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� �� m � � .� '•'., rte` ._ ..i r�, � � -. �` -„ _q - �A 00 Woodward-Clyde Const.tttaitt 8.0 RECOMMENDATIONS AND CONCWSIONS The results of this investigation show that a hydrocarbon plume that originated from the Underground storage tanks at the Caltrans Shop 10 Facility at 1603 South B Street, in Stockton, California forms a concentric plume beneath the original underground storage tanks location that is migrating very slowly toward the west, south- and northwest. The plume has contaminated unsaturated zone soils in the area immediately adjacent to the underground storage tank cavity, and groundwater to a depth of about 55 feet. The southern and vertical limits of this contamination is not known. The available subsurface information is insufficient to develop the final design of a remedial treatment system, although do evaluation of the existing data indicates that groundwater extraction with discharge of the treated water to a POTW in combination with enhanced insitu biodegradation of soils is the most cost effective treatment option. WCC recommends that the following work be conducted before contaminated soils and groundwater are removed or treated at the site: 1 ) Install one additional groundwater monitoring well to a depth of about!!-' feet to the south of well MW-9 to delineate the extent of contamination h' this direction, 2 ) Install one dual cased deep zone monitoring well,be installed to a depth of about i00 feel to evaluate the vertical extent of groundwater contamination and determine the verlicai groundwater gradient, 3 } Advance and sample soils from 4 additional soil borings placed around the limits of the soil gas plume to verity the extent of unsaturated zone soil j contamination. These results can be used to evaluate the amount of soil around the tank cavity that is contaminated and required insitu treatment or removal. 3 ) Sample,and analyze soils and groundwater from these borings and wellsk and evaluate the results to determine the extent of conlamindtion, 4 ) Survey the vertical elevations of the wells and measure groundwater levels to evaluate the vertical groundwater gradient, 33