ity (e.g., see Guilbeault et al., 2005), the requirements for additional site characterization can represent a considerable burden on site management.

Transformation Capacity

As discussed in Chapter 7, monitored natural attenuation (MNA) is the dominant process during long-term management at sites not relying on physical or hydraulic containment. Knowledge of the biogeochemical environment and the identification of potentially important reactive pathways for the target contaminants are necessary prerequisites for initiating MNA after the transition to long-term management has occurred. Relevant considerations include bulk aquifer properties such as mineral composition and pore water chemical constituents, as well as the presence of the necessary microbial consortia. Contaminant transformation during MNA can occur through microbial pathways, abiotic mechanisms, or in many cases a combination of both.

Of critical importance to the aquifer “transformation capacity” for MNA is the spatial pattern of redox zonation. Redox zonation occurs as a result of microbial metabolism where in a homogeneous system terminal electron acceptors with the most favorable free energies are preferably used before the next one can be utilized (termed the “redox ladder” by Borch et al., 2010). Complex sites, however, may have areas of overlapping or patchy redox zonation whereby microbial communities that utilize different terminal electron acceptors can co-exist. Determining whether the site is fully oxic, has extensive zones of anoxia, or is comprised of these patchy suboxic/anoxic regions in conjunction with the target contaminant composition is critical to determining the appropriateness of MNA (Rugge et al., 1998; Hofstetter et al., 1999).

Another important parameter in contaminant transformation is the presence of reactive minerals associated with aquifer solids, such that characterizing these chemical factors can yield clues about the potential effectiveness of MNA. A variety of naturally occurring iron and manganese oxides, iron sulfide minerals, and clays with iron moieties have been shown to be highly reactive and can act as respective reductants and oxidants in abiotic attenuation pathways (Kappler and Straub, 2005; Hofstetter et al., 2003; Neumann et al., 2009; He et al., 2009). Microorganisms play an important role in the controlling both the type and stability of these minerals since many organisms are capable of utilizing mineral oxides as terminal electron acceptors (Lovley, 1993; Tebo et al., 2004). Under some circumstances the microbial population can convert iron oxides to reactive media useful for MNA by producing Fe(II), which can either be chelated by natural ligands, be adsorbed to the remaining iron oxides to create highly potent reductants, or react with sulfides (if sulfate is in abundance as a ter-



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