Chlorinated solvents have been used in large quantities by a diverse range of industries, including chemicals production, metalworking, automotive, aerospace, electronics and dry cleaning. They account for about 30% of groundwater pollution incidents in England and Wales. Trichloroethene (TCE) is reported to be the most frequent contaminant within this group.
In the subsurface, all except very small releases may result in dense non-aqueous phase liquid (DNAPL) contaminant source areas, which will persist for decades and act as long-term sources of groundwater contamination - 7l of TCE can contaminate 100M.
l of groundwater to 100µg/l.
Current methods of remediation are usually very expensive, with uncertain results.
Under anaerobic conditions, dehalorespiring bacteria (eg Dehalococcoides ethenogenes) use TCE and other chlorinated ethenes as terminal electron acceptors for respiration. This metabolism (reductive dechlor involves a step-wise removal of chlorine atoms from the molecule, ultimately yielding ethene. The reductive dechlorination reaction has been shown to be viable for field bioremediation of dissolved chlorinated solvent plumes and laboratory studies have suggested it is effective in the presence of chlorinated solvent DNAPL.
The overall objectives of Sabre (source area bioremediation), the consortium running the project, are to develop and demonstrate the application of anaerobic bioremediation for the insitu treatment of chlorinated solvent DNAPL source areas, and develop numerical models and performance assessment tools. The project aims to generate scientifically robust, multiple lines of evidence to support these objectives.
Sabre is a collaborative project being carried out by a multidisciplinary consortium from the UK, USA and Canada, supported through the Department of Trade and Industry LinkBioremediation programme. The consortium is comprised of 11 industrial companies from the UK and USA, three UK research institutions, the Environment Agency, the US Environmental Protection Agency (EPA) and Cl: aire.
The project value is about £3.3M, with half the funding sourced from the industrial members of the consortium. Sabre began in October 2004 and will run until the end of 2008 with field demonstrations starting in July. Findings will be circulated throughout the project by presentations at major conferences, through Cl: aire technical bulletins and reports, by a technology guidance document and a dedicated conference at the end of the project.
Concept The Sabre project will comprise laboratory and field pilot scale development of an accelerated anaerobic bioremediation process for DNAPL source areas based upon reductive dechlorination by dehalorespiring bacteria.
This will be achieved by the controlled addition of growth substrates to support dehalorespiring bacteria under conditions that ensure the maximum rate of reductive dechlorination to ethene.
In view of the properties of DNAPL, preference will be given to the use of substrate formulations that partition into the organic DNAPL phase and are subsequently slowly released to support biodegradation at the DNAPL-groundwater interface and downgradient of the DNAPL source.
Consideration will also be given to bioaugmentation with a mixed culture containing D ethenogenes, as this may be the most effective way of reducing treatment time in future full-scale applications of the technology.
Laboratory studies are being used to determine appropriate process conditions for effective DNAPL bioremediation at the test site and to generate design data for the field pilot scale treatment test. Field pilot scale studies will by done in two closely coordinated test cells: one contained cell that enables intensive monitoring of the treatment process and one uncontained unit that represents a realistic model of how such a process could be costeffectively implemented at full scale.
Development of any bioremediation technology requires a high level of confidence in the field performance of the process if it is to gain acceptance. For DNAPL remediation, this need is greater due to the complexity of source areas, the difficulty in demonstrating performance of existing competing technologies, and their ineffectiveness.
Site workers will monitor the contained - eld pilot test cell using traditional and novel chemical, biological and geophysical techniques, supported by statistical data processing and interpretation.
The uncontained pilot test area will be operated using information generated in the contained cell but in a way that is representative of future non-research operational constraints.
The team will use modelling in the design and validation of the process to provide essential information and tools to enable future development.
In this way, Sabre will identify the best implementation approaches for further development.
Site selection The site, in central England, was chosen because of the presence of a relatively pure TCE source area in a shallow aquifer that can be costeffectively characterised, treated and monitored in a pilot scale experiment. There is evidence of intrinsic biodegradation of TCE by reductive dechlorination, making the enhancement of bioremediation relatively straightforward.
Research issues, objectives and programme of work The overall hypothesis to be tested is that enhanced anaerobic bioremediation using reductive dechlorination can result in the effective treatment, through solubility enhancement and degradation to non-toxic end products, of chlorinated solvent DNAPL source areas.
Specific research objectives and study methodologies have been established in six work packages (WPs) defi ned for the project (see Table 1).
One of the innovative features of the site characterisation work package is the use of 4D Electrical Resistivity Tomography (ERT) to measure DNAPL mass before, during and after treatment. As described below, the inclusion of ERT placed limitations on the materials that could be used for the construction of the test cell.
Design and construction of the contained test cell A test cell was designed to aid detailed and scientifically defensible measurement of reductive dechlorination and other processes in the groundwater system. It was built partly within the DNAPL source zone, aligned with the direction of groundwater flow, with the upgradient end in the DNAPL source.
The test cell is 4m wide by 30m long, with the long axis parallel to groundwater flow. Its walls found in a low permeability mudstone at about 7m depth.
Building of the test cell had to satisfy a number of criteria (Table 2). In particular, for ERT to be effective, the walls of the test cell had to be constructed from materials which do not conduct electricity.
This prevented the use of steel sheet piles.
A number of grouted-wall systems, including slurry trenches, jet-grouting and secant pile walls, were compatible with ERT and offered well proven installation solutions. But there was a concern that penetration of bentonite or bentonite-cement grouts into the formation would alter formation properties in the vicinity of the walls, which is a potential problem, given the small width of the test cell.
Also, the grout materials would potentially interact chemically with the contaminants.
The Sabre team chose plastic sheet piles for the test cell walls because they provided the required containment with relatively little compromise in terms of chemical interaction, use of ERT and formation damage.
However, although lightweight driven plastic piles have been used in soft soils in the UK previously, there was no track record of driving them through relatively dense granular soils like those at the Sabre research site, with SPT N values up to 40.
Experience in driving plastic piles with a patented mandrel system in similar ground conditions in the USA provided reassurance that there was a reasonable likelihood of success.
Three types of plastic sheet pile of the appropriate strength are produced in the USA: uPVC, polyethylene and glass-fibre reinforced polyester composite (GRP). At the time of ordering the materials, polyethylene piles were unavailable from the single supplier.
Tests carried out on samples of uPVC indicated this material was likely to be adversely affected by high concentrations of TCE, with probable loss of strength and impermeability. This meant GRP piles were selected (see feature page 16).
Acknowledgements Sabre is funded via the UK Bioremediation Link programme, receiving fi ancial contributions from each participating organisation as well as from BBSRC, DTI, the Environment Agency, EPSRC and NERC.
The industrial members of the consortium are Acetate Products, Dupont, ESI, General Electric, GeoSyntec, Golder Associates, Honeywell, ICI, Scientifics, Shell, and Terra Systems. The research organisations are British Geological Survey, University of Edinburgh and University of Sheffield. Other participating organisations are CL:
AIRE, the Environment Agency, the US Environmental Protection Agency (EPA) and the Research Technology Development Forum, which was set up by the EPA and US industry to develop new remediation technology.
White Young Green Environmental is the site owner's consultant for design and implementation of Sabre. It has project managed construction of the contained test cell and will be responsible for implementation of the bioremediation technology in other areas of the site.
Archon Environmental Consultants is acting as the site owner's representative on Sabre and advising on system design and operation.
Table 1 Sabre work packages Package Activity WP1 Site characterisation and operational monitoring - detailed site characterisation and monitoring during treatment operations using traditional and innovative methodologies, including quantification of DNAPL mass before and after treatment WP2 Microbiology - bench testing using laboratory microcosms and column studies, with statistical analysis of data WP3 Engineering - construction of test cell and uncontained area WP4 Performance assessment - provision of database and statistical support to the project and development of performance assessment tools WP5 Flow and Process modelling - comprising (i) hydrogeological flow modelling to support cell engineering and operational design and (ii) modelling of key physical and biological processes WP6 Technical and quality management - establishment of appropriate data quality objectives and development of QA/QC programme to ensure compliance Table 2 Construction alternatives for the test cell containment system Containment design alternative Cost Disturbance/ formation Chemical interaction Containment performance Interference with ERT Remarks Hydraulic control H + + - - + Ineffective containment. Would require more extensive monitoring Slurry wall I - - ++ + The main disadvantages of grout-based approaches was formation plugging adjacent to the walls and chemical interaction with groundwater in the cell Secant piles I - - ++ + Sheet piles - steel I - 0 + - - Steel piles offered predictable installation performances, but would have prevented the use of ERT Sheet piles - non-metallic L - + + + Non-metallic sheet piles provided the optimum combination of reasonable cost, adequate containment, low chemical interaction and compatibility with ERT. The main disadvantage was they were unproven in the UK in anything other than very soft sediments H = high cost, I = intermediate, L = low, + = favourable performance, 0 = neutral, - = unfavourable, - - = poor performance