How safe is Wellington's water supply?
The contamination of Havelock North’s drinking water is a crisis for both residents and the council. It begs the questions…. could it happen here? What are councils and Wellington Water doing to make sure it doesn’t?
The Wellington water network
Wellington Water’s catchment area covers Lower Hutt, Wellington, Upper Hutt and Porirua cities. It has three sources of drinking water; the Hutt River, the combined flow of the Wainuiomata and Orongorongo rivers and the Waiwhetu Aquifer - a natural underground reservoir that’s fed by river-water filtered through the ground.
Wellington’s water is sampled and tested in accordance with the Ministry of Health’s Drinking Water Standards for New Zealand. The standards set out the sampling rate for the population served and the tests that must be carried out on all samples.
We carry out about 5,700 samples a year, at nearly 500 sites across the four cities. Sampling rates are also determined by whether or not the water is chlorinated. As most of Lower Hutt’s water supply is not chlorinated, it’s subject to more sampling than water for the other cities.
Because it is impracticable to monitor water supplies for all potential pathogens (bacteria that cause disease), E. coli is used as an indicator organism for faecal contamination of drinking water.
If a sample tests positive for E. coli, the water is resampled at the original and adjacent sites, an investigation and physical inspection is carried out on the affected area, and corrective action taken. Depending on the number of bacteria (or colony forming units) counted in the sample, this could include a boil water notice being issued and a reservoir or network could be dosed with chlorine to kill any bacteria present.
Wellington Water continues to monitor and manage the water supply system to ensure its safety, and will take close note of the learnings from Havelock North.