No cause for concern: slightly elevated levels of arsenic in treated Waikato River water

Two drinking water suppliers who treat water from the Waikato River have reported to Taumata Arowai slightly elevated levels of arsenic in recent samples of treated water. There is no immediate risk to public health.

The ‘maximum acceptable value’ for arsenic in New Zealand drinking water is 0.01milligrams per litre.

Watercare reported a result to Taumata Arowai of 0.011 milligrams per litre in a sample taken on November 25. Hamilton City Council’s testing on 19 and 20 November recorded results of 0.0113 and 0.0115 milligrams per litre, which were also reported to Taumata Arowai. 

Both water providers followed standard procedure in notifying the agency and are keeping it informed as to their operational response. The agencies have extensive sampling programmes in place that have served their purpose in detecting the issue.

Further sampling has been carried out and Taumata Arowai will receive the results of these tomorrow. 

Previous samples taken earlier in the month by both water suppliers all showed levels of arsenic below the limit in the drinking water standards.

National Public Health Service regional clinical director Dr William Rainger says: “People who are drinking water sourced from the Waikato River do not need to be alarmed. The limits in the New Zealand Drinking Water Standards are based on the level of risk from a lifetime of drinking the water. 

“Drinking water with slightly elevated levels for a short period of time is unlikely to impact people’s health.

“You do not need to boil your drinking water. It is safe to drink.”

Arsenic is known to be elevated in the Waikato River, primarily due to geothermal activity, says Waikato Regional Council Environmental Science manager, Dr Mike Scarsbrook.

“We undertake monthly freshwater testing on the Waikato River. Testing was last carried out on 7 November and the arsenic levels were not unusual. These levels are typically above the drinking water standard, which is why treatment is required before drinking water is supplied to households.” 

As a result of water suppliers alerting the regional council to elevated arsenic levels after treatment, environmental monitoring staff have collected samples from a number of locations along the Waikato River – Lake Ōhakuri, Lake Arapuni, Karāpiro (upstream and downstream of the dam), Narrows Landing, Horotiu and Rangiriri. 

“The laboratory is prioritising testing and results are expected during the day on Thursday (November 28) which will be analysed by scientists from the council and Taumata Arowai.”

Watercare’s response

Watercare chief operations officer Mark Bourne says the Waikato River supplies about 20 per cent of Auckland’s drinking water.

“We treat the water at our Waikato Water Treatment Plant in Tūākau and distribute it throughout the metropolitan network. 

“In most parts of Auckland, treated water from the Waikato River is blended with treated water from our dams. However, Pukekohe, Buckland, Clarks Beach/Waiau Pā, Patumahoe, Glenbrook Beach, and Drury south receive water solely sourced from the Waikato River.”

Since detecting the elevated result, Watercare has reduced production at its Waikato Water Treatment Plant and increased production at its major Ardmore plant, which treats water from four dams in the Hunua Ranges.

“This extends the processing time at the plant, and we expect it to improve our ability to reduce the level of arsenic in the treated drinking water,” Bourne says.

“We’ve also stepped up our sampling at the treatment plant and within the distribution network, which includes sampling our blended water at the Redoubt Road reservoir.

“We’re working closely with other agencies and will continue to share information as we learn more.”

Hamilton water safe to drink, testing shows minor increase to arsenic level 

Hamilton City Council’s water supply remains safe for drinking, bathing, cleaning and cooking, following test results which showed small increases in the level of arsenic present.   

Testing during a stage of the treatment process on 19 and 20 November recorded results of 0.0113 and 0.0115 milligrams per litre, slightly above the MAV.

Arsenic levels have been stable in Hamilton’s drinking water for many years at below 0.0100 milligrams per litre.  

The current minor exceedance of the standards does not present any immediate concern for public health. 

“Ensuring the quality of our drinking water is a top priority for Council and our waters team,” said Unit Director – Three Waters Maire Porter. “While we are conducting additional testing, we want to reassure the community that we are taking the advice of Taumata Arowai, the drinking water regulator, and working diligently to make sure our decisions are based on the most accurate information available.” 

Waikato District Council’s response

In the Waikato District Council area, Watercare operates the local water networks under contract. Pōkeno and Tūākau are supplied by Watercare’s Waikato Water Treatment Plant. There are also treatment plants that serve the townships of Huntly, Te Kauwhata, Meremere, Taupiri and Ngāruawāhia. Samples from these plants are currently being tested, with results due tomorrow.