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a) Is there evidence of too much air flow through the soil such that travel times are too small, or <br /> that diffusion limitations hamper the effectiveness of the high extraction rates? (If extracting air <br /> at higher rates does not increase the mass removal rate, the system may be moving more air than <br /> necessary) <br /> b) Is there evidence of short-circuiting along the well casing, through nearby utility corridors, or <br /> through soil fractures or other subsurface features? Consider well replacement or relocation <br /> e) Have the VOC concentrations in the extracted gas (in combined influent or from most wells) <br /> approached an asymptotic concentration value9 This may be caused by diffusion limitations, <br /> continuing source, or poor well placement Consider whether reduced flows, system pulsing, <br /> additional wells thermally enhanced SVE, or bioventing might remove additional contaminant <br /> mass <br /> d) Are there portions of the target zone in which contaminant concentrations have failed to <br /> decline in response to SVE? This may be due to inadequate air flow in that portion of the target <br /> zone, diffusion limitations, high soil moisture content perhaps to due to surface irrigation, or a <br /> continued source of contaminants in that area Consider increasing air extraction rates from <br /> certain wells or the entire system, adding wells in areas of inadequate extraction, limiting <br /> irrigation, or excavating hot spot soils <br /> e) Are extremely high concentrations of VOCs, at or near explosive levels, present in the <br /> extracted gas? This may be due to a continuing source, floating product, high levels of residual <br /> contaminants, or accumulation of methane Consider adding dilution air, reducing the flow rate <br /> from "hot spot"wells, or replacing the SVE off-gas treatment system with a flare or internal <br /> combustion engine (ICE) treatment system <br /> fJ Do the VOC concentrations in the extracted gas vary significantly over time? This maybe <br /> due to ground water fluctuations, soil moisture variations due to precipitation, or a periodic <br /> continuing source Consider controlling groundwater levels, installing a surface cover, or other <br /> source removal alternatives <br /> g) Have the total extraction rates failed to reach the expected design rates or the extraction rates <br /> needed for efficient operation? This may be due to poor well installation, improper well design, <br /> 06/07/99 <br /> Page 6 of 8 <br />