minal electron acceptor) to produce potentially reactive iron sulfide minerals (Hakala et al., 2007; Hakala and Chin, 2010). In other cases, however, reduction of manganese oxides (which can mediate oxidation reactions) may result in a decrease in potential MNA. In aquifer pore waters, reactive species such as natural organic matter and reduced sulfur species (bisulfide, polysulfides, and organic thiols) play an important role in MNA by acting as reductants and electron mediators (Kappler and Haderlein, 2003; Hakala and Chin, 2010). Natural organic matter significantly increases the reactivity of reduced sulfur species by acting as an electron mediator, and is an important reductant in sulfur-rich aquifers (Dunnivant et al., 1992).
An example of a well-characterized site with high transformation capacity amenable to MNA is Altus Air Force Base, which has abundant levels of both sulfate and Fe(III) (Kennedy et al., 2006). Microbial metabolic activity at this site produced potent reactive reductants such as reduced sulfur compounds, Fe(II), and iron sulfide minerals, which were capable of abiotically transforming TCE and its derivatives. These investigators reported the absence of sulfate in the area of the TCE plume and the existence of abundant iron sulfide minerals. Further they found no TCE in the area where iron sulfides are abundant and only trace levels of by-products, suggesting that MNA was occurring.
While much is known about the biological/abiotic conditions necessary to effect contaminant transformation during MNA, there is not yet a complete protocol to determine the extent to which such conditions are present at a site and whether contaminants are being degraded. The tools discussed later in this chapter represent important initial steps toward the development of such a protocol.
Vapor Intrusion Issues
As described in Chapter 5, the vapor intrusion pathway is increasingly considered at complex sites with DNAPL contamination. This pathway can be conceptualized as three distinct zones (Figure 6-1): (1) the source zone where contaminant is immobilized, (2) the subsurface migration pathway, and (3) the influence zone of the building. The management of vapor intrusion requires expanded site characterization, an interpretation of the several types of vapor concentration measurements in the context of site-specific conditions, and, if necessary, development of appropriate mitigation strategies if source removal measures are insufficient to reduce exposure to acceptable levels.
Characterization of the vapor pathway is challenged by the fact that each component is subject to considerable spatial and temporal variability. Fluctuating water table conditions controlled by recharge, pumping, and stream–aquifer interactions can result in transient vapor flux generation at