Chemicals in your water

At the treatment plant chemicals are added to the water to kill or help remove bugs, or to improve its look, smell or taste. Traces of these chemicals may remain in the water, and some have aroused concerns about possible harmful side-effects. However, the health benefits of these chemicals are regarded as outweighing the possible risk.

To comply with the drinking water standards, authorities must monitor the levels of these chemicals to ensure they remain within World Health Organisation (WHO) guidelines.

THMs

Chlorine can react with organic matter such as decayed vegetation in water to form potentially harmful by-products called trihalomethanes (THMs). The most common THM is chloroform. Some studies have suggested a possible link between chlorinated tap water and a slightly higher risk of some forms of cancer.

Aluminium

The aluminium in drinking water is just a small proportion of our dietary intake. It occurs naturally in river and lake water and is often used in chemical coagulation to remove suspended particles from water, making it clearer. While most of the aluminium can be filtered out of water, small amounts may pass through.

Fears that aluminium in water supplies could cause Alzheimer's disease have not been backed by research.

Fluoride
Many water authorities add fluoride to water, as it has a proven record of reducing tooth decay. It's also an ingredient in many toothpastes. Critics say fluoridated water is unnecessary and that it leads to dental fluorosis (mottled teeth) in people who ingest too much. Some claim that at high levels it can cause cancer, but the WHO has not been able to establish a link. Negatively charged ion exchange resins will remove fluoride.

Natural contaminants

Some chemical contaminants occur naturally, for example, heavy metals in some areas, or nitrates in farming districts. In areas where there is likely to be an unacceptable level of chemical contaminants in the water supply, water authorities are required to carry out regular checks.

Problem supplies

People in rural areas, or with private supplies, may face problems of contamination by agricultural runoff, pesticides and herbicides. In particular, nitrates from fertiliser runoff or sewage may contaminate groundwater supplies.

While nitrate is harmless in itself, it can be converted into nitrite which is hazardous. It mainly poses a problem to babies and young children as it can decrease the blood's ability to transport oxygen, but there have also been claims it is linked to cancer.

Systems which collect water from the roof are vulnerable to contamination by birds and animals. Bore water, particularly shallow bore water, may contain nitrates, pesticides and harmful bacteria.

If you're concerned about the quality of your water, you can check it online. Public libraries also hold copies of the Register of Community Drinking-Water Supplies in New Zealand, published each year by the Ministry of Health.

If the grading suggests there is a problem, or your supply has not been graded, talk to your local water supply authority or to the health protection officer at your local hospital. They can tell you what contaminants are most likely to be present in your supply, and advise you on how to treat them. If you wish to have your water supply tested, they'll recommend a laboratory accredited to carry out the required testing.

For more information on rural water supplies, see Water Collection Tanks and Safe Household Water, a resource from the Ministry of Health.