ARCHIVED REPORTS XR0009904 - Laserfiche WebLink

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Corrective Action Plan Page 11 Due to the high permeability of the soils (particularly the loose sands) and the extent of the required excavation, it was estimated that to de-water the site in order to permit controlled excavation of all affected soil and to backfill the pit in the dry would require groundwater pumping at a rate on the order of 300 gpm (gallons per minute) Even if the obvious risk to neighboring un-reinforced masonry structures due to removal of fine material from the sand strata by piping were discounted, such a rate of steady pumping would produce a volume of contaminated water that could not be contained in temporary on-site storage and, even if continuous treatment with discharge directly to the sewers were economically feasible, such an operation would not be permissible under the terms of CRWQCB Order No 92-150 Due to the restrictions that the site-specific conditions impose on an excavation designed to remove all traces of contanunated soil from the sub-surface, at the time that the original corrective action plan was prepared, pumping of groundwater from an array of recovery wells and treatment of the recovered water by air-stripping and carbon adsorption was judged to be the preferred means of remediating the site The original corrective action plan was submitted to the SJCPHS for review in June 1994 Following their review, SJCPHS staff expressed concern that groundwater extraction and treatment methods, while effective in providing hydraulic control and capable of extracting large volumes of contaminated groundwater, are expensive to install and operate and may be inefficient with respect to their ability to remediate the mass of hydrocarbons affecting the subsurface (Ref 4) They also noted, as might have been anticipated from the evaluations performed in preparing the original corrective action plan for the 152 East 11 th Street property, that studies of the efficacy of a soil vapor extraction system installed at a similar site in Tracy, had proven to be inconclusive It has been the SJCPHS staffs experience that removal of contaminated soil and aggressive pumping of groundwater from the resulting excavation has been the only remedial procedure successfully used in the Tracy area to date The SJCPHS staff also expressed concern that shallow soil between the ground surface and the water table may be contaminated in the vicinity the former dispensing-pump island that remains intact on the 152 East 11th Street property (see Figure 2 for location) The possibility of shallow soil contamination in that area is suggested by the distribution of concentrations of components of hydrocarbons in samples recovered from borings drilled at nearby locations (see Figure 4) However, because no borings have been drilled immediately adjacent to the pump island, conditions in the specific area of concern are not precisely known The SJCPHS staff also noted that, if piping associated with the former underground storage tanks was not removed when the tanks were exhumed, it should be removed as part of the currently planned remediation