16 Sep 2016

Options for reducing the potential public health risk to the water supply in the northwest of Christchurch will be outlined to Councillors when they meet on Thursday.

Council Head of Three Waters and Waste John Mackie has prepared a report for the Council which recommends it fast-tracks the drinking water supply well improvement programme in the northwest of the city, with the aim of decommissioning most of the shallow, unconfined wells by the end of March.

The shallow, unconfined wells are considered the most susceptible to microbiological contaminants as the ground water can be affected by land use activities above as surface water can seep into the aquifer.

Northwest water zone

The Northwest water zone. Click to view as a pdf [3.2 MB]

A water sample is taken for testing.

A water sample is taken for testing.

In the northwest water supply zone – which stretches from Belfast through to Yaldhurst and includes parts of Riccarton and Addington – water is drawn from shallower wells that are vulnerable to contamination and have a D grading. In most parts of Christchurch drinking water is sourced from deep wells that carry the highest possible grading for a secure, unchlorinated groundwater supply, which is a B.

About 80,000 people get their water from those shallow wells.

A programme is already in place to replace the shallow wells with deeper ones, but currently it is not scheduled to be completed until the end of June 2018.

Mr Mackie’s report recommends the Council fast-track that work so that most of the shallow wells can be decommissioned by the end of March, 2017, which should reduce the potential public health risk.

In the meantime Mr Mackie is recommending the Council step up its monitoring of the water quality at the Farrington, Grampian and Avonhead pump stations and begin talking to affected residents about other preventive measures that could be considered, including disinfection of the water using chlorine until the deeper wells are commissioned.

Mr Mackie’s report also recommends the Council look at using water conservation measures to reduce the reliance on shallow bore water supplies and consider feeding water into the northwest zone from more secure neighbouring zones.

Mr Mackie has put forward some alternative options in his report.

The Council could:

  • Fast-track the well improvement programme and put in place additional monitoring and safeguards that do not involve chlorination.
  • Continue with the existing well improvement programme, but put in place additional monitoring and safeguards, including consideration of temporary chlorination.
  • Leave things as they are and continue with the existing well improvement programme, with no additional safeguards.

The safety of water supplies across the country has been in the spotlight since last month when it was revealed that thousands of people had fallen ill in Havelock North after testing for E.coli indicated that there had been bacterial contamination of the town’s water supply.

In the wake of that incident Christchurch City Council received a letter from Canterbury Medical Officer of Health, Dr Alistair Humphrey, asking Council to provide details of the steps it was taking to manage the potential risks in the shallow, unconfined wells in use in the northwest of Christchurch.

What happens when E.coli are detected in the water?

E.coli are used as markers or an indication of contamination in the water supply. Most E.coli strains are harmless, although some can be harmful to health.

The presence of E.coli in the drinking water means the water has been contaminated with faecal matter from animals or humans. In that instance immediate steps are taken to isolate the affected supply and then to identify and eliminate the cause.

Learn more about Christchurch's water supply.