Vadose zone bioremediation

A moisture ranging between 25 and 85% of complete saturation is considered to be adequate for soil bioremediation.12 In many cases, the soil moisture in the vadose zone is below or at the lower end of this range, so the addition of water is often needed to maintain good operating conditions. 

Bioremediation as a remediation procedure within the vadose zone can take several forms including bioventing as previously discussed. When conditions of low or inadequate permeability exist, the site may not be suitable for ventilation, and the contaminants may be best degraded by anaerobic reactions. 

In its basic form, bioremediation of the vadose zone involves introduction of nutrients and electron acceptors necessary to stimulate the indigenous bacteria and provide for removal of waste products generated by the reactions. This sometimes takes the form of a series of injections of a soup of nutrients and electron acceptors into the vadose zone through wells, or infiltration galleries. Other sites may require pressure fracturing of the soil before the stimulant blend can be injected. 

The primary application of the BERT technology will likely be to complement active soil vapor extraction efforts by removing residual contaminants after active methods become insufficient. It could also be used on the edge of unsaturated zone contaminant plumes where concentrations of volatile contaminants are low or for enhancement of bioremediation activities. The primary advantages of the technology application are low capital costs and minimal operating costs. The system is well suited for applications in low-risk contaminant settings, where rapid response and remediation are not necessary. Suitable applications include volatile contaminants at relatively shallow depths (less than 20 ft) in the vadose zone, such as ... 

Figure 3. Common bioventing system for treatment of vadose-zone contaminants using oxygen as a terminal electron acceptor. Reprinted from In Situ Bioremediation When Does It Work Copyright 1993 by the National Academy of Sciences. Courtesy of the National Academy Press, Washington, DC.

There are a number of engineering variations of in situ bioremediation strategies potentially applicable for soil and/or groundwater contaminated by PAHs. Recent reviews of in situ bioremediation technologies by Norris et al. (1993) provide excellent sources of references and offer case studies for myriad in situ bioremediation applications. These include bioventing vadose zone soil, biosparging saturated zone soil, vacuum-vaporized well (UVB) technology, and in situ bioreactors. 

Vacuum-vaporized well (German Unterdruck-Verdampfer-Brunnen abbreviation UVB) technology for in situ treatment of the capillary fringe, phreatic zone, and vadose zone contaminated with volatile organic compounds, including NAPLs, represents to us one of the most promising in situ bioremediation technologies (U.S. 

Molecular-level studies of chemistry in solution and at interfaces, including mineral interfaces (e.g., the behavior of metal ions in aqueous solution and on metal oxide or clay surfaces for vadose zone, tank, and groundwater remediation and catalysis)—a detailed understanding of redox (electron transfer chemistry) is broadly needed studies of the interactions of biological molecules with surfaces for bioremediation are also needed and being pursued. 

The vacuum-vaporized well technology is a proprietary process that combines vacuum extraction, bioremediation, and soil flushing to mobilize contaminants in the vadose, capillary, and saturated zones. A schematic of the double-screened well with the equipment contained within is shown in Figure 10.12. 

Based on the above and related experimental results, it was recommended that (1) the major effect of the SSNM added is the enhancement of the mobilization and desorption from the soil matrix and solubilization/dispersion of the released entrapped NAPL in the aqueous soil solution and (2) the bioremediation process is positively related to time duration, and a specific-site-tailored, preoptimized SSNM should be used for ensuring the sustainable bioremediation of both vadose and saturated zones in NAPL-contaminated aquifers [8]. 

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