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CALIFORNIA GEOPHYSICAL GROUP, INC. <br /> P O Box 578341, Modesto, CA 95357 Tel (209) 527-1247 <br /> November 5, 1998 Fax (209) 527-4810 <br /> Public Health Services <br /> San Joaquin County <br /> P O Box 388 <br /> Stockton, CA 95201 <br /> ATTN Ms Carol Oz <br /> RE Work Plan Addendum to September 23, 1998 Addendum <br /> Darpetro, Inc USA Service Station <br /> 749 E Charter Way <br /> Stockton, CA <br /> Site No 1060 <br /> We have written a Work Plan dated August 21, 1998 and a Work Plan Addendum dated <br /> September 23, 9998 This addendum will cover the points raised in the EHD letter dated October <br /> 12, 1998 The points will be addressed in the same numerical order as those raised in the <br /> October letter <br /> SOIL VAPOR EXTRACTION AND AIR SPARGE SYSTEM MODIFICATION <br /> 1 The equipment used to measure the vacuum in each vapor well is a manometer made by <br /> Dwyer Instruments Inc of Michigan City, Indiana Each vapor well has a valve and fitting <br /> connected below the shut off valve This fitting and the extraction well can be isolated from the <br /> vapor extraction system by closing the shut off valve while the system is operating Therefore, <br /> each vapor well can be isolated from the operating system We have 5 vapor wells at this site and <br /> it is possible to shut off two of the wells at the same time If more than 2 wells are shut off the <br /> vacuum increases to the point where the system shuts down <br /> The manometer is connected to the fitting and the vacuum is allowed to equalize, which takes <br /> about 5 minutes The equalizing vacuum is recorded, then the manometer is allowed to return to <br /> atmospheric pressure and then connected to another well fitting <br /> As noted in the Addendum to the Work Plan, the groundwater is above the slotted casing in the <br /> monitoring wells, so the only wells we could measure the vacuum was in the Vapor Extraction <br /> wells which are shown in Figure 1 The vapor wells are called VW-Number in this plot The <br /> closest well, as noted in the Addendum, is the vapor well closest to the well being measured If <br /> VW-3 is being read then the closest vapor well is VW-1, 26 feet away The distance measured in <br /> the Addendum was 22 feet, however, it is 26 feet The new distances are shown below <br /> Well Closest Well (Feet) Vacuum in Hg" <br /> VW-1 26 230 <br /> VW-3 26 320 <br /> VW-4 27 250 <br />