The Council is replacing the remaining shallow wells that supply water to Christchurch’s northwest with deep wells to reduce the small risk of contamination from those shallow wells.

The northwest zone has a D grading under criteria set out in the Drinking Water Standards for New Zealand(external link). This is because of the shallow wells, which have a small risk of contamination from surface influences. 

In 2012, the  Council began a programme to replace the shallow wells in the northwest with deeper wells. The work is being reprioritised to reduce the risk of contamination through the more shallow wells.

All 22 shallow wells on the replacement programme are turned off and most of them will be fully decommissioned and sealed by the end of June 2018.

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About the northwest zone

  • The northwest zone supplies about 80,000 people.
  • There are 12 pump stations and 53 wells supplying the zone. Of these, 22 shallow wells are being replaced as they have a small risk of contamination from surface influences.
  • The 22 wells to be replaced range in depth between 16m and 43m:
    • 15 have already been decommissioned
    • 7 are turned off and won’t be used unless absolutely necessary during peak demand

Treatment options

The Council has decided not to treat water from these shallow wells with chlorine while work is done to decommission them.

Treatment with chlorine will be used in the following situations:

  • when new infrastructure such as reservoirs are being introduced
  • after a reservoir has been taken offline for routine planned cleaning
  • in response to positive E-coli tests, where typically the contamination is from bird droppings mixed with rain seeping through very small cracks.

As well as speeding up the replacement programme, the Council will increase its testing regime for E.Coli and will check wellheads for possible vulnerability to contamination.

The Council is also cleaning and coating reservoirs with a waterproof membrane to remove the risk of direct contamination in rain events. 

How the work will be funded

The additional cost of fast-tracking of the northwest shallow wells replacement programme is expected to be about $480,000. This will be paid for by bringing forward spending from future budgets and deferring other less critical projects.