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Report Finds Low Risk Of Contamination

January 29, 2013

Report Finds Low Risk Of Contamination

Environment Canterbury has released a report and recommendations on managing the possible risk to Christchurch drinking water from treated timber piles used in building foundations.

“The aim of the report was to better understand the possible risk to Christchurch’s drinking water from leaching of treated timber piles before construction work starts in TC3 areas,” said Ken Taylor, Environment Canterbury’s Director Investigations and Monitoring.

“The report informs councils, government agencies and builders of the issue so they can take the necessary steps to minimise any risk to Christchurch’s drinking water.”

The Environment Canterbury study – Groundwater quality risk assessment for treated timber housing foundations used in the Christchurch rebuild – was completed with help and information from the building industry, CERA and the Ministry of Business, Innovation and Employment.

“The report found there is generally a low risk to groundwater quality throughout Christchurch because in most areas the drinking water aquifers are much deeper than any potential piling from building construction.”

The maximum economic depth for treated timber piles is reported to be 12 metres.

As a result the report looked for at-risk areas of TC3 land in Christchurch where the main groundwater aquifer was less than 13 metres deep – which allows for the maximum pile depth of 12m plus a 1m buffer.

The report found – based on worse-case scenario modelling and a literature review – there was a possible risk to groundwater quality in shallow gravels from leaching of the chemicals most commonly used to treat timber piles.

“In the TC3 areas where the gravel beds are close to the surface, the report recommended minimising the use of copper chromium arsenic (CCA) treated timber piles.

“As an alternative, steel or concrete or non-CCA treated wooden pile foundations can be used to minimise the risk to groundwater.”

Concrete and steel are relatively inert – compared with treated timber – with a low risk of contamination of groundwater.

The locations where both TC3 land and shallow gravels coincide are limited to small areas in Kaiapoi, in Hoon Hay, and near Prebbleton (see figure 4-1 in the report).

“Any risk to Christchurch’s drinking water from deep piling of treated timber can be minimised by good practice and co-operation between agencies and builders, now that we have this report and recommendations,” said Ken Taylor.

Mike Stannard, Chief Engineer Building System Improvement Programme, said MBIE will consider the findings and recommendations in the Environment Canterbury report, which will inform MBIE’s next update for repairing and rebuilding houses in Canterbury.

Technical facts

TC3 land in Christchurch is likely to sustain moderate to significant liquefaction damage in an earthquake. As a result any new buildings on TC3 land will require site improvements, deep piles, or surface structures with shallow foundations. The best option will depend on a site-specific geotechnical investigation.

The CCA treated timber likely to be used in deep piling is treated to hazard class 5 and contains a mixture of copper, chromium, and arsenic. These chemicals can leach into both soil and groundwater.

Worse-case modelling showed the arsenic concentration leaching from a row of 20 piles would be between 0.0047 and 0.009 mg/L (at a distance between 22 and 29 metres downstream and with a 1 metre per second aquifer flow) which is below the maximum acceptable value for drinking water of 0.01 mg/L.

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