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

Recent test results received by Council on 26 November showed a slightly elevated reading for arsenic in Hamilton’s drinking water.  The first result with an elevated reading was taken on 19 November, and previous tests all returned Arsenic at normal levels. Further water samples have been sent to a laboratory for analysis. The elevated level exceeds limits set under Water Services (Drinking Water Standards) Regulations 2022, so the incident has been reported to the regulator, Taumata Arowai.

As at 29 November 2024, the cause appears to be a change in the type of naturally-occurring arsenic in the Waikato River. Arsenic in water can take different forms but a very simple explanation is we normally see arsenic attached to other things in the water at a very fine level. This means when we remove those other things, we also remove the arsenic. When we have a more dissolved arsenic, it is mixed with the water and harder to remove. This is what we seem to be seeing. We don’t know the cause but Waikato Regional Council are investigating.

Water treatment is a very complex process and we have multiple systems to make sure the water that reaches our homes is safe to drink. Any changes we make to our processes need to be carefully planned so we don’t impact other steps in the process. Since our routine testing identified this issue we have taken several steps to address the elevated levels:

• Optimised the current processes in the treatment plant that can remove Arsenic
• Brought in an independent expert who has been working with others in the industry to join our own specialists on site.
• Brought forward the use of a new treatment process which was already planned and completed shortly before this issue arose. The new plant uses a membrane treatment with hundreds of thousands of tiny filters to treat the water in a different way to our existing plant.

We’re also working with national agencies, other councils and experts in the sector as multiple water users along the river continue investigations.

Arsenic is a naturally occurring heavy metal found in the Waikato River. Some of it comes from the geothermal activity near Lake Taupo and other natural sources downstream.

Hamilton gets its drinking water from the Waikato River, and it is thoroughly treated before it reaches our taps. Hamilton’s treatment plant removes around 80% of river water’s natural arsenic levels.

The slight variation noted recently may be due to changes in the river arsenic caused by environmental or other factors. Waikato Regional Council are investigating the variation.

Our current advice, supported by Ministry of Health and Taumata Arowai, is that Hamilton’s water remains safe to drink.

Limits set under Drinking Water Standards are very conservative and are based on health impacts at high doses over a lifetime. Health impacts from drinking water with slightly elevated levels for a short period of time are not considered likely to result in similar impacts.

We received test results showing a slightly elevated level of arsenic on 26 November 2024 from samples taken on 19 and 20 November 2024. The previous test, on 5 November 2024, showed no elevated levels. The maximum possible time the water supply had elevated levels before it was identified was 21 days.  Council typically tests for arsenic in drinking water once each month to monitor levels.

As it does with any non-compliance or a risk of a non-compliance of Drinking Water Standards, Council immediately notified the drinking water regulator, Taumata Arowai. Council commissioned additional tests to confirm the initial result and established an incident response team which first met at 4pm on 26 November 2024. Developing information indicated the issue was region-wide and likely due to changes in natural arsenic in the river. Council sought further information from neighbouring councils and relevant agencies, put in place a communication and response plan, and notified the public once the additional test results were received on 27 November 2024.

Our advice, supported by Ministry of Health and Taumata Arowai, is that Hamilton’s water supply is safe to drink.

While the risk is extremely low, people who may be especially vulnerable, such as dialysis patients, pregnant people or babies, could use bottled water for drinking or cooking, and should contact their GP or Healthline if they have concerns.

No. Unlike bacteria, boiling water does not remove arsenic.

There are multiple check and test points every day during the treatment process as well as specific sampling and testing for biological issues and bacteria as they can change quite quickly. Testing for levels of chemical components, particularly heavy metals like arsenic, is done less frequently as these are naturally in the environment and any changes following treatment are usually very, very small and gradual. In this instance our routine testing picked up a change which exceeded the limit by around one thousandths of a gram per litre of water. That’s still safe to drink and we’re investigating different ways to address it through our process.

Yes, Council’s fluoride-free taps at Claudelands and Taitua Arboretum are operating as normal. They are not affected by the elevated arsenic-levels detected in the main water supply.