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27 June 1996 <br /> AGE-NC Project No. 95-0118 <br /> Page 17 of 23 <br /> 8.1.2. BASIC SYSTEM DESIGN <br /> Injection wells are screened within the saturated zone and normally have short screened intervals, <br /> perhaps 5 to 10 feet. Since most of the air escapes from the upper part of the interval where head <br /> pressure is the least, little is gained by increasing the length of the screen interval. <br /> The radius of influence of a sparging well is dependent on site lithology, the depth of the sparging <br /> well and the length of the screened interval. Sandy soil tends to produce a larger radius of influence; <br /> clayey soil tends to produce a more limited radius of influence. <br /> y'1 Unless injection wells are properly located, channeling of 'air flow streams may occur and <br /> contamination pockets may be bypassed..This problem is more common where fine-grained strata <br /> with low permeability are interbedded with.coarse-grained strata in the saturated zone. Air flow is <br /> retarded within the fine-grained strata, and hydrocarbon recovery is poor. <br /> r^ <br /> i Two of the wells at the site were installed for the specific purpose of its-situ air sparging. Use of this <br /> technology could possibly-require the installation of 2 or 3 additional IAS injection points, screened <br /> at depths between 45 and 50 feet bsg. Considering the sand-rich soil at the site and the relatively <br /> favorable results of the SVE pilot test,we estimate that all the wells could create a cone of influence <br /> of approximately 35 feet under adequate vacuum. The IAS system must be combined with vapor <br /> k_ extraction to capture vapor stripped from the ground water. <br /> r# 8.1.3. MONITORING ACTIVITIES <br /> ka <br /> One of the potential problems associated with air sparging is lateral spreading of the contaminant <br /> it= plume. This is due either to mounding of ground water above injection points or lateral air flow away <br /> from sparging wells. To detect this problem, ground water elevations in monitoring wells and <br /> extraction wells (if appropriate) should be monitored on a monthly basis (minimum) to detect <br /> mounding and to be monitored for organic vapor. <br /> ` = 8.1.4. FEASIBILITY <br /> `i Remediation of small ground water plumes require the installation of a few injection and extraction <br /> wells. Air injection and extraction rates must be balanced to prevent the forced migration of <br /> contaminants. Sufficient piping must be installed to injection and extraction wells. As in vapor <br /> extraction, air sparging is not effective in the remediation of longer-chain hydrocarbons. <br /> jData from the air sparging pilot test indicted that movement of hydrocarbon vapors to the extraction <br /> . J <br /> i`r <br />