Rules determining where firefighting foam can be used in the Dunedin district may be introduced after contamination of the city's main water catchment.
The Deep Creek catchment was yesterday confirmed as unaffected by the massive vegetation fire that burnt across 5000ha of land at Te Papanui Conservation Park on Old Dunstan Rd at the weekend.
However, the Deep Stream catchment, which usually provides 80% of the city's water supply, was contaminated with firefighting foam and ash.
Dunedin City Council 3 Waters group manager Tom Dyer confirmed yesterday the council had no agreements in place with Fire and Emergency NZ (Fenz) concerning use of the foam.
Formalising an arrangement would "be wise'', he said.
"We will review that in the future, and certainly it's going to be one of the lessons from this.''
Fenz special risks, safety and urban search and rescue manager Ian Duncan said 1200 litres of class A foam was used. The foam concentrate was mixed to a very low level of one part concentrate to 500 parts water.
Mr Dyer said testing on Deep Stream to determine contamination levels began yesterday.
The samples had to be sent to Australia, and results should be known by the end of the week.
Bringing the catchment back into use was estimated to take between three months to one year.
Although there were some treatment options, it was likely they would just "wait the contamination out'', he said.
The foam used by firefighters was highly soluble, so over time it would be diluted and taken downstream. If the summer was dry and hot, the process would be slower, but if the area experienced a "lush spring'', it would be faster, he said.
The council stopped taking water from the Deep Creek catchment on Saturday as a precaution, but aerial photos yesterday confirmed it was not contaminated.
"Deep Creek is one of our key water catchments so it's great to know it can continue to supply safe drinking water to Dunedin residents,'' Mr Dyer said.
Dunedin residents are being asked to continue to limit their water use while staff assess the overall implications of the fire for the city's supply in the medium term.
It is likely voluntary restrictions will remain in place over summer, and more formal restrictions may have to be considered based on the weather and water use over coming months. The city's usual water use is about 44,000cu m a day.
"With Deep Stream out of action, we'll be taking about 15% of our water from the neighbouring Deep Creek catchment and 85% from different sources, such as the Silverstream and Taieri bores from the Taieri River,'' Mr Dyer said.
The council had considered fire scenarios in the catchment, but in terms of what would happen if 10% of the catchment was burned, not 75%.
"So this is a slightly new consideration and it's quite an extreme event. Having this catchment offline for this amount of time is a significant inconvenience, but we're still able to supply the city with water,'' he said.
He confirmed there had been no infrastructure damage as a result of the fire.
The cost of the incident was still to be determined, he said.
Meanwhile, the environmental impacts of the fire remain unknown.
Department of Conservation coastal Otago operations manager Mike Hopkins said more information about the environmental and conservation impacts would be known once the affected area had been fully assessed.
Te Papanui protects a large amount of native tussock grassland which includes 60% of Dunedin's water catchment area.
The park is also home to a huge variety of other native plants and animals, including 547 species of native insects, the narrow-leaved snow tussock, and many rare wetland plants.
The 21,000ha park opened in 2003, and was created from existing protected areas, land purchased by the Nature Heritage Fund, and land from tenure review.
In outline
- Water is collected in the catchment, which acts like a sponge.
- The Deep Stream and Deep Creek pipelines, 58km and 64km respectively, use gravity to take water to the Mt Grand reservoir.
- From there it is treated at the Mt Grand treatment plant.
- The Taieri bores and Silverstream catchment flow into the Southern Reservoir.
- Treated water is carried from the plants to reservoirs around the city via a network of large mains, pumps and valves to maintain or reduce pressure.
- From the reservoirs, 726km of pipes bring water to properties.
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