monitoring and sampling program, to evaluate the groundwater conditions downgradient of the site. MtBE was also identified in a soil sample collected from well boring MW-19 at a depth of 10 feet bgs. Based on the static groundwater depth in well MW-19 at approximately 9 feet bgs, it appears that the soil sample may have been collected from the saturated zone. Laboratory analysis of a saturated soil sample may lead to a result where concentrations of hydrocarbons originating in the groundwater are reported as being present in the soil. GR believes that the presence of MtBE in soil at the location of MW-19 is highly unlikely, and GR considers the reported result to be inconsistent with observed conditions at and in the vicinity of the site. GR recommends that four quarters (1 year)of groundwater analytical results be used to evaluate the dissolved hydrocarbon concentrations beneath and in the vicinity of the site. The additional groundwater sampling events will be used to gain a more accurate understanding of the groundwater flow direction and gradient. Following one year of quarterly groundwater sampling and system operation, it will be determined if additional delineation is necessary. 140193.14-2 9