Water Quality

• Selwyn District Council takes water samples to monitor microbial contents (regular bacterial testing) of the source water, treated water and from reticulation according to the Drinking Water Standards for New Zealand requirements.
• Currently Selwyn District Council tests all water sources for chemical determinands on an annual basis. Please refer to Annual Chemical Test Results.
• Our water quality monitoring is governed in compliance with Drinking-water Standards for New Zealand. 
• Our Water Supply Bylaw 2008 applies to water supplies under the care, control and management of, or water provided by, the Selwyn District Council, except for water supplies subject to the Selwyn District Council Water Race Bylaw.

• Treatment methods differ across supplies and  include secure ground water, Ultraviolet (UV) disinfection, selective abstraction and filtration, and chlorination.
• UV (ultra violet light) disinfection treats the water where it enters our supply network. 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.
• Chlorine is used as a preventative measure against contamination, particularly in the pipe network. This is done on a risk basis and in line with new national water safety requirements. Chlorination has been widely used internationally and here in Aotearoa where over 90% of water supplies are permanently chlorinated.

Selwyn water supplies generally draw water from the following two sources:

1. Ground water (aquifers):
   1. Deep secure ground water. These sources are protected from the influence of surface water contamination and are usually deemed to better source of raw water supply.
   2. Shallow or insecure ground water. Shallow bores and wells are generally at a higher risk of contamination and may require a higher level of treatment.
2. Surface water (general)
   1. Sourced from rivers, streams, creeks, water races and irrigation races. These open catchment sources are at higher risk of contamination and need higher levels of treatment before distribution.

The well-head or bore-head is where the water is extracted from the ground. It should comprise of a suitably designed and constructed concrete pad and has to be well sealed. This protects the bore and prevents any contamination from the surface, e.g. during surface flooding or heavy rainfall periods.

Water from non-secure ground water (shallow bores or wells), and surface water is treated under most circumstances. Treatment involves mostly filtration and disinfection. Filtration removes suspended matter (particles) from the water and this enables effective water treatment to remove harmful microorganisms such as bacteria (e.g. E.coli) and protozoa (e.g. Cryptosporidium and Giardia).

There are various types of filtration. These include cartridges, filter bags, candle filters, sand filters and reverse osmosis. Selwyn has many supplies with cartridge micron filters that have proven to be an effective medium to maintain a good quality water for an advance level of treatment.  There are various methods of water treatment. These include: Ultra Violet Light sterilisation, chlorination and ozone. The main method is UV light. Treatment systems such as UV light need to be of a suitable design and well-maintained. Selwyn has 13 supplies which have a state of art validated UV treatment system.

Whilst UV treatment system ensures that the Water is disinfected at the treatment plant, there is still the possibility of contamination from infiltration on the pipes or other factors. Therefore some level of residual disinfection is required in the reticulation network. Chlorine is added in the water to achieve this end result from time to time. Surface water supplies like Acheron, Malvern Hills Dalethorpe, and Springfield & Hororata have permanent chlorination as an additional protection barrier.
Other sources have temporary chlorination in line with national drinking water regulations.

Backflow prevention is important, particularly on farms and on properties where commercial or industrial activities are being undertaken. Backflow prevention ensures that no contamination is able to backflow into the water source or the reticulation. There are different methods of backflow prevention. Examples are: air gap separation by ball cock, a range of back flow prevention devices depending on the water pressure and purpose. Back flow prevention systems and devices need to be maintained to ensure they are fully operational and effective.

More information about Backflow prevention

• Traces of metals – in particular lead – are of concern because over time they pose a potential health risk, especially if consumed from childhood. The whole phenomenon is technically known as ‘Plumbosolvency’.
• The Ministry of Health therefore recommends that people do not drink the first mugful of water if their tap has not been used for several hours overnight, or after long absence from your property. Instead, people should run the tap for a second or so, which will quickly flush-away any water that has become contaminated with lead or any other metals from the plumbing fittings.
• Additional details on this issue can be sourced from the Ministry of Health website – www.moh.govt.nz or the Plumbosolvency Q & A Sheet [PDF, 34 KB]