Using monitoring data to measure nitrate concentrations in groundwater and in rivers, it found that 12% of the 120 monitored wells in the region had nitrate concentrations exceeding the New Zealand drinking water standard of 11.3mg/L and 20% had concentrations above half of this.
For community supply wells, which are not used for drinking water supply, just under 4% exceeded the NZ standards.
It was presented to the Environmental Performance Committee at its meeting on May 6 by its freshwater science team.
The NZ standard is set according to WHO guidelines and is much higher than the 0.87mg/L recommended in a well-publicised 2018 Danish study, which linked a statistically significant increase in colorectal cancer at nitrate levels in water.
If the NZ drinking water standard for nitrate was raised to the Danish study threshold, there would need to be considerable reduction in nitrate concentrations, or introduction of expensive drinking water treatment systems.
“The Waikato Region would face significant challenges in meeting a drinking water standard as low as 0.87mg/L the majority of monitored river and groundwater sites have high nitrate concentrations and trend analyses demonstrate worsening trends in nitrates at many sites,” the report said.
Councillor Jennifer Nickel asked the scientists what some of those challenges would be for the council.
WRC senior scientist John Hadfield says anything to reduce nitrate concentrations by land-use control would take a long period of time.
While councillor Stu Kneebone says there would still be a significant lag effect of nitrate levels even if cow numbers were reduced in the region in an effort to reduce those levels.
“If there was a desire to lower that drinking water level, there’s not a lot we can do about it in terms of land-use change to have an impact on that,” Kneebone said.
WRC science manager Mike Scarsbrook agreed, saying in the upper catchments around South Waikato, there were very long lag times around nitrate leaching.
“The things that change on land now, will take decades before they are reflected further downstream,” Scarsbrook said.
Councillor Fred Litchtwark asked the scientists what they required from the councillors to push this process forward.
Scarsbrook told him that councillors had expressed their concerns about nitrate levels in the region and it was appropriate for them to get more information on this issue because it could have consequences for the region.
“What we would like councillors to do is recognise that we have an issue with nitrate levels in our waterways due to the intensification of land-use change and land-use in our region,” he said.
Hadfield says the highest monitored nitrate concentrations among groundwater supplies was in the Pukekohe, Hamilton Basin and southern Hauraki Plains areas associated with horticulture and dairy farming.
Trend-wise, there was a mix between increases and decreases across the 120 wells.
WRC senior scientist Eloise Ryan says it was difficult and expensive to remove nitrates from water.
Ryan says nitrate levels in the Waikato River treated by the Hamilton City Council for the city’s drinking water fluctuated throughout the year.
The council monitored 115 river sites monthly and over the past 10 years the maximum nitrate concentration had not exceeded the 11.3mg/L limit, but 69% of sites had recorded a concentration that was greater than 0.87mg/L.
Ryan says from trends perspective, there were many rivers where N concentrations were increasing and degrading.
The overall trend analysis indicates that nitrate-nitrogen concentrations are increasing in many sites in the Waikato region, the report concluded.
Hadfield says nitrate contamination clearly is an issue in parts of the Waikato.
“Many of our monitored rivers and groundwaters have elevated nitrate concentrations and the trend analysis shows they are increasing on many sites,” he said.
