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1 4cologko(7-echnics/Au Page 10 Groundwater Monitoring Report Project No 723 2 October 25,2005 Recommendations I 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 (1 10 - 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 infixing with shallower clean water The CMTTM well MW-108 is screened (4") at 52 feet bgs and it contains 2,360-ug/l TPH-G The Shell site report ' also indicates that the two wells in the southern portion 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-8/108/2081308 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 CPTTM to collect discrete groundwater samples rather than installing additional monitoring wells A work plan to accomplish this task has received SJC/EHD approval and is in the process of being implemented ' 4 We recommend 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-1 & 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 4.0 LIMITATIONS This report was prepared in accordance with the generally accepted standard of care and practice in effect at the time Services were rendered It should be recognized that definition ' and evaluation of environmental conditions is an inexact science and that the state or practice of environmental geology/hydrology is changing and evolving and that standards existing at the present time may change as knowledge increases and the state of the practice continues to ' improve Further, that differing subsurface soil characteristics can be experienced within a small distance and therefore cannot be known in an absolute sense All conclusions and recommendations are based on the available data and information.