Waikato still has nitrate levels that exceed drinking water guidelines, according to a report from the Waikato Regional Council.
The areas with the highest nitrate levels are those associated with intensive agriculture and horticulture.
The report, “Groundwater Nitrates – potential hotspots, legacies and lag”, says the areas with the highest concentrations are aquifers in Pukekohe and Pukekawa and parts of the Hauraki Plains.
The report is due to be presented to the council’s Environment Performance Committee meeting on Thursday.
Nitrate levels are monitored through 110 state of environment (SOE) wells and a network of 80 “community” or rural school water supply wells.
New Zealand Drinking Water Standards set a maximum acceptable value (MAV) of 50 milligrams per litre (mg/l) for nitrate, which is equivalent to 11.3 mg/l nitrate-nitrogen for drinking water.
However, recent studies have raised health concerns related to lower concentrations of nitrate than those allowed by the standards.
Most notably, a Danish study identified a correlation with colorectal cancer risk and recommended an upper limit of 0.87 mg/l for nitrate-nitrogen.
Within the SOE wells, it found that three-quarters of these wells had concentrations greater than 0.87 mg/l. For the community wells, 50.6% had nitrate concentrations over 0.87mg/l.
The report found that from 2016-2020, 22% of groundwater in SOE wells exceeded MAV levels at least once. The median concentrations from the same five-year period also exceeded guidelines at 11% of SOE wells and 5% of the community wells.
“The highest concentrations are associated with market gardening and dairy farming activities,” it said.
Nitrate levels are monitored and reported by the council as one of a number of environmental indicators on a biennial basis. Its 2020 report showed that nearly 12% of SOE wells exceeded the MAV.
A further 20% of wells had nitrate concentrations over half the MAV and the remaining two-thirds (68.18%) had “low” concentrations below half the MAV.
Within the community supply wells, nitrate levels exceeded the MAV at 3.75 % of the community supply wells, none of which are used to supply drinking water.
A similar percentage are described as elevated and 92.5 % have concentrations below half MAV.
The highest nitrate-N concentrations occur in northern areas, Hauraki and Hamilton Basin.
These are the areas of traditionally most intensive agriculture and horticulture, the report says.
Sub-regionally, it says that groundwater quality trends are mixed. Two-thirds of northern Waikato aquifers had increases in nitrates while the shallow monitoring wells of the Hamilton Basin are decreasing by the same amount.
Other areas are more balanced, although there are more increases (37%) than decreases (-22%) in nitrate-N concentration trends in the Hauraki area.
The report also says that new research shows that aquifers in deep groundwater have a smaller influence on nitrogen loads in surface waters than previously thought.
Near-surface and shallow groundwater pathways are more dominant on that loading, suggesting that changes made on the land now will be more quickly reflected in surface waters.
Long lag times are largely restricted to catchments with young volcanic geology around the Upper Waikato/Lake Taupo sub-region
“According to modelling results, deep groundwater is a less important contributor of the total nitrogen load than may have been previously thought,” it said.
“This has implications for assessing policy effectiveness, as the time frames for changes in land management or land use are likely to be observed sooner than we might have expected,” the report says.
“This also means that the ‘load to come’ is also less than we originally thought.”
