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Healthy circulation

Directional drilling was used to install a groundwater remediation system for a massive contamination plume beneath a Danish town.Dan Simpson reports.

A2km long contamination plume beneath the town of Hedensted, 60km south of approximately =rhus is being tackled by a joint venture of two Danish firms.

The £900,000 (DKK 11M) contract brings together geotechnical contractor Per Aarsleff and groundwater remediation technology firm Kruger to install a series of pipes, filters and pumps.

The trichlorethylene plume built up between 1960 and 1980, discharging from a factory south west of the town.The chemical permeated a 4m-12m thick glacial sand and gravel layer and came to rest on a relatively impermeable clay layer below.

A hot spot directly under the former factory site is being cleaned by Danish consulting engineers Cowi and NCC in a separate contract under the overall £2.5M cost of remediation, pumping in steam and extracting the trichlorethylene in its volatile form.

Groundwater flow carried the chemical north east and then south east to form a 2km long, 400m wide 'dog-leg' contamination plume in the sand and gravel layer beneath a large part of the town, with concentration levels falling away from the source.

Concentrations of 0.23mg/kg of soil and 0.7mg/litre of water were found in the worst affected areas. And in samples taken from houses above the worst polluted areas, large concentrations of the chemical were found in the air.

Trichlorethylene is believed to be carcinogenic.

The clean up system designed by Kruger works by pumping out contaminated groundwater and sending it to cleaning stations, where it is passed through large cylinders of active carbon to extract the trichlorethylene.The water is then pumped back into the ground 200m from where it was removed and the cycle repeated.

The water is circulated by centrifugal pumps, capable of pumping 1,000m 3a day, buried 1m below ground level.The plume is split into five cells, each circulating its own water through a cleaning station. Concentrations are constantly monitored.

Because groundwater is circulated from low to high concentrations of pollution, reversing the natural groundwater flow, further spread of pollution is prevented.

Per Aarsleff is contracted to install the network of pipes in the plume using directional drilling to minimise disruption to the town.This is the first time the remediation method has been used with directional drilling in Denmark.

The pipes are installed just above the clay in stretches of between 250m and 420m, often beneath roads and buildings. Pilot holes were bored using a bentonite drilling mud and then widened using a 350mm reamer. A starchbased slurry, developed by Aarsleff, was used to aid reaming rather than bentonite which would have impeded the flow of water to and from the drainage and recharge pipes.Starch is organic and decomposes.

Once the reamer had widened the bore, it passes back through the bore, pulling the pipe behind it. Three 63mm slotted HDPE pipes were encased within a larger 200mm diameter pipe, to stop them snapping as they were pulled through. This was withdrawn after installation, exposing the smaller pipes to the soil and allowing them to drain or recharge the groundwater freely.

Per Aarsleff began drilling in mid-December last year and was due to complete the 5.5km length at the end of last month, with about 15km of pipe and 2.5km of filter in place.

The firm has specialised in directional drilling over the last four years and uses a compact unit, including control and monitoring equipment and a site cabin for operatives, mounted on a specially adapted lorry.

When GE visited the site at the beginning of May, some of the cells were already circulating and cleaning groundwater.The entire system should be working by the summer.

It is estimated that the water will have to be circulated four to five times and the clean-up could take between three and five years.

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