Arsenic naturally occurs in the Waikato River. (File image) Photo: Leah Tebbutt
Waikato River samples will be sent to Australia for testing, as authorities investigate the reason behind the slightly elevated levels of arsenic in the water.
Last week, slightly elevated - but safe - levels of arsenic were detected in treated water from Waikato and Auckland's drinking supply.
A joint statement released by the affected councils, Watercare and the the Water Services Authority said further testing from the Waikato River returned results between 0.0189 milligrams per litre and 0.0256 milligrams per litre.
Those results were within the "usual range" for the same sites, which was between 0.0123 milligrams per litre and 0.03 milligrams per litre, the statement said.
The maximum acceptable value for arsenic in treated water is 0.01mg/L.
Waikato Regional Council environmental science manager Mike Scarsbrook said there had been a small increase in the amount of dissolved arsenic, which could be harder for treatment plants to remove.
The council would collect samples from the river to be tested in Australia, to determine the relative levels of arsenic 3 and arsenic 5 in the water, a council spokesperson said. The process was expected to take a month.
Water quality scientist Dr Jenny Webster-Brown, who has researched arsenic in the Waikato River previously, said this was important, because arsenic 3 - which indicated a state of oxidation - was harder to filter out.
"When arsenic is in that particular form, it is more difficult for treatment plants to remove it from water,
"So if arsenic 3 is there in a greater proportion than it normally is, that would be the answer that would tell us why the treatment plant is less able to remove it at the moment."
It was not not unusual to see the concentration of arsenic in the Waikato River vary significantly across the seasons - rising in spring and peaking in late summer, Webster-Brown said.
"The odd thing, is really that the treatment plants are not able to remove it as well as they normally do at the moment.
"So the oddity is not the level in the Waikato, it's why treatment plants are having difficulty, and that's got to be something about the form of the arsenic in the water."
While it was a concern when standards for drinking water were not being met, people should bear in mind that those standards were designed to protect them over a lifetime period, she said. A slightly higher concentration of arsenic for a period of days, or even weeks, was unlikely to be a safety issue.
The Water Services Authority Taumata Arowai and Health New Zealand earlier confirmed that drinking water with slightly elevated arsenic levels for a short period of time was s unlikely to impact health.
Arsenic levels in treated water across affected councils, as of 29 November:
- Auckland: Watercare said treated water samples taken at Waikato Water treatment plant had exceeded the limit by 0.001 mg/litre on 25 November, but was under the safety limit between 26-28 November.
- Hamilton: The city council said arsenic levels remained slightly elevated, but were safe. It said it was optimising current arsenic removal processes, including adding a new membrane filtration system.
- Waikato: The district council said test results from 29 November showed treated water supplied from Ngāruawāhia, Huntly and Te Kauwhata plants were below the 0.01mg/litre safety limit.
- Waipā: Test results on 29 November recorded 0.0128 and 0.0131 mg/litre from the two main treatment plants.
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