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Bank of Stockton Page 14 BofS (Cuter)-Prehminary Soil and Groundwater Investigation Project No 102 2 April 8, 1997 ' accomplish this goal because of the very poor transmissivity normally associated with clay and clayey silt Highly contaminated soil is in direct contact with groundwater, and will be a contaminant source able to bleed hydrocarbons into the groundwater for ' the foreseeable future Until this contaminant source is removed, the groundwater plume will be unlikely to begin to degrade naturally ' 5.2 Recommendations ' Based on the data collected during the course of this investigation, Geo-Phase Environmental Inc makes the following recommendations ' 1 Additional work will be needed to define the vertical and horizontal extent of gasoline in the groundwater This can best be accomplished by installing groundwater monitoring wells at selected points in the area At least three shallow monitoring wells ' outside the immediate area of the release point should provide a much better understanding of the distribution of petroleum hydrocarbons in the groundwater In order to determine the vertical extent of hydrocarbon impacted groundwater, a deep ' monitoring well may also be needed In order to establish the extreme lateral limit of the plume, two or three additional soil borings for the purpose of collecting on-time- only groundwater samples would be helpful 2 In order to stop the ongoing impact to groundwater from highly contaminated soil directly beneath the tank pit, the source of contamination should be removed It is our professional opinion that the very impermeable nature of the soil will render vapor extraction techniques unlikely to be effective in adequately removing hydrocarbons ' from the soil A better option is believed to be to conduct a limited excavation By removing the soil in the immediate area of the tank pit, down to the level of the groundwater surface, the hydrocarbon source available to continue to enter the ' groundwater, will largely be eliminated The process of natural degradation should then begin to stabilize and reduce the groundwater plume ' 3 Although groundwater monitoring wells will be needed, there would seem to be little point in installing a well in the immediate area of the release point until the suspected free product, and its source, have been removed Once the source of hydrocarbons is ' reduced to more manageable levels, it would then seem reasonable to install a monitoring well in the area of the former tank pit 1 2937 Veneman Ave ,#11240 Geo-Phase Environmental Inc Phone (209) 569-0293 Modesto, CA 95356 Fax (209) 569-0293 r