ESTCP Project #1011, Rhizosphere
Final Report
costs, although some re-seeding may be needed annually, and even some watering may be
beneficial during seedling establishment. Annual fertilizer can be added but may not be
necessary. Again, this is specific to the site and the goals. We have found that in year two (and
even the first season), many volunteer plants established themselves. This is usually beneficial
and, in our experience, the vegetation will shift with time to resemble the local vegetation.
Typical sampling and monitoring techniques used for tracking more aggressive treatments are of
little use for monitoring rhizosphere-enhanced remediation of contaminated surface soils. Data
are too heterogeneous for firm conclusions to be made. Useful tools for obtaining more
meaningful data and reducing variability include composite samples, fraction specific
hydrocarbon analysis (FSH), biomarker normalization, and temperature normalization. Using
these tools for a longer time but with greater intervals between sampling times emerged as a
reasonable monitoring plan.
2.4 Advantages and Limitations of the Technology
2.4.1 Advantages. The primary advantages of this technology are cost, ease of
implementation, and applicability where others technologies cannot be used, such as surface-
contaminated soils covering large areas. Essentially, the costs for implementing rhizosphere-
based treatment include:
1.
Initial site characterization
2.
Permitting
3.
Transportation to the site
4.
Seed and nutrient procurement and application
5.
Monitoring
Because this is a new technology, the frequency and intensity of monitoring are not well defined;
two goals of this project were to confirm that rhizosphere-enhancement provided a benefit and to
provide initial monitoring techniques and cost data. Cost and performance data are provided in
this Final Report and in the Cost and Performance Report.
Cost advantages are valid only in context of comparison to the cost of other alternatives (such as
bioreactors, landfarms, and biosparging), which typically involve mobilizing heavy equipment,
excavation and handling, on-site operation and maintenance, and demobilization. In extreme
winter conditions, mechanical hardware systems must be protected from freezing or operations
must be suspended during winter. For remote sites in cold regions, these operational costs can be
prohibitive.
Natural attenuation is perhaps the definitive low-cost, passive treatment alternative. It has been
successfully demonstrated for BTEX-contaminated groundwater systems where an electron
acceptor is present to drive contaminant oxidation. Such systems are relatively well mixed,
thereby minimizing enzyme-substrate contact and related mass-transport limitations. However,
observations demonstrate that substantial contamination still remains although many World War
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