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Installing the treatment zone

A box 6m by 4m by 12m deep was excavated, under a cement-bentonite slurry, downgradient of the contaminant plume and a 12m deep by 1.2m diameter reactor vessel placed within it. This reactor formed the treatment zone and was filled with 15t of iron imported from the US. The reactor design was based on an expected 50 years lifetime for this quantity or iron. The contaminated groundwater plume was contained within a cement-bentonite cutoff wall with a design permeability of 10-9m/s. To improve the flow through the reactive zone an upstream gravel filled collector and a downstream gravel filled distributor were installed on either side of the reactor. The reactor was designed so that groundwater entered at the base of the reactor and was discharged at the top (Figure 3). This design ensured that the contaminated groundwater flow was in contact with the full mass of iron at all times irrespective of seasonal variations in groundwater level. The excavation for the insitu reactor was formed under a cement- bentonite slurry to prevent the excavation forming a pathway for contamination from the source to deeper permeable strata which underlay the site. For deeper localised contaminant plumes hydrofracture techniques have now been employed by Golder in the US to place active materials as horizontal lenses and vertical walls (Figure 4).

Monitoring points were installed to measure concentrations of chlorinated solvents in the collector, reactor and distributor. Figure 5 illustrates the changes in concentration of TCE as the groundwater moves through the reactor. Groundwater analysis had shown a gradual reduction in TCE concentrations in both the source area and down gradient wells since the reactor was installed though concentrations downgradient of the reactor have reduced more slowly than within the reactor as there was contamination downgradient of the reactor prior to its installation. As there is some unused capacity in the reactor, groundwater from this contaminated area is now to be pumped back to the reactor using pumps powered by a combination of solar and wind power.

Jefferis concluded that a significant reduction in chlorinated solvents had been achieved so far, and a steady clean up would continue. The project was undertaken by Nortel on a voluntary basis without regulatory drive as part of its environmental programme. Work such as this maintains a client's sound environmental image and the value of the positive publicity can far outweigh the cost of the works.

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