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Geological rerkit&s Inc Page 9 Groundwater Monitoring Report Project No 723 2 June 20, 2005 ❑ Figures 10— 12 also show that the spring increase in groundwater elevation for the event ' is almost twice that evident in the preceding four years ❑ Oxidation-Reduction Potential (ORP) and Dissolved Oxygen (DO) measurements (Table 7) show that most of the wells monitored are screened in areas of the groundwater plume ' that are affected by the biodegradation reactions occurring within the plume The low to highly negative ORP values indicate that the wells are within the halo of reaction depleted water caused by biodegradation of the contaminants The low DO values of the ' wells also reflected this condition ' 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 The deepest set of CMTTM wells displayed a southwesterly gradient for the event and this anomalous to the other aquifer intervals ' Since this is only the second time the interval has been monitored additional data is needed before reaching any 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-1 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 third consecutive event This probably represents the edge of the advancing plume ' 5 Down gradient wells MW-8/108/208 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 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 once in the past ' 8 The low levels of contamination in well MW-101 and lack of contaminants in MW-201 suggests that these wells define the vertical linut 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 1