Laserfiche WebLink
CHASE CHEVROLET <br /> 424 FREMONT,STOCKTON <br /> PAGE 4 <br /> The horizontal soil vapor extraction wells (horizontal wells) do not appear to be very effective in <br /> capturing the vapor from the vadose zone. HowtoEurlmteAlterrmatize C6msp Tia for <br /> Urdb7=d Storage Tang Sits prepared bythe Environmental Protection Agencylisted a range of 3 to <br /> 100 inches of water(vacuum) pressure at the extraction well head and 10 to 100 cfm(cubic feet per <br /> minute) of vapor flow rate. The PID readings and vacuum pressures of horizontal wells such as <br /> HW-1,5,and 8 were minimal Although the vapor flow rates of the horizontal wells were similar <br /> comparable to the vertical vapor extraction wells (i.e.VW-5),the PID and vacuum pressures of the <br /> horizontal wells were by far lower than the vacuum pressures of the regular vapor extraction wells. <br /> An increase in vacuum pressure at HW-5 in March 2000 reduced the air flow rate to trace level <br /> instead of improving it. The low air flow rate,vacuum pressures,and PID readings could be <br /> reflective of the low permeable soil type in the vadose zone. Compounding the problem is the length <br /> of the horizontal vapor wells (60 feet). Since the vacuum pressures and air flow rates at the well <br /> heads are already at a minimum,the vacuum pressures and vapor extraction potential probably <br /> become negligible as the length of the well away from the well head increases. <br /> PHS/EHD agrees with the work plan to install two groundwater extraction wells to evaluate the <br /> feasibility of 1) depressing the groundwater table,and 2) ground water extraction as a corrective <br /> technology to be used in addition to the existing soil vapor extraction system at the site. In addition <br /> to the goal of testing for groundwater depression,the proposed aquifer pump test must be tailored <br /> so that the rate of groundwater flow can be calculated. Presently,there are an insufficient amount of <br /> monitoring points for that at this site. MW-9 is the only monitoring point on the north portion of <br /> the property. No monitoring point exists 140 feet south,east and west of MW-9. Additional <br /> monitoring wells as observation points for the pump text are required north,northwest and <br /> northeast of MW-10. Submit a workplan addendum that will address these issues by March 21,2001. <br /> Also submit a proposal to PHS/EHD that will improve the SVE/AIS system and address the <br /> groundwater contamination on the north portion of the property by March 25,2001. <br /> If you have any questions contact JefferyWong at (209) 468-0335. <br /> Donna Heran,RENS,Director <br /> Environmental Health Division <br /> /effy Wong, Senior RENS Ivlatgare�,REFIS <br /> LOP/Site Mitigation Unit IV Supervisor <br /> Q Advanced GeoEnvironmental,Inc.—Paul Dotsun <br /> C RWQCB,Central Valley Region—Marty Hartzell <br />