Christchurch's drinking water contaminated 125 times in four years
E coli bacteria has been found in Christchurch's drinking water 125 times in the past four years.
The number of times contamination has occurred is falling though, as the Christchurch City Council completes work to reduce the public health risks.
A report on the council's drinking water monitoring programme – to be discussed at the infrastructure, transport and environment committee on Thursday – reveals 43 water samples tested positive for E coli in 2012-13 and 39 the following year. Another 29 positive results were found in 2014-15 and 14 during 2015-16. During this time, 24,872 samples were taken.
The information has been released in the wake of the Havelock North water contamination crisis, where thousands of residents became sick after drinking the town's water.
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Council three waters and waste head John Mackie said in the report that sampling data suggested water supply reservoir tanks were the main contributors to the positive tests.
"Corrective actions always proved successful and over the last years council has made significant improvements to its reservoir maintenance programme to reduce the risk."
Mackie said positive E coli results from groundwater wells or pump stations were rare and were related to shallow non-secure wells, which were being replaced as part of a $16 million upgrade of wells in Christchurch's northwest. This work was due to be completed by June 30, 2018, but council staff were considering accelerating the programme to reduce the contamination risk even further.
Equipment failure at water treatment plants had resulted in E coli being present, but upgrades at all Banks Peninsula plants had reduced the risk of future transgressions, Mackie said.
The council had embarked on several other programmes and initiatives to reduce the risk to public health. It already tested more than the minimum number of water samples required under law, but proposed increasing this even further in response to the Havelock North situation to provide "additional assurance to the community".
Christchurch's water supply is graded "b", or "satisfactory, very low level of risk".
Without treating the water with systems such as chlorination, UV light, and membrane filtration, it is impossible for Christchurch to get an "a" rating.
That does not mean the water is bad, but means it carries more risk.
Canterbury medical officer of health Alistair Humphrey said last week that the positive tests should not cause alarm, but highlighted a risk.