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N%Tc 1117 Lone Paim Avenue, Suite Modesto, CA 953511 www.atcassociates.com 209-579-2221 ' ASS O C I ATE S I N C . Fax 209-579-2225 Environmental, Geotechnical and Materials Engineers of benzene, 3 ,600 micrograms per liter (gg/L) and 2,600 gg/L, were detected in the groundwater samples collected from MWl and MW8 , respectively. TPHg was detected in the groundwater samples collected from MWl , MW2, MW3 , MW5 , MW6, MW8 , and MW9 . The highest concentrations of TPHg, 120,000 µg/L and 53 ,000 pg/L, were detected in the groundwater samples collected from MWl and MW8 , respectively. MTBE was detected in the groundwater samples collected from MW2 and MW3 at concentrations of 19 gg/L and 19 lig/L, respectively. 1 ,2-DCA was detected in the groundwater samples collected from MWl through MW6, MW8, MW9 , and MW12 at concentrations ranging from 1 . 1 µg/L to 540 µg/L. The highest concentration of 1 ,2-DCA was detected in the groundwater sample collected from MW3 . TBA was detected in the groundwater samples collected from MW2 and MW3 at concentrations of 47 µg/L and 29 µg/L, respectively. Analytical results of groundwater samples are summarized in Table 3 . Laboratory data sheets and chain-of-custody documentation are contained in Appendix E. Isoconcentration maps depicting benzene, MTBE, and 1 ,2-DCA concentrations in the groundwater samples collected from the shallow zone during the fourth quarter of 2006 are shown on Figures 6, 7, and 8 , respectively. Isoconcentration maps depicting benzene and 1 ,2- DCA concentrations in the groundwater samples collected from the intermediate zone during the fourth quarter of 2006 are shown on Figures 9 and 10, respectively. 5.0 CONCLUSIONS The laboratory analytical results of groundwater samples collected from the monitoring wells in November 2006 indicate the lateral extent of petroleum impacted groundwater in the shallow zone appears to be defined in all directions except to the south. MTBE was detected in the groundwater sample collected from MW3 and 1 ,2-DCA was detected in the groundwater samples collected from MW3 and MW9 . These wells are located in the southern portion of the site. Monitoring wells MW 1 and MW8 continue to be the most impacted wells . The lateral extent of petroleum impacted groundwater in the intermediate zone is not well defined. The apparent groundwater flow direction in the intermediate zone as indicated by depth to water measurements collected on December 7, 2006, is to the north. This flow direction is inconsistent with historical data that generally indicates a southerly groundwater flow direction in this zone. Monitoring wells MW4 and MW12 are slightly impacted, and MW5 and MW6 continue to be the most impacted wells. Petroleum - hydrocarbon have not been detected in groundwater samples collected from MW7 locate�a +a the former source area since December 20055 and were not detected in the groundwater sample collected from MW15 located in the southern portion of the site and potentially downgradient of the former source area. Monitoring wells MW7 and MW15 are screened from approximately 116 to 121 feet bgs and 141 to 151 feet bgs, respectively. The laboratory data associated with groundwater samples collected from these wells indicates the vertical extent of petroleum contamination in groundwater lies between 90 and 116 feet bgs. Si\Env iromcntal\28349treports\sum.doc 6 ATC Associates Inc.