Field trials were done at the Western Storage Area (WSA) of Harwell in Oxfordshire, a former nuclear research site being decommissioned by Research Sites Restoration Ltd (and formerly by the United Kingdom Atomic Energy Authority).
The site had been contaminated with various chemicals including mineral oil, chloroform, trichloroethane (TCA), carbon tetrachloride and tetrachloroethene (PCE). The contamination had affected the unsaturated zone, groundwater in the chalk aquifer and was a continuing source of groundwater contamination below the WSA.
We have significantly reduced the concentrations of chlorinated solvent in the unsaturated zone by removing over three tonnes of chlorinated solvents from the chalk aquifer through the full-scale application of TESVE – an in-situ thermal desorption (ISTD) technology.
The work aimed to show the potential for biologically mediated contaminant reduction in assisting thermal remediation projects. During and post active thermal remediation this approach can help reduce total treatment costs by providing an inexpensive final polishing step or by being a complementary process within the perimeter of heated zone and inside hot-spots during the cool-down phase.
The elevated temperature environment at the site has effectively stimulated the microbial populations, providing further reduction in chlorinated solvents through a reductive dechlorination process in the immediate vicinity and outside the effective zone of thermal influence.
Findings of the work to date have indicated an increase in Dehalococcoides sp at the site some six months after thermal treatment, suggesting that thermal enhancement created more favourable conditions for dehalogenating bacteria to adapt, and reduced the number of well established colonies competing for available electron donors.
The sequential transfer from PCE to DCE was evident following the thermal treatment, and the accelerated kinetics of the dechlorination reactions was observed during the cool down phase and six months after cessation of the heating process, when the rock mass and groundwater temperature remained in the range of 35-40˚C.
The in-situ passive diffusion sampling device, BioTrap, proved to be a robust microbial sampling tool, which allowed precise sampling of a well acclimatised and biologically active microbial population present in the real site environment.
The most advanced testing method in molecular biology –qPCR was applied to detect and quantify the presence of specific DNA sequences of Dehalococcoides sp and Dehalobacter sp responsible for the reductive dechlorination process.
Thermally enhanced monitored natural attenuation (TEMNA) can be an economic and sustainable technology in many circumstances. Sites with the following constraints would be worthy of appraisal for TEMNA:
- Mainly VOCs (chlorinated solvents) affected sites where reductive dechlorination is the dominant biological step
- Expensive or impractical excavation
- LNAPL sites where aerobic degradation is the dominant biological step
- Rapid remediation is not required.
Generally TEMNA has the advantage that a significant increase in degradation rates can be achieved with a relatively small energy input (heating up to about 35-40˚C). Therefore costs associated with fuel/energy consumption can be substantially reduced, since the duration of heating could be limited to about two weeks. However where remedial targets need to be achieved within a shorter time frame or where there are site characteristics like a mixture of highly persistent hydrocarbons, the applicability is reduced and TESVE will be the preferred approach.