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aerobic cometabolism involves co-injection of an electron donor (methane, propane, phenol, <br /> toluene) and an electron acceptor (oxygen). While aerobic cometabolism was discussed as an <br /> emerging in situ bioremediation technology in guidance documents published in late 1990s <br /> and 2000 (EPA, 1998; ITRC 1998; EPA, 2000), recent in situ bioremediation guidance <br /> documents have focused on reductive dechlorination, an anaerobic process (AFCEE, 2004; <br /> ITRC, 2005; ITRC, 2007; ITRC, 2008), which is more commonly used for chlorinated solvent <br /> remediation. Commercial application of aerobic cometabolic treatment for full scale <br /> groundwater remediation has been very limited, probably because of practical limitations of <br /> injecting oxygen and a cometabolic inducer. <br /> 2.3 DESIGN CONSIDERATIONS <br /> A primary design consideration for biosparging is the spacing (horizontal and vertical) of the <br /> sparge wells and the injection flow rates. Injection rates and well spacing should be designed <br /> to assure an adequate density of air channels because of the diffusion limiting processes that <br /> control COC removal rates from the saturated zones between the air channels. If the air <br /> channel density is too low, a large portion of the target treatment volume could be left <br /> untreated (NAVFAC, 2001; ESTCP, 2002; USACE, 2008). <br /> 2.3.1 Zone of Influence <br /> Both ESTCP (2002) and USACE (2008) suggest the term `zone of influence' (ZOI), as <br /> opposed to radius of influence, to refer to the area around a well where the density of air <br /> channels is sufficient enough to promote contaminant removal from the saturated zones <br /> located between the air channels. Clayton (1996) (cited in USACE (2008)) proposed a working <br /> definition of ZOI as the area around an injection point where the air saturation exceeds 10%. <br /> USACE (2008) suggests a minimum air channel density of 3%, although in practice, the <br /> distribution and density of air channels in the field is extremely complex and difficult to <br /> measure. Design guidance has relied upon laboratory studies and a few detailed field <br /> experiments to come up with expected ZOls for typical air injection rates. USACE (2008) <br /> suggests the effective ZOI is likely to be no more than 15-feet. ESTCP (2002) and NAVFAC <br /> (2001) suggest similar ZOIs. <br /> Biosparging is typically used to treat shallow groundwater impacts (ESTCP, 2002) and there is <br /> little guidance available on vertical spacing for multi-depth sparge well clusters across large <br /> saturated thicknesses such as those that exist near the City of Ripon WWTP. ESTCP (2002) <br /> and NAVFAC (2001) recommend that injection wells be installed to inject oxygen <br /> approximately 5-feet below the target treatment area. ESTCP (2002) also recommends that <br /> injection wells be set no greater than 10-feet below the target treatment area due to increased <br /> risks that an adequate supply of oxygen will not reach the target treatment area. <br /> AMEC Geomatrix, Inc. <br /> \\oad-fs1\doc_safe\9000s\9837.006\4000 REGULATORYTS Assessment_Apx B_012711\Attachment B.1\Attach B1.doc 1314 <br />