Former coal gasification sites often have recalcitrant contamination within groundwater that cannot be easily or effectively remediated in-situ to appropriate standards using active remediation techniques (pump-and-treat, source oxidation/sparging). Recently, however, reactive barrier technology has been showing promise for remediation of groundwater plumes. It is also proving to be a sustainable, cost-effective means of managing risk at sites that have a moderate net value, particularly at problem sites where the costs associated with many treatment methods significantly exceed the site’s net value.

These issues of remediation, sustainability and cost will continue to become more acute in the near future with the full implementation of the European Community Landfill Directive, which will severely limit disposal to landfill of contaminated soils and other substances such as tar. In addition pretreatment of all waste will have to be undertaken before disposal will be permitted.

Figure 1: SEREBAR Treatment System—Former Gasworks, Haven Road, Exeter

Figure 2: Groundwater Flow model

Advancing Reactive Barrier Technology

PB’s Environmental Safety and Risk Management Group based in Bristol works for SecondSite Property Holding Limited under a term commission. As part of this agreement, we have been asked to manage, specify and supervise the construction of a new system for the sustainable treatment of groundwater at a former gasworks site in Exeter with groundwater contamination comprising cyanide, polyaromatic hydrocarbons and petroleum hydrocarbons.

The project, funded by Department of Trade and Industry (DTi), SecondSite, PB and the Biological and Biotechnological Science Research Council, expanded reactive barrier technology to include sequential treatment steps that may be combined with either active or passive hydrogeologic control of groundwater. Termed the Sequential Reactive Barrier (SEREBAR) technique, this will provide a potential break-through for solving problems in contaminated land and groundwater remediation.

Sustainable treatment of the groundwater means that no pumping or surface treatment system is required. In addition, free-phase liquids (such as petrol and tar) will be collected and recycled as part of the project.

The system uses a bentonite slurry impermeable barrier (Figure 1) to channel groundwater toward a treatment zone. This barrier was modelled using a groundwater flow model (Figure 2) that predicted the impact on the local groundwater table. The treatment system uses granular activated carbon and sand filters that are inoculated with aerobic and anaerobic bacteria.

The flow of groundwater through the treatment units can be altered using weirs and flow switches, thereby enabling development of optimum treatment conditions. This will then treat the groundwater which flows through the system by gravity. PB designed an oil interceptor that will remove tar from the system, and a free-phase collection system (using belt skimmers) has also been installed.

Following treatability studies, the full scale treatment began construction in January 2004. The SEREBAR project was completed in May 2004. The first set of analyses has shown that the system is successfully removing polyaromatic hydrocarbons, cyanide and petroleum hydrocarbons. The concentrations within the groundwater leaving the site are now very low and within regulatory approved limits.