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Jf <br /> a Jt <br /> 08 October 1997 <br /> AGE-NC Project No 95-0128 <br /> Page 2 of 8 <br /> soil has been delineated, however the lateral extent of hydrocarbon-impacted ground water has not <br /> been delineated to the satisfaction of the PHS-EHD Data collected from the site during the <br /> investigation of the former UST site indicated that hydrocarbon-impacted soil is present in the vadose <br /> zone from depths of approximately 5 to 55 feet bsg Rising ground water elevation has greatly <br /> exceeded the screened intervals of monitoring wells MW-1, MW-2 and MW-3, rendering data <br /> collected form these wells suspect Hydrocarbon-iinpacted ground water has been encountered north <br /> and east of the property in the down-gradient direction from the suspected former source of the <br /> unauthorized hydrocarbon release, but is not well constrained in these directions These factors <br /> necessitate installation of the additional ground water monitoring wells <br /> A vapor extraction test was conducted in December 1993 Since that time, ground water has risen <br /> from approximately 55 feet bsg to as high as 30 feet bsg, reducing the vertical extent of the vadose <br /> zone open to the screened interval of the vapor extraction well The rise of ground water elevation <br /> has also eliminated most of the coarse-grained soil from the vadose zone, leaving predominantly fine- <br /> grained soil in contact with the reduced open screened interval of the vapor extraction well These <br /> changes can potentially drastically reduce the zone of influence of the vapor extraction well(s) and <br /> reduce the concentrations of hydrocarbon vapor recovered by the extraction process For this reason, <br />. plans for an updated vapor extraction test are included in this addendum work plan, and will be <br /> performed if approved <br /> 3.0. SCOPE OF WORK PLAN ADDENDUM <br /> This work plan details procedures for the installation of five additional ground water monitoring wells <br /> (one on-site well and four off-site), seven soil vapor extraction (SVE) wells, six in-situ air sparging <br /> (IAS) wells, and conduction of a vapor extraction test and an air sparging test The vapor extraction <br /> wells will be installed on the site The additional ground water monitoring wells will be installed <br /> south, north and east of the known portions of the dissolved hydrocarbon plume One well will be <br /> required on the McDonald property north of the site, three in the City of Stockton right-of-way on <br /> Pacific Avenue and one on the site <br /> i U11 LLL1VgV 1 11114"% SvJLA 01 iUylrLilVLr <br /> Notify the PHS-EHD 48 hours prior to commencement of any work Secure proper city/county soil <br /> boring permits Boring locations will be marked at least 48 hours in advance for an Underground <br /> Service Alert (USA) check and marking of underground utilities or pipes Soil borings will be <br /> advanced utilizing a truck-mounted, hollow-stem drill ng equipped with 8 25-inch or 10-inch hollow- <br /> stem augers Advanced 8 25-inch borings to depths specified in Sections 4 1 , 4 2 and 4 3 , at <br />. planned locations for single-completed soil vapor extraction wells, in-situ air-sparging points and <br /> Advanced GeoEmironmenW,Inc <br />