Research showing how water moves under and through South Auckland has revealed groundwater and streams around Pukekohe and Bombay can contain high nitrate levels.
It also found nitrate levels take a long time to reduce.
Auckland Council will use the GNS study to help with long-term water planning in Franklin.
Franklin Ward Councillor Andy Baker says councillors have been briefed on the work, with excessive nitrate levels having the potential to damage freshwater environments.
“The study is useful as a tool for managing the environment, because it tells us more and provides valuable insights.”
Auckland Council Natural Environment Strategy Manager Dave Allen says the study provides more understanding around how old the water in the area is, where it is flowing, and where nitrate levels are concentrated.
“Levels tend to be higher upstream, and diluted downstream, and that tells us what we might need to do in certain places, including what land use activities might need to change.”
Baker says nitrates from Franklin eventually end up in the Manukau Harbour.
“The cause of these nitrate levels has been well investigated and can generally be tracked back generations.
“Pukekohe and Bombay lose nitrates into groundwater, then the springs release it into our streams. The good news is that most of our farmers and growers are cutting fertiliser use and changing what they do as they adapt to gaining knowledge and information like this.”
In simple terms, most deep groundwater is more than 100 years old, and has low nitrate levels given biological processes at that depth. Shallow groundwater and springs are younger, less than 80 years, and usually have higher nitrate levels.
The report will be presented to iwi representatives, the Pukekohe Vegetable Growers Association and Horticulture New Zealand.
Allen says the Manukau Harbour receives a range of other nutrients, and the relative contribution of horticulture nitrates compared to other sources needs more evaluation, but the report will help in discussions around what responses to land use might be needed.
“We need to know if any changes we contemplate have a positive bearing on future nitrate levels within the groundwater system.”
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