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 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 of plots that had been both vegetated and
fertilized. TPHgc losses on the vegetated and fertilized sites 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 an indirect measurement of contaminant concentration changes, is a more direct
measurement of the governing mechanisms.
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 mechanical 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.
We also conducted field demonstrations at two DoD locations in Korea. Although the constraints
that these installations faced were caused by limited manpower and funding to treat excavated,
contaminated soil using traditional approaches rather than the location and budget constraints
typical of northern cold-region sites, the constraints manifested themselves in similar ways. The
field user needed a low low-cost, low low-maintenance, self-repairing treatment approach for
contamination in near surface soils.
2.4 Advantages and Limitations of the Technology
The expected benefits of implementing rhizosphere-enhanced bioremediation are:
1. Costs may be reduced dramatically in treating sites that are remote from infrastructure
such as roads, power, and transportation.
2. Rhizosphere-enhanced treatment can be used at active installations, releasing scarce
cleanup resources for more urgent contaminated sites.
3. The technology avoids the mechanical problems caused by freezing temperatures.
4. Human and environmental risks related to POL-contaminated soils will be reduced at
these sites.
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