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ffMVVAA Working To Restore Nature Report on Vapor Extraction Test January 29, 1993 i Coca-Cola Former Distribution Facility, Stockton, California 5400601 Pro ected Gasoline Removal Rates Ideally, a well designed and placed vapor extraction system should only affect the areas of concern, both laterally and vertically It is neither practical nor cost-effective to induce a significant vacuum or an extraction flow rate over an area that includes clean soil The primary area of concern for this site is within a 20 foot radius of the former tank pit, and + down to a depth of about 25 feet Three wells are proposed to be brought on line for the vapor extraction system The estimated effective radius of influence for these wells is shown } in Plate 6, and covers the lateral area of petroleum hydrocarbon impacted soil shown in the TPHg concentration contour map (Plate 4) Wells VEW-2, VEW-3, and VEW-7 are recommended to be brought on line because they are in, or near, the former tank pit, and because all three wells, at a minimum, are screened in the shallow zone VEW-2 is screened from 8-28 feet below grade, VEW-3 from 8-68 feet, and VEW-7 from 7 5-47 5 feet Because the screened intervals of wells VEW-3 and VEW-8 are substantially deeper than 4 the vertical extent of the area of concern, it is recommended that these two wells be grouted from the bottom of the well up to 30 feet below ground surface The wells should be grouted using a tremied mixture of neat cement and 5% bentonite, or equivalent as approved by the regulatory agencies This grouting will focus the vapor extraction in the vertical area of concern, which is a depth of about 25 feet below grade Although relatively high vapor concentrations were detected in well VEW-4 (19,000 mg/m3), well VEW-4 is not recommended to be added to the VES for several reasons A review of soil boring analytical results for B-17/VEW-4, summarized in Tables A-1 and A-2 in Appendix A, indicate nondetectable TPHg, trace amounts of BTEX and TPHd, and nondetectable PNAs in the soil in this boring The soil may not be the source of the high vapor concentrations extracted from VEW-4 VEW-4 is screened in the deeper zone (43-68 4 feet below grade), and the petroleum hydrocarbon impacted soil is primarily present only down to a depth of 25 feet This appears to indicate that vapors may be originating to a Iarge extent from the groundwater table The proposed vapor system needs to be designed and operated such that subsurface vapors are maintained at the source (especially since the contamination is highly localized around the tank pit), and vapors should not be drawn down to the groundwater or spread laterally away from the source For these reasons, only the three above-named wells VEW-2, VEW-3, and VEW-7 are recommended to be part of the proposed VES ' The amount of TPHg in the subsurface soil in the vicinity of the former underground gasoline storage tank was previously estimated at about 9,560 pounds of TPHg (USTEC, September 1991) Assuming a density of 50 lb TPHg/cubic foot, this is about 1,430 gallons of gasoline in the subsurface soil Depending upon the type of soil at the site, about 90% 14 � I i