Enhanced Bioremediation as a Cost Effective Approach Following Thermally Enhanced Soil Vapour Extraction for Sites Requiring Remediation of Chlorinated Solvents

Abstract

Thermally enhanced bioremediation can be a more cost-effective alternative to full scale in-situ thermal treatment especially for sites contaminated with chlorinated solvents, where reductive dechlorination is or might be a dominant biological step. The effect of Thermally Enhanced Soil Vapour Extraction (TESVE) on indigenous microbial communities and the potential for subsequent biological polishing of chlorinated solvents was investigated in field trials at the Western Storage Area (WSA) – RSRL (formerly United Kingdom Atomic Energy Authority - UKAEA) Oxfordshire, UK. The WSA 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. During TESVE the target treatment zone was heated to above the boiling point of water increasing the degree of volatilization of contaminants of concern (CoC), which were mobilised and extracted in the vapour phase. A significant reduction of concentrations of chlorinated solvent in the unsaturated zone was achieved by the full-scale application of TESVE – In Situ Thermal Desorption (ISTD) technology. The rock mass temperature within target treatment zone remained in the range of 35°–44° C, 6 months after cessation of heating. The concentration of chlorinated ethenes and other CoC were found to be significantly lower adjacent to the thermal treatment area and 1 to 2 orders of magnitude lower within the thermal treatment zone. Samples were collected within and outside the thermal treatment zone using BioTraps® (passive, in-situ microbial samplers) from which the numbers of specific bacteria were measured using quantitative polymerase chain reaction (qPCR) methods of analysis. High populations of reductive dechlorinators such as Dechalococcoides spp. and Dehalobacter spp., were found within the zone that was subjected to thermal remediation and moderate levels of Dehalobacter sp were found outside the treatment area. These results confirm dehalogenating bacteria are present within the site and suggest populations have bounced back following thermal treatment. The thermally treated zone showed a greater number of active indigenous bacteria — indicating that the conditions following TESVE treatment selectively promote the growth of desirable bacteria. This might result from elimination of micro-organisms competing for hydrogen as an electron donor, increased biovailability of CoC or a reduction in its inhibiting properties. This paper aims 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 hotspots during the cool-down phase.