Waiwhetu Stream clean-up trial starts next week

The first stage of the works to clean-up the Waiwhetu Stream will start in the week beginning Monday 18 February. This is the beginning of the project to remove the contaminated sediment from the stream bed and other works which will protect the local residents from devastating floods such as the one that swept through the community in 2004.

This trial is testing the clean-up method that will involve installing sheet-pile walls to block off the stream, pumping the stream's flow around the walls while the stream bed is excavated to remove the contaminated sediments.

This is the first time this innovative clean-up method has been used in New Zealand. It has been successfully used before on the Little Scioto River, Ohio, USA. This river is about the same size and had about the same amount of contamination as the Waiwhetu Stream. The clean-up method there was approved by the US Environmental Protection Agency.

The trial is expected to cost $500,000 and is jointly funded by Greater Wellington, Hutt City Council and the Ministry for the Environment. The trial will help fine tune the method in our unique New Zealand environment, as well as refining the total cost for the full clean-up works.

The work will take place in an 80 metre stretch of the Waiwhetu Stream, between the Bell Road bridge and the rail bridge. Sheet-pile walls will be driven into the stream bed to block off the stream flow, and create two areas – one which will have the contaminated sediment excavated out and the other will be a settlement area where the water from the first area will be pumped into, to let any sediment settle to the bottom of the stream, and not escape into the normal stream flow.

The sediment will be transported to the Silverstream landfill in sealed trucks to ensure no contaminants are accidentally released into the community. A number of other measures, such as washing down of the trucks before they leave the site, will also ensure this.

The Waiwhetu Stream has a legacy of contaminated sediment from undiluted and untreated industrial discharges being pumped directly into the stream up until 1978 when a trade waste sewer was installed. Heavy metals such as lead, zinc, copper and cadmium, as well as pesticides are found in the contaminated sediment.

ENDS

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