Drinking water

In New Zealand, we take it for granted that our tap water is safe to drink. But, where does it come from, what’s in it and what do we do to make sure it’s safe?

How good does water taste? See Taste Sources in Water, link at the bottom of the page.

Where does it come from?

Your water supply comes from three sources: the Hutt River, the combined flow of the Wainuiomata and Orongorongo Rivers and the Waiwhetu Aquifer – a natural underground reservoir beneath the Hutt Valley.

What’s in it?

Water naturally contains a variety of chemical elements. It can also contain contamination from dirt, decaying vegetation and animal waste.

Because it’s much easier to provide you with high quality treated water if the water is relatively clean to begin with, river water is collected from areas that are upstream of human settlement.

Public access to these areas is limited and, where needed, pest animal controls are put in place to limit contamination from animal waste.

River and aquifer water also absorbs chemicals from the land it flows over and through. Because some of these elements can be harmful if consumed at high levels, the levels of a number of chemical elements that occur naturally in our water are actively monitored.

Hard or Soft?

The water supplied to Upper Hutt, Lower Hutt, Porirua and Wellington by Greater Wellington can be described as soft.

Water with high calcium and magnesium content is characterised as hard, while water with less calcium and magnesium content is soft.

The sum of all calcium and magnesium compounds in water results in the total hardness, measured in milligrams of calcium cabonate per litre (CaCO₃, mg/L) (1mg/L equals one part per million, or 0.01millimoles (mmol)/L).

Water hardness is described as an "aesthetic determinand" in the Drinking-water Standards for New Zealand 2005 (Revised 2008), meaning a property that can adversely affect taste, colour or general appearance of water. Guideline values in the standards for total hardness are 200mg/L CaCO₃. and levels below 100mg/L can be characterised as soft.

During the 2010/11 operating year (to 30 June 2011), the mean value for total hardness in treated water leaving each of Greater Wellington's four water treatment plants was:

• Te Marua: 20mg/L
• Wainuiomata: 41mg/L
• Waterloo: 51mg/L
• Gear Island: 24mg/L

What do we do to it?

Water from river supplies is strained through dams to remove sticks, leaves and silt and then piped to treatment plants in TeMarua and Wainuiomata. As river water enters the treatment plant, carbon dioxide and lime are added to improve the following treatment processes:

• coagulation - attracting negatively charged contaminants to a positively-charged chemical.

• flocculation - gently stirring the particles to encourage them to form into larger clumps, or ‘flocs’.

• separation and filtration - to remove the ‘floc’ particles.

Water from the Waiwhetu aquifer is naturally filtered during the twelve months it spends underground, making it free from disease-causing micro-organisms. This means it only needs to be treated with lime at the Waterloo treatment plant, to reduce its corrosive effect on pipes and fittings.

Once the water has been treated, fluoride is then added to help protect your teeth.

Is there chlorine in the water?

All of the water supplied to Wellington, Porirua and Upper Hutt is also chlorinated. Wainuiomata, Stokes Valley and Manor Park also receive chlorinated water; the rest of Lower Hutt is unchlorinated.

Your water is chlorinated to kill water-borne bacteria and viruses that cause diseases and could make you ill. This treatment process has two benefits: not only does it remove germs and protozoa (like giardia and cryptosporidium) from your water supply, it can also kill bacteria and viruses if they get into the supply system between the treatment plants and your taps, for instance through a cracked pipe.

Most areas of Lower Hutt can receive an unchlorinated water supply because the water comes from the secure underground aquifer beneath the Hutt Valley.

Because the water taken from this aquifer has been in an air-tight environment underground for at least 12 months, the Ministry of Health describes this water as being free from microbiological contamination, since bacteria, viruses and protozoa can’t survive under these conditions.

How do we make sure it’s safe?

We have a rigorous monitoring programme in place to make sure that there are no bacteria, viruses or protozoa (such as cryptosporidium and giardia) left in your water once it’s been treated. We also monitor the concentrations of a range of chemicals in your water, to make sure that there are no metals or other chemicals at levels that are considered unsafe. To do this, we test your water at the source, at treatment centres and in the pipes to your home.

These monitoring and testing processes are designed to prove that we meet the standards for water safety contained in the Ministry of Health’s Drinking Water Standards for New Zealand, so you can be sure your water is safe to drink.

Just how good is it?

Water quality standards are set by the Ministry of Health. Each year they publish a grading for every public water supply indicating how safe the water supply is from contamination.

The grading system has two parts:

• A grading for the source of the water and the treatment plant (‘A1’ to ‘E’), and

• A grading for the distribution system (‘a1’ to ‘e’).

The gradings range from ‘completely satisfactory’ (‘A1’ or ‘a1’) to ‘completely unsatisfactory’ (‘E’ or ‘e’).

We are very fortunate to have ‘A1’ ratings for the TeMarua and Wainuiomata treatment plants and ‘B’ for Waterloo. (Even though the aquifer water supply at the Waterloo treatment plant is considered to be free of contamination, it can’t be rated higher than a ‘B’ because it is not treated with chlorine).

What’s more, the bulk distribution system ratings are all ‘a1’.