ESTCP Project #1011, Rhizosphere
Final Report
Because we usually are not able to identify the sequence of limiting nutrients at a site without a
series of treatability studies, and the cost of conducting these studies is usually greater than the
benefit gained from them, applying an appropriate level of fertilizer may be as important as
using a proprietary fertilizer. Our demonstrations were successful with the use of standard
agricultural fertilizer.
6.4.5 Monitoring: Sampling
Monitoring is perhaps the most difficult aspect of rhizosphere enhancement. For sampling, the
goal is to determine if there is a decrease in petroleum through time. Problematically,
contaminants in surface soil are not uniformly distributed, and trying to quantify the amount of
contaminant in a volume of soil at any time is not trivial. The "error" or variability associated
with samples is large, and estimates for the total amount of contaminant in the soil are based on
the results of the samples that you take. In many instances, taking a set of random samples and
using these to estimate the contaminant in the soil, and then taking samples again, exactly the
same way but on the next day, would likely yield very different results for the concentration or
total amount of petroleum.
At the ESTCP field demonstrations, we used both composite samples and "soil sock" samples.
The soil socks consisted of a series of net tubes or socks containing premixed soil placed into
each test plot. This approach was used to minimize the variability at the initiation of the study.
Soil socks are useful for research, but are too labor and cost intensive and are therefore
impractical for routine field use.
We have found that composite samples are helpful. Composite soil samples have been
unacceptable in some areas, probably due to the fact that the regulations and guidance on
sampling have been based on very aggressive (and costly) remediation methods such as
incineration. The concern was that composite sampling would "dilute" possible hot spots and
grab samples therefore were required. In reality, for many current remediation methods, grab
samples would tend to miss the hot spots. For surface soils that are being rhizosphere-
remediated, there is essentially no natural mixing, as is the case for samples from saturated or
groundwater zones. For the field demonstrations in Korea, as well as those in Alaska, we took
six to eight samples in each plot and mixed them together prior to analysis. This reduced the
variability significantly. Our results suggest that composite sampling provides useful data.
6.4.5 Monitoring: Analysis
For what do you analyze? This is another difficult question that these demonstrations, as well as
related projects, have tried to address. Not surprisingly, the easy-to-degrade compounds will
degrade readily. Although there can be a rhizosphere benefit for essentially all petroleum
compounds, the benefits of rhizosphere-enhancement are most observable for recalcitrant
compounds, such as PAHs. We have seen this in our laboratory studies, in the field in Alaska,
and also at demonstration trials Korea.
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