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Geological rechma Inc Page 9 Groundwater Monitoring Report Project No 7212 April 24, 2006 ' aroundwater mound in the vicinity of MW-101 with divergence to the northwest and g Y southwest It appears the deepest groundwater level monitored experiences periodic ' reversals from the easterly directions exhibited in the shallower levels Additional data is needed before reaching any definitive conclusions ' 2 High levels of BTEX and TPH-G are present in the groundwater of all six on-site shallow and intermediate monitoring wells (MW-I through MW-6) 3 The main mass of contaminants has migrated to a depth of 45 to 70 ft bgs, indicating that the contaminants are mobile 4 Cross gradient deep well MW-207 is displaying an increase in contaminant concentration since October 2004 This probably represents the edge of the advancing plume ' 5 Down gradient wells MW-81108/208 contain low levels of TPH-G and BTEX as compared to the on-site wells 6 The groundwater contaminant plume is not defined laterally down gradient of the MW- ' 8/108/208 well set 7 Well set MW-9/109/209 historically defines the lateral extent of the plume in an up gradient direction However, all three aquifer levels shallow, intermediate and deep ' (MW-9, MW-109 & MW-209) currently contain TPH-G The mechanism by which the contamination has mobilized up gradient is unknown but may be a result of preferential pathways or dispersion down concentration gradient lines, and/or the low groundwater gradient slope value could lend to ineffective gradient control parameters 8 The low levels of contamination in well MW-101 and lack of contaminants in MW-201 suggests that these wells define the vertical limit of the plume beneath the site ' Recommendations 1 Continue quarterly groundwater monitoring ' 2 The lateral extent of the groundwater plume is undefined down gradient of CMTrm well set MW-811081208/308 A limited site investigation should take place to define the down ' gradient extent of the plume This should consist of direct push probe drilling methods such is CP Fm to collect discrete groundwater samples rather than installing additional monitoring wells A work plan to accomplish this task has received SJC/EHD approval ' and was completed on February 21 — 23, 2006 A report on this work is forthcoming 3 GTI recommended that interim remediation be considered before this plume migrates any further and the cost for cleanup continues to increase Interim remedial action can be ' initiated by installing a high vacuum dual phase extraction unit to address the on site contamination Extraction should be performed on the existing wells EW-1, VEW-I & VEW-2 and a new extraction well set at a deeper depth The treatment system should ' include air stripping and carbon filtration of extracted groundwater to be discharged to City of Stockton public treatment works A work plan to accomplish this task has been sent to SJC/EHD and has received approval The implementation of the work plan is in ' progress 1 1