Operational changes at Waikato District Council’s Ngaaruawaahia treatment plant have ensured arsenic levels in treated water sourced from the Waikato River are now within the maximum acceptable value (MAV).
This plant is the only one in the district that has recorded slightly elevated levels of arsenic in the drinking water supply during the past week.
After a test result of 0.011 mg/litre was recorded on Sunday, just above the 0.010 mg/litre MAV under Drinking Water Standards, Watercare Waikato has adjusted the treatment process by introducing chlorine at the front end to optimise existing arsenic removal procedures.
This is working, as test results since the change have measured 0.010 mg/litre, 0.0061 mg/litre and today’s result of 0.0081mg/litre.
Meanwhile, all test results from the Huntly and Te Kauwhata treatment plants have been well within the MAV, with today’s being 0.0054 mg/litre and 0.0024 mg/litre respectively.
Waikato District Council Waters Manager Keith Martin is pleased that operational changes at Ngaaruawaahia are showing plant improvement and a downward concentration of arsenic in the treated water.
“We have seen arsenic values fall below the MAV since Watercare Waikato made process changes over the weekend, and we will continue to monitor our plants on a daily basis.
“Our operations and maintenance contractor, Watercare Waikato, are also exploring other ways we can continue to improve the operation of the Ngaaruawaahia plant specifically and our other plants generally.
“It’s also been really useful all the different agencies coming together to share information and respond in a coordinated way, while having the reassurances of Taumata Arowai and the National Public Health Service about the safety of the treated water supplied to our communities.”
The arsenic level in treated water supplied by Watercare to residents in Tuakau and Pookeno also remains well below the MAV, with today’s result being 0.0064mg/litre.
And the latest results from Hamilton City Council, who supply treated drinking water to the Tamahere-Woodlands Ward, have showed a reduction in arsenic levels with the city’s drinking water supply once again meeting the applicable Drinking Water Standards.
Arsenic is a naturally occurring heavy metal found in the Waikato River. Some of it comes from the geothermal activity near Lake Taupō and other natural sources downstream.
At all times treated water supplied to Waikato district residents has remained safe to drink and for all normal use.
News from Waikato Regional Council
Meanwhile, Waikato Regional Council is investigating the feasibility of stepping up its monitoring of the Waikato River over the next two years to improve understanding of the drivers of arsenic concentration and speciation.
Testing of samples collected from a number of sites on the Waikato River between Lake Ōhakuri and Rangiriri last week found that total arsenic levels were within the normal long-term range.
While there had been a small increase in the amount of dissolved arsenic in the water, Waikato Regional Council scientists felt the current situation was likely due to a change to an alternative chemical form of arsenic – from arsenic (V) to arsenic (III) – which can be harder for treatment plants to remove.
Dr Mike Scarsbrook, Environmental Science Manager at Waikato Regional Council, says the regional council is investigating the feasibility of expanding the suite of testing on the Waikato River over the next two years.
“We believe a long-term study would enable us to gather the robust data needed to build on historical studies on arsenic in the Waikato River, including in hydro-lake sediments which are a known source of different forms of arsenic.
“It would give us a better picture of what’s happening in the river and perhaps get us to a point where we might be able to predict incidents, like the one we are experiencing, in the future.”
