5. Rhizosphere-enhanced remediation is, to a large degree, a self-sustaining or self-repairing
technology. Volunteer plants and, potentially, native species can eventually populate a
site.
Rhizosphere-enhanced remediation has known limitations. It is applicable to surface
contamination that is within the rooting zone (generally about 4 ft) but not for deeper
contamination. While this limits its use for deeper zones of contamination, it makes it useful for
contaminant source zones that may be releasing contaminants by periodic leaching or for soil
that has been excavated and stockpiled. It is also potentially useful for treating less mobile but
carcinogenic contaminants, such as PAHs, which tend to remain near the surface. It may also
have applicability above permafrost, where application of other technologies may not be feasible
(see Figures 5 and 6). For other situations, such as trichloroethylene (TCE) in shallow
groundwater, other forms of phytoremediation that rely on different mechanisms have shown
success.
Obtaining regulatory approvals and developing suitable monitoring plans are perhaps the most
difficult problems associated with using rhizosphere-enhanced biotreatment. The technical risks
associated with demonstrating this technology are primarily difficulties in getting sufficiently
precise data to show treatment effects in a relatively short period. Although choosing the
appropriate sample analysis is important, research overwhelmingly and clearly demonstrates that,
due to the spatial variability of contaminants in the soil, a much greater error arises from field
sampling. In brief, the success of representing the situation in the field is limited by obtaining a
representative sample from the field rather than the sample analysis. We used replicated,
statistically valid, field studies and multiple sampling and analyses methods to address these
issues. Each site included appropriate replicated treatment controls.
Another limitation is the relatively longer treatment times compared to more aggressive
treatments (Figure 7). Longer treatment times are offset by the reduced costs associated with
rhizosphere-treatment.
Also unknown are the final concentrations that can be attained using rhizosphere remediation.
The tendencies for concentrations to become asymptotic to a concentration greater than desired
are well documented. At present, we do not know the final attainable contaminant concentration
in soils for various soils types and contaminants. Moreover, we do not know how rates vary in
different climates, different soils, different contaminants, or for different plants.
Because this is a root-interface phenomenon, the root must explore the soil being treated. Depth
of rooting is obviously important and is an aspect we addressed in the demonstration. In
laboratory studies, we can readily grow the roots of annual ryegrass to 4 ft within approximately
two months. The optimum plants for site remediation are, to some degree, those plants with
prolific root growth. Permafrost barriers and the sorption capacity of soils for many PAH
compounds help to keep these compounds near the surface where root penetration is likely. In
our research site at Fairbanks, we observed little difference in the concentrations at lower depths,
suggesting that rhizosphere treatment was reasonably effective in the lower portion of the root
zone (Reynolds et al., 1997).
Wet or saturated soils may be difficult to remediate using this method. There are older sites that
have been vegetated for some time and yet are still contaminated. In poor quality, well-drained
soils, the carbon provided by root exudations apparently satisfies the carbon limitation to the
6
Previous Page
