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ESTATE OF CALIFORNIA-BUSINESS,4RANSPORTATION AND HOUSING AGENCY PETE WILSON, Governor <br /> DEPARTMENT OF TRANSPORTATION 9 +tr 1 <br /> SP.O. BOX 2048 TOCKTON CA(1976 9520�E. CHARTER WAY) � (Ex��� NTAL 4������4'_Tjt o��1,w.,.•4� <br /> (209) 948-7449.. I <br /> 95 HAR 30 Pig 3; 4 <br /> March 28, 1995 <br /> i <br /> Ms. Mary Meays <br /> Environmental Health Division <br /> Public Health Services <br /> P O Box 388 <br /> Stockton, CA 95201 <br /> L <br /> Vie:`._ Stockton 1Vlairitenarice'S'tation� "` �"' '�" " �Site-`Cod"e:1-0193-1604 South B Street <br /> Stockton, CA 95201 <br /> Dear Ms. Meays: <br /> We have received your letter dated March 21, 1995 and wish to thank you for reviewing the <br /> "Site Assessment Workplan" dated March 15, 1995. We offer the following explanation for the <br /> questions you have raised in your letter. <br /> 1. We have chosen to use the 4 inch diameter monitoring well pipe size for the ease of <br /> installation due to stability, better well development and versatility in conversion to <br /> groundwater extraction and treatment well. .We feel the slight difference in cost of material <br /> between 2 and 4 inch diameter pipes is more than offset by the advantages accrued due to the <br /> use of larger size pipe. <br /> 2. The justification for five perimeter monitoring wells is that Caltrans wants to determine the <br /> vertical and lateral extent of contamination in soil andg roundwater at this site. The <br /> groundwater-gradient,at_the-site.isabout..0.0013,ftLft.towards.north/north.northeast., There__._._._ <br /> are four proposed monitoring wells at four corners and one (MW-6) to cover the north <br /> northeast spread of contamination from MW-2 direction. Furthermore, locations of these <br /> proposed monitoring wells will be adjusted in the-field during drilling when location of j <br /> underground utilities is determined. <br /> 3. Monitoring well MW-9 is being installed for possible future use as a vapor extraction well. <br /> Based on the limited lateral extent of soil with residual hydrocarbons beneath the former <br /> underground storage tanks (UST) location, one vapor extraction well will likely be capable of <br /> remediating impacted vadose zone soil. The installation of 60 feet of slotted casing is <br /> appropriate for this purpose. It is important that the screen section be installed to below the <br /> groundwater surface such that the removal of residual hydrocarbons from the capillary fringe <br /> is possible. An adequately sized blower(e.g. 5-horsepower) will be capable of providing a <br /> vertical vacuum distribution throughout the slotted interval sufficient to remove residual <br /> hydrocarbons down to the groundwater surface. <br /> I <br /> i <br /> i <br />