Waikato nitrate trends and levels ‘highly variable’

Nitrate trends and levels are highly variable across the Waikato region, with monitoring of groundwater quality by Waikato Regional Council providing regular and publicly available data.

Groundwater nitrate levels are not a new issue – research published in 1982 identified nitrate contamination of groundwater in parts of the Waikato region – and it will be addressed through the council’s freshwater review process.

“The highly variable concentrations of nitrate and trends reflect differences in aquifer hydrogeology, aquifer hydrochemistry and land use,” said Waikato Regional Council Senior Water Scientist, Nicki Wilson.

Council monitoring finds high nitrate concentrations in the Waikato are widely associated with market gardening and dairy farming activities, which account for all exceedances of the maximum acceptable value (MAV) except for one point-source location related to effluent from a woolshed.

A 2022 council report identified that “nitrate commonly exceeds the drinking water guideline, with median concentrations being over the maximum acceptable value at 11% of the SOE monitoring network sites”.

The highest nitrate concentrations in Waikato are in the Pukekohe area, Pukekawa area, and parts of the Hauraki Plains and Hamilton Basin.

In general, increasing nitrate trends have been observed in the Pukekohe and Pukekawa areas and decreasing trends in the Hamilton Basin.

By contrast, areas of relatively low intensity land use activity such as the Coromandel Peninsula and Taupō, have noticeably lower nitrate concentrations.

The council’s groundwater quality monitoring programme derives information from two monitoring networks, with sampling wells widely distributed throughout the region.

The principal state of environment network comprises 110 wells, the majority of which are monitored annually, many since the mid-late 1990s.

A second ‘community’ network was established in 2000 and comprises a minimum of 80 rural schools which are monitored every two years.

Nitrate concentrations and trends for particular sites in the Waikato can be found on the LAWA website – www.lawa.org.nz/explore-data/waikato-region – and are further detailed in Waikato groundwater quality state of environment to 2020.

The findings have been corroborated by research undertaken by Earth Sciences New Zealand, which has just published a journal article, Nitrate contamination in New Zealand's domestic drinking water with a focus on rural groundwater-sourced self-supplies.