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receptors in the microbial degradation process A decrease in microbial colonies from the upgradient to downgradient edges of the plume is interpreted to represent diminished colonies having previously affected the plume in the vicinity of MW-1 Since the plume is depleted in BTEX, the colonies have decreased given the lack of fuel "substrate " Summary of Work to Date and Rationale for Future Site Closure The site investigation has been completed by the work presented in this report and the previous reports Six soil and groundwater sampling borings have been drilled and four groundwater monitoring wells installed The underground storage tank as a contaminant source has been removed However, a limited amount of contaminated soil appears confined to the vicinity of the former underground tank pit Groundwater sampling in the street revealed that contaminants were not observed Dissolved contaminants have been observed at MW-1, however, these are highly depleted in volatile components (BTEX) and are not moving, although they are interpreted to occur in and around the capillary fringe at well MW-I Further excavation into the street will not be allowed nor approved by the City of Tracy The highly limited occurrence of contaminants and the depletion of BTEX and clayey sediments makes soil vapor extraction not feasible nor cost effective for this small area The interpreted occurrence of capillary fringe contaminants further complicates a possible groundwater and soil vapor approach to the problem Groundwater occurrence is very shallow and this area is constrained by urban commercial and roadway uses The affected groundwater is adjacent to and under 10th Street This shallow groundwater is not used for drinking or any or other use and there are no known wells in the area The overlying clayey soil is of relatively low hydraulic conductivity and acts to cap the affected area Since the volatile BTEX components are depleted and degraded, very low dissolved concentrations are observed at MW-1 The data collected in the work herein indicate that biologic processes are operating on the contaminants degrading the volatile components With the source removed, continued degradation is anticipated on the remaining BTEX However, the residual concentrations at MW-1 may take some time to degrade Therefore, Wright recommends a limited excvavation in the immediate vicinity of the former pit and MW-I to remove these reisduals It is our opinion that this would result inj very low groundwater contamination Following the excavation, we recommend groundwater monitoring for one additional quarter If wells MW-2, 3, and 4 continue to show contaminants are not detected, then Wright will request that this site be closed