ESTCP Project #1011, Rhizosphere
Final Report
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, within each treatment, we observed little difference in the
petroleum concentrations at lower depths relative to petroleum concentrations at the more
shallow 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
system. We believe that carbon additions are a major part of the success of rhizosphere
treatments in such soils. In wet, somewhat anaerobic soils, carbon accumulates and is probably
not limiting. Therefore, root additions of carbon may not result in increased biotreatment rates.
2.5 Available Treatability Guidance
Although efforts to provide treatability guidance have been developed and are being updated,
there are few examples of well-documented field studies published. Below are some documents
that provide overviews of phytoremediation.
Brownfields Technology Primer: Selecting and Using Phytoremediation for Site Cleanup.
Published: 2001. http://www.clu-in.org/download/remed/phytoremprimer.pdf.
This primer explains the phytoremediation process, discusses the potential advantages and
considerations in selecting phytoremediation to clean up brownfield sites, and provides
information on additional resources about phytoremediation. This document is not limited to
rhizosphere remediation of petroleum in surface soils. Although treatability studies are
suggested, specific information on treatability studies is not provided. A general overview of the
many mechanisms potentially involved in phytoremediation is included and useful information
on plant selection based on rooting depth.
Phytoremediation Decision Tree, Published: 1999.
http://www.clu-in.org/download/partner/phytotree.pdf.
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