• Dalethorpe Water Supply
  • Hororata - Acheron (Hororata) Water Supply
  • Hororata - Acheron (Acheron) Water Supply
  • Sheffield/Waddington Water Supply
  • Springfield Water Supply
  • Castle Hill Water Supply
  • Hartleys Road (Malvern Hills) Water Supply

Water chlorination in Selwyn

Selwyn District Council is committed to providing our communities with safe drinking water. The Council currently uses a variety of water treatment methods for its 30 water supplies and all supplies have Water Safety Plans which are approved by the Canterbury District Health Board. Treatment methods used in various supplies include secure ground water (to meet current New Zealand Drinking Water Standards), Ultraviolet (UV) disinfection, selective abstraction and filtration, while chlorination is currently used on six supplies: Acheron, Castle Hill, Dalethorpe, Hororata, Sheffield/Waddington and Springfield.

For more information on the ways that we treat water, visit Water Quality in Selwyn District.

During Selwyn District Council’s 2018/19 Long-Term Plan deliberations Council voted to develop a risk based assessment for determining which of Selwyn’s 30 water supplies should be chlorinate.

Why the Council is adding chlorine to water supplies

We want to make sure that everyone in Selwyn has safe drinking water. The Health Act makes the Council responsible for providing a safe and wholesome drinking water supply and to do everything practicable to meet the NZ Drinking Water Standards. In December 2017 The Director-General of Health issued a formal statement reminding water suppliers of their statutory responsibilities set out in the Health Act 1956. The formal statement recommends reconsidering reliance on ‘secure bores’ and warns that the public risk is increased if drinking water is untreated. Councils and District Health Boards have been advised to seek information from public health units on the need to disinfect any drinking water supply that is currently not being treated. The Ministry strongly advises water should be treated, with chlorination the safest treatment option.

Chlorination as a preventative measure

Chlorine is used as a preventative measure against contamination.  Chlorine provides residue treatment within the reticulation which has the ability to treat any contamination that enters the network after the water leaves the treatment plant.  In unchlorinated supplies, chlorine can be added to treat transgressions once detected. However, it is far better to prevent contamination than to react to an event with emergency chlorination.  Chlorine has the potential to provide additional protection to all water supplies regardless of the age and condition of the network.

We have made the decision to add chlorine, as well as UV treatment because

UV (ultra violet light) treats the water where it enters our supply network. It is very effective as long as the water is not turbid (discoloured). UV treatment does not treat the water once it is in our reservoirs and pipes. There is always the potential for contaminants to get into the water reticulation system, for example through cracks in the reservoir, broken pipes – tree roots growing through pipes are a potential problem, as are unlawful connections from households where people have done their own plumbing.

The amount of chlorine in water

Typically we will use a dose of 0.4mg to 1.0 of chlorine for every litre of water. This will give a residual dose of 0.2mg per litre in what comes through your taps.

History and safety of chlorine

Chlorine has been used safely all over the world for around 120 years. It keeps millions of people all round the world – including most of New Zealand – safe from waterborne disease. The amount of chlorine dosed into the water supply will be carefully managed to ensure levels of chlorine in the water people drink are absolutely minimised. The use of filters will mitigate any risks for those on dialysis. This can be arranged by the Canterbury District Health Board.

If you are concerned about the taste

You can either: Fill a jug of water and leave it on the bench or in your fridge overnight. The chlorine will dissipate naturally over a few hours, or install an under-bench filter. Chlorine and any associated by-products can be removed by using a granulated, activated carbon (GAC) filter. These are available from hardware supplies stores and water filter companies.

What do to if you don’t want to shower or wash your clothes in chlorinated water:

You can buy at your own cost a carbon filter that attaches to your water supply where it enters your property. It will remove all the chlorine from the water to your home.

Animals and chlorination

If you have fish in outside ponds you will need to either turn down in-coming water to an absolute trickle (this dilutes the chlorine level to a safe amount for your fish), or fill up drums of water and let them sit for at least 24 hours before using (the UV of the sun evaporates chlorine). For fish tanks or bowls inside, fill up a container of water and let it sit for at least 24 hours and then only replace 1/3 of this water at a time with what is in the tank already. If you’re still worried, you can buy de-chlorinating kits (sodium thiosulfate) at pet supplies stores.

New Zealand drinking water standards

The Drinking Water Standards for New Zealand are issued by the Ministry of Health (external link) under the Health Act and set out the requirements water suppliers need to meet to provide safe water to their communities. The standards specify:  Maximum amounts of substances, organisms, contaminants and residues that may be present in drinking water. Criteria for demonstrating compliance with standards.  Remedial action to be taken in the event of non-compliance with standards.

Feedback on chlorination

In more recent history, the Hororata, Acheron and Dalethorpe rural water (including Russells Flat) supplies have been chlorinated with complaints about chlorine being uncommon.  In fact, some users of these schemes have reported that they were unaware of the scheme being chlorinated.

It is likely that modern precise injection of chlorine and regular contractor checking of chlorine residual has overcome the issues experienced by some users in the past.

The water around New Zealand

Over 80% of the country is being supplied with chlorinated water.

Risk Assessment of Water Supplies

Following consultation on the 2018-2028 Long-Term Plan the Council voted to develop a risk-based assessment for determining which of Selwyn’s 30 water supplies should be chlorinated. We engaged BECA consultants to develop the assessment, and representatives from the Council, Sicon (the water supply operators), Food and Health Standards (Environmental Health Officer) and Drinking Water Assessor (Community & Public Health) also contributed.

The assessment looked at the source, treatment, reticulation and end users of each scheme, and considered a number of factors such as well depth, treatment, turbidity, risk of flooding, stock exclusion, population including vulnerable populations, growth in number of connections, condition of pipes, and water pressure.

Each water supply has been ranked from Priority 1 (highest risk) to Priority 5 (lowest risk). Chlorination has been recommended for supplies ranked Priority 1, along with some Priority 2.

Read the full report on Risk assessment of water supplies