ESTCP Project #1011, Rhizosphere
Final Report
II era sites in cold-regions have been "naturally attenuating" for over 50 years. Several
conditions that occur frequently at older sites in cold regions affect this persistence and may be
overcome by rhizosphere-enhanced treatment.
Firstly, for surface spills and the subsequent contamination that characterizes cold regions sites,
less soluble--and therefore less mobile--compounds such as PAHs tend to predominate.
Volatile fractions frequently have either volatilized or leached. If they have leached, either they
are in the groundwater and subject to natural attenuation in a mixed system or they have been
retarded by permafrost. The remaining PAHs would not mix within the upper soil to a great
extent because there is no driving force for mixing. Recent data have shown that one of the
benefits of rhizosphere-enhancement is more effective treatment of heavier compounds, such as
PAHs (Reynolds et al., 1999; Reynolds et al., 2001). These data support the hypothesis put forth
by Donnelly and Fletcher (1994) that suggests that root exudates may beneficially influence
degradation of recalcitrant compounds.
Secondly, if the mean annual temperature is below 0 oC, as is the case in much of Alaska, sites
may be underlain by either continuous or discontinuous permafrost, which serves as a natural
barrier to leaching. A permafrost barrier may be an advantage for successful rhizosphere
treatment as the barrier serves to keep contaminants near the surface of the soil where roots can
penetrate.
Thirdly, during the spring and fall transitions between the more constant winter and summer
temperatures in Alaska, soil temperatures are much more variable, fluctuating on several scales
including diurnal. We have measured significant diurnal fluctuations in root-zone soil
temperatures at Fairbanks. The effect of frequent temperature fluctuations on soil microbial
activity is not fully understood, but there appears to be a lag time of diminished microbial
activity following reduced temperatures. Our working hypothesis is that microbial stimulation
from root exudates reduces the lag time following any temporary period of diminished microbial
activity. We think that the lag phase in microbial activity, and hence the benefits gained from
reducing the lag time, occur frequently, perhaps daily, for a portion of the year. If so, seemingly
small benefits resulting from reduced lag times associated with rhizosphere treatment could be
multiplied by daily temperature cycles.
2.4.2 Weaknesses. Obtaining regulatory approvals and developing suitable monitoring
plans are perhaps the most difficult problem associated with using rhizosphere-enhanced
biotreatment. The technical risks associated with demonstrating this technology are primarily
difficulties in getting sufficiently precise data to show treatment effects in a relatively short
period. We used replicated, statistically valid, field studies and multiple sampling and analyses
methods (described in Section 3) to address these issues. Each site included appropriate
replicated treatment controls.
Another limitation is the relatively longer treatment times compared to more aggressive
treatments (Figure 3). Longer treatment times are offset by the reduced costs associated with
rhizosphere-treatment.
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