Safe water

Supplying safe drinking water is the most important task for water authorities. Water catchment areas are increasingly under threat from animals and recreational use, which can contaminate the water, and from logging which results in increasing runoff and sediment in the water. Large-scale conversions to dairying in Southland and Canterbury have raised concerns about pollution of local water supplies.

To comply with the Drinking Water Standards for New Zealand, water supply authorities must monitor for bacteria and demonstrate that treatment can remove protozoan cysts.

Bacteria

Water authorities monitor for the bacterium E. coli, which is found in animal gut. While it's not necessarily harmful, it indicates the potential presence of other more dangerous organisms. Cholera and typhoid are just two of the diseases which can be spread by contaminated water. In New Zealand water supplies, campylobacter is the most common cause of illness.

Chlorination at the treatment plant is the most common means of killing bacteria in source water. But bacteria in the water mains are another threat, so water suppliers try to ensure there's a residue of chlorine to protect the water on its way from the treatment plant to your home.

Because deep underground water sources (such as in Christchurch) are usually protected from animal and human contamination, their water is usually not chlorinated.

Protozoan cysts

Giardia and cryptosporidium belong to a group of microscopic single-celled organisms called protozoa, which live in the intestines of humans and animals. Their cysts are passed out in faeces. If they get into the water source they can cause severe diarrhoea or worse. Cryptosporidiosis may be fatal in people with lowered immunity, and in small children and the elderly.

It's very difficult to check for these organisms - they're not distributed evenly, but exist in the water in clumps. They may settle to the bottom in still water, to reappear in the water flow when disturbed, perhaps by heavy rain in the catchment area.

To comply with the drinking water standards, there must be a level of treatment which can remove the cysts, or the source must be secure from contamination by animals or humans. Chemical coagulation causes particles, including giardia and cryptosporidium cysts, to form a sludge which can be filtered out.

Despite these measures, giardia and cryptosporidium may pop up in water supplies. In 2003 a cryptosporidium scare forced Masterton residents to boil their water.

Local water supplies are required to issue a "boil water" warning if there is a risk of your drinking water becoming contaminated.