KEY POINTS:
Drinking water collected from home roofs carries a significant risk of illness, according to a five-year study from Massey University.
The survey found that more than half of 560 samples from private dwellings exceeded the minimal standards for contamination and 30 per cent showed evidence of heavy faecal
contamination.
"I'm utterly amazed at the number of roof water supplies that fail the New Zealand drinking water standards," said Stan Abbott, a microbiologist at the Institute of Food, Nutrition and Human Health.
Roof-collected rainwater consumption was popular because the public believed that rainwater was pure and safe to drink, said Mr Abbott, who is director of the roof water research centre at Massey's Wellington campus.
More than 400,000 people depend on roof-collected rainwater systems for their drinking water, especially those living on farms, lifestyle blocks or baches that are not served by town water supplies.
The likely sources of the contamination were faecal material deposited by birds, frogs, rodents and possums, and dead animals and insects, either on the roof or in gutters or water tanks.
Mr Abbott said contamination could lead to gastrointestinal diseases from pathogens including salmonella, campylobacter, giardia and cryptosporidium.
"Simple steps such as installing down-pipe debris screens and a first-flush diverter will reduce the risk of contracting waterborne diseases," he said.
A first-flush diverter is a device that reduces contamination of the tank water by diverting the first flush of contaminated water after a rainfall so that contaminants do not enter the tank.
Recent research at Massey University has shown improvements in water quality in the storage tanks linked to first flush diverters.
Mr Abbott said that while few disease outbreaks linked to contaminated roof-collected rainwater had been reported, there was under-reporting of illnesses associated with contaminated roof water.
"The lack of evidence linking illness and poor quality roof water inhibits moves to improve systems delivering rainwater for consumption."
Although it was the homeowner's responsibility to ensure drinking water was clean, information on the safe collection and storage of roof-collected rainwater seemed not to be reaching many people, he said.
The Building Act requires premises to be provided with drinkable water.
Mr Abbott urged roof water users to reduce risks of disease from contaminated rainwater consumption by regular maintenance and using a well-designed system.
- NZPA
