Consequently health issues related to the using rhizosphere-enhanced treatment of contaminated
soil are minimal.
Our demonstrations included seeding and fertilization of cold-tolerant grasses and legumes in
POL-contaminated soils at three locations in Alaska. We used a replicated design to test seeding
and fertilization, seeding only, fertilization only, and a no treatment control. Figures 1 through 4
show treatment plots at our three field locations.
To implement this technology at other sites, operational activities would include preliminary soil
sampling to define the contaminated area and obtain baseline measurements of contaminant
composition and concentration. For large areas, standard seeding and fertilizing equipment may
be used, although seeding and fertilization can be done by hand or with hand-held seeders. In our
demonstrations, neither seeds nor nutrients were mixed into the soil, eliminating the need for
heavy equipment mobilization to remote sites. Once the area has been seeded and fertilized, the
only remaining activity, beyond any needed reseeding and fertilization, is sampling for the
monitoring process.
Plant selection for these sites was based on hardiness and high potential for stand establishment
without constant maintenance. Relatively large seeded grasses, such as annual ryegrass, excel in
these criteria. Following initial growth, we have found that volunteer plants are abundant.
Monitoring is a challenge, due to the relatively non-aggressive nature of rhizosphere-enhanced
treatment, spatial variability of contaminants in surface soils, lack of mixing, and temperature
differences among sites that impact biological processes. We have found that using composite
samples and biomarker-normalized data help reduce the data variability associated with
concentration and spatial differences. Normalizing data by adjusting for growing degree-days
also helps for comparing data among field sites at different temperature regimes. All of these
approaches are based on changes in contaminant concentration. An option that has significant
potential is to monitor microbial activity and use the results to make inferences about the site.
For petroleum, which is relatively easy to degrade, it is possible that general microbial activity
would be useful. Two problems associated with this approach are: i.) General microbial activity
indicates that microorganisms are active, but does not identify the carbon source that they are
using, and ii.) Alternative measurements, such as soils taken to a laboratory for
radiorespirometry of labeled compounds, are very good measures of what the microorganisms
are using in the laboratory, but do not identify what is being used in the field. To address these
issues, molecular techniques are being developed and evaluated. Their use is not fully accepted
at this time.
The frequency and duration of monitoring for rhizosphere-enhance remediation likely differs
from more aggressive treatments. In general, it would make economic and practical sense to
monitor less frequently but for a longer period of time. Extending the interval and duration of
monitoring needs to be balanced against the need to know that the system is "working".
2.3 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 experiments have shown
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