ARCHIVED REPORTS_2011 REVISED FEASABILITY STUDY

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plumes because it would be cost prohibitive to install a grid-type network of sparge wells— which essentially would be multiple biobarriers spaced very closely together. For example, if the area of VOC-affected groundwater targeted for treatment is defined by monitoring wells M- 30C1, TH-10, M-31 C1 and M-1 7C1 (Figure B.1-1) in a grid type pattern for"hot-spot" remediation, it is clear that a grid over this area would involve a much greater number of sparge points than would be needed for the biobarrier alignment. This scenario would clearly be cost-prohibitive and would not provide any additional protection of the closest downgradient receptors south of the Stanislaus River. Key biosparging processes that should be considered when evaluating the potential effectiveness, implementability, and cost of AS for a particular site include: • Delivery of the amendment to the target treatment area (in this case, oxygen); • The amenability of the CDCs to aerobic biodegradation processes Oxygen delivery considerations are discussed in Section 2.1 below. Treatment processes (i.e. the amenability of the COCs to aerobic biodegradation processes) are discussed in Section 2.2. Design considerations (e.g. well spacing, flow rates) are discussed in Section 2.3. 2.1 DELIVERY OF OXYGEN TO THE SUBSURFACE The effectiveness of biosparging depends greatly on the uniform delivery of gaseous oxygen to the target treatment zone so that a stable population of aerobic bacteria can thrive and degrade target CDCs. Laboratory and field studies however have shown that the delivery of gaseous oxygen (or any other gas, including air)to the subsurface can be highly irregular resulting in an uneven distribution. Subsurface gas flow characterization experiments have indicated that the injection of air into a saturated porous media during AS does not result in a uniform distribution of air bubbles around the injection point, but rather in a limited number of discrete air channels that are heterogeneously distributed and difficult to predict (Ji et al., 1993; Ahfield et al., 1994; Hein et al., 1997; Brooks et al., 1999; Elder and Benson, 1999). These air channels typically form an inverted cone-shaped net around the sparge point in relatively homogeneous systems; in layered systems, air typically will accumulate in pockets beneath low permeability layers (Ji et al., 1993; Lundegard and LaBreque, 1998; Tomlinson et al., 2003). With continued injection, air trapped beneath a low permeability layer will spread laterally until a coarser grained vertical migration pathway is encountered or the localized pressure increases to a point where air can penetrate the overlying low permeability layer (ESTCP, 2002). This situation could cause spreading of CDCs and could also result in impacts to soil gas which would have to be managed appropriately. The majority of research evaluating contaminant removal mechanisms in AS applications has adopted a conceptual model similar to Ahlfield (1994) (ESTCP, 2002). These conceptual models generally acknowledge the following (ESTCP, 2002): AMEC Geomatrix, Inc. \\oad-fs1\doc_safe\9000s\9837.006\4000 REGULATORYTS Assessment_Apx B_012711\Attachment B.1\Attach B1.doc 131-2