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amec�9 <br /> sediments could limit the transport of injected microbes to within several feet of the injection <br /> well. <br /> Attachment of bacteria to the surfaces of sand grains also plays a critical role in limiting the <br /> transport of bacteria (microbes) in porous media. Classical clean-bed filtration theory <br /> describes the filtration of entrained bio-colloids in water (microbes) moving through uniform <br /> porous media (Yao et al., 1971 in Li and Logan, 1999): <br /> %0= exp(—a/,L) Equation 1 <br /> where C is the bio-colloid concentration in aqueous suspension that has traveled a distance L <br /> through granular media, Co is the initial concentration of bio-colloids (i.e. injected microbes), a <br /> is the collision efficiency (also known as the sticking coefficient), and X is the filtration <br /> coefficient, which is related to the porosity, and attributes of stationary porous media (collector <br /> efficiency and the irreversible kinetic rate coefficient for attachment; Rajagopalan and Tien, <br /> 1976). Therefore, Equation 1 describes a logarithmic decrease in mobile bio-colloids <br /> (microbes) caused by filtration during transport, where this immobilization increases <br /> exponentially with the number of collisions between microbes and aquifer particles (equal to <br /> XXL) and the fraction of microbes that become attached to porous media after each collision <br /> (a); Li and Logan, 1999). Therefore, colloid theory suggests that injected microbes should not <br /> migrate far beyond the injection location in aquifer sediments, and that the sticking coefficient <br /> (a) is an important control on the transport distance. <br /> Li and Logan (1999) note that filtration of injected microbes by aquifer sediments is a <br /> substantial impediment to bioaugmentation because the population of injected microbes <br /> typically decreases by several orders of magnitude within 0.3 to 3 feet of the injection well. The <br /> logarithmic decrease described in Equation 1 has been observed in field studies of bacterial <br /> and virus transport in sandy aquifers (Harvey et al., 1989; Schijven et al., 1999). Limited <br /> microbial transport has also been observed in several recent bioaugmentation studies, where <br /> the resulting decrease in contaminant concentration has been measured in observation wells <br /> that were only a few feet from the injection point (Dybas et al., 1998; Ellis et al., 2000; Lendvay <br /> et al., 2003; Smith et al., 2005). Efforts to enhance microbial transport are being researched, <br /> and these include the use of adhesion-deficient bacteria, starved bacteria, surfactants and low <br /> ionic strength delivery solutions; these methods need further development and remain <br /> unproven (Li and Logan, 1999; Alvarez and Illman, 2006). <br /> Although important challenges associated with applying bioaugmentation have been identified <br /> by the research community, examples of reportedly successful case studies do exist (Alvarez <br /> and Illman, 2006). This apparent disconnect may be related to how success is defined and <br /> interpreted, and because it is often difficult to distinguish treatment as a direct result of <br /> AMEC Geomatrix, Inc. <br /> \\oad-fs1\doc_safe\9000s\9837.006\4000 REGULATORYTFS Assessment_Apx B_01 2711\Attachment B.2\Attach B-2.doc 1324 <br />