ESTCP Project #1011, Rhizosphere
of representing the situation in the field is limited by obtaining a representative sample from the
field rather than the sample analysis.
Our design to demonstrate the efficiency of rhizosphere-enhanced remediation and obtain
meaningful samples from the field is described in Section 3 of this document.
2.2 Previous Testing of the Technology
Our earlier laboratory and field studies in Alaska suggested that the rhizosphere effect increases
in importance as the recalcitrance of the compound in question increases (Reynolds et al., 1999;
Reynolds et al., 2001). Recent carefully conducted and replicated field experiment have shown
that significantly greater petroleum reductions can be verified in vegetated plots relative to non-
vegetated plots. On many remediation sites, total petroleum hydrocarbon (TPHgc) commonly is
used a as a dependent or response variable. TPHgc analyses are relatively inexpensive and readily
available. TPHgc provides a single value that integrates all peaks and unresolved portions of a
chromatogram. The compromise is that TPHgc is not as sensitive as some other measurements.
Nevertheless, TPHgc data are useful.
In earlier Alaska field research using soil recently contaminated with diesel, we measured
significant TPHgc decreases during a three-year study from plots that had been both vegetated
and fertilized. TPHgc losses were greater than the plots receiving only fertilizer or vegetation,
and greater than losses from the control treatments. The effects were similar but less dramatic for
crude-oil contamination (Reynolds et al., 1997). There is some evidence that the major benefits
from the rhizosphere effect, relative to non-vegetated soil, are likely greatest for heavier, more
recalcitrant compounds (Reynolds et al., 2001). Resistance to degradation of heavier PAH
compounds may result in longer treatment times being required before rhizosphere effects can be
measured. Measuring changes in the soil microbiology, although not a direct measure of
contaminant concentration changes, may be a more direct measurement of the underlying
One approach to measuring treatment effects would be to conduct a two-dimensional
contaminant spatial characterization at initial and subsequent sampling times. In our prior
research at a one-acre landfarm site, we measured contaminant concentrations on a 25-node grid
and developed spatial (two-dimensional) concentration profiles at four separate sampling times
(Reynolds, 1993). Even though the soil was mechanically tilled approximately every two weeks,
half-lives calculated from the concentration data varied by a factor of seven. We have concluded
that costs for developing two-dimensional profiles would be prohibitive and the resulting data
may not be sufficiently precise to observe changes in concentration.
2.3 Factors Affecting Cost and Performance
The greatest cost for rhizosphere-enhanced bioremediation typically is in sampling and
monitoring, and that is specific to the frequency of sampling, the type of analysis done, and cost
of analysis per sample. The transport, spreading, seeding, and fertilizing are essentially one-time