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' Grob ical T¢ckxicsl�u Page 4 g g Groundwater Monitoring Report Project No 723 2 February 28,2005 ' 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 CMTrM 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 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 tthe 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 CMTrm well MW-8/108/108/308 A limned 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 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 The placement of these components at the site will be problematic due to limited space 1 4.0 LIMITATIONS 1 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. The tasks proposed and completed during this project were reviewed and approved by the ' local regulatory agency for compliance with the law No warranty, expressed or implied, is made 1 1 1