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S C401410l Tecluucs lac Page 9 Groundwater Monitoring Report Project No 723 2 March 21,2005 3.0 CONCLUSIONS & RECOMMENDATIONS ' Conclusions 1 Groundwater flow trends northeast to east The southerly flow direction recorded on December 4, 2000, appears to be anomalous ' 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 contained low levels of TPH-G for the second consecutive event This probably represents the edge of the advancing plume 5 Down gradient wells MW-81108/208 contain low levels of TPH-G and BTEX 6 The groundwater contaminant plume is not defined laterally down gradient of the MW- 8/108/208 W- 8/108/208 well set 7 Well set MW-9/109/209 defines the lateral extent of the plume in an up gradient direction The intermediate and deep levels (MW-109 & MW-209) both contained trace ' BTEX for the first time during the last event 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 SJC/EHD records for the adjacent Shell gas station should be reviewed to determine the degree of co-mingling of the plumes beneath the subject and adjacent Shell Oil sites Mr Jeff Wong of the San Joaquin County Environmental Health Department (SJC/EHD) supplied GTI staff with a copy of a July, 2003 groundwater monitoring report for the Shell Service Station facility across N El Dorado Street The report indicates that the wells on the northern portion of the Shell site contain low levels of TPH-G (110 - 2,500 ' ug/1) Since these monitoring wells are 50 feet deep, it is likely that the subject plume is diluted in these wells and mixing with shallower clean water The CMTTM well MW-108 is screened (4") at 52 feet bgs and it contains 6,010-ug/l TPH-G The Shell site report ' also indicates that the two wells in the southern portionn of the site (near the existing USTs) have MTBE above the laboratory detection limits The two northern wells that lie along the subject site plume axis do not contain MTBE above the laboratory reporting ' limit (0 5 ug/1) This suggests that the Shell facility has an MTBE problem not related to the subject site Further delineation of the plume in this area may be warranted 3 The lateral extent of the groundwater plume is undefined down gradient of CMTTM well ' set MW-8/1081208/308 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 as CPTrm to collect discrete groundwater samples rather than installing additional monitoring wells A work plan to accomplish this task has been sent to SJC/EHD for approval