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6(YA 1117 Lone Palm Avenue, Suite B Modesto, CA 95351 www.atcassociates.com 209-579-2221 AS s o c 1 A T e s I N C . Fax 209-579-2225 Environmental,Geotechnical and Materials Engineers CONCLUSIONS The laboratory analytical results of the groundwater samples collected from all the monitoring wells in March 2006 are generally consistent with recent historical analytical results of groundwater samples collected from these wells. MW1 and MWS located in the central portion of the site are the most impacted wells. The concentrations of benzene, ethyl benzene, 1,2-DCA, and TPI-Ig detected in the groundwater sample collected from MW1 increased with respect to the previous sampling event. The toluene and xylenes concentrations in this sample were unchanged with respect to the previous sampling event. The concentrations of toluene, ethyl benzene, xylenes, and TPI-Ig detected in the groundwater sample collected from MW8 increased with respect to the previous sampling event. The benzene and 1,2-DCA concentrations in this sample decreased with respect to the previous sampling event. Petroleum hydrocarbons were not detected in the groundwater sample collected from deep well MW7. This represents the first time petroleum hydrocarbons have not been detected in samples collected from this well and suggests that the vertical extent of impacted groundwater in the vicinity of this well is close to 120 feet bgs. The shallow groundwater plume appears to be adequately defined to the west and north in the vicinities of MW10 and MW11R, but is not defined to the east and south. The intermediate groundwater plume appears to be generally defined to the northwest in the vicinity of MW 12, but not • well defined in other directions. Groundwater flow in the shallow zone was variable with eastward and northward components flowing toward the center of the site. Groundwater flow in the intermediate zone was towards the south. Additional investigation to the east and south is necessary to evaluate the extent of petroleum hydrocarbon impact. ATC is in the process of implementing the Subsurface Investigation Workplan, dated October 31, 2005. Please contact our office at (209) 579-2221 if you have any questions or comments. Respectfully submitted, �p14AL. GFpI ATC Associates Inc. x000 Q HIAA -4 Todd Hafher, P.G. CA Professional Geologist No. 8090pa'z"t Pr2 David W. Ashcom, P.E. CA RCE No. 22868 cc; Mr. & Mrs. Ernest Foppiano Mr. Jim Barton, CRWQCB 5:lenviromenta11283491reports12006 IQMR.doc 4 ATC Associates Inc.