Dr Adam Canning

22.10.2019

Having attended many of the Ministry for Environment Action Plan for Healthy Waters roadshow events, in the capacity of the science and technical advisory group members, it was clear that there was confusion about the proposed dissolved inorganic nitrogen (DIN) bottom-line of 1.0 mg/L, what it meant nationally and regionally, as well as the extent of reductions in nitrogen loss it would require.

Firstly, some important clarifying points:

  • A DIN concentration of 1.0 mg/L does not mean pristine. A pristine river typically has around 0.1 mg/L – a ten-fold difference.
  • A river that is found to have 1.0mg/L is a river that has been impacted by land use but that retains some life-supporting capacity for fish and invertebrates.
  • 1.0mg/L is not a target, it is a bottom-line and councils may set more stringent standards, depending on the local circumstances. Many regional councils are already managing DIN to more stringent levels, for example, Horizons Regional Council are managing to 0.44 mg/L – less than half the proposed bottom-line.
  • The 1.0 mg/L is not the concentration of water to be found in soils or in farm drains. It is the maximum median (so half the time can be higher) concentration to be found in rivers. River concentrations are a lot lower than soil and farm drain concentrations due to uptake on route to the river and dilution.
  •  1.0mg/L does not mean that all farms must reduce their nitrogen leaching by 80%. The amount of reduction required for a catchment is unlikely to be the same as the amount required for a specific farm (i.e., a 40% catchment reduction does not mean a 40% N leaching reduction on all farms). The future allocation system will determine who cuts by how much. The only time all farms might require proportional reductions is if the allocation system adopted is grandparenting. 

I have done a broad assessment across the country (see report below) and found that:

  • Approximately 18% of monitoring sites (these are sites that councils already regularly monitor) exceed the proposed DIN bottom-line;
  • Using modelled data, outside conservation land, approximately 10% of rivers, by length, exceed the proposed DIN bottom-line.
  • After giving effect to the existing NPS-FM 2017 requirements to manage DIN for periphyton, 9% of dairy farms (by area), 2% of other livestock (e.g., sheep and beef), and 12% of cropping are in catchments that exceed the proposed DIN bottom-line.
  • A very small fraction of dairy (1% by area), other livestock (1%) and cropping (5%) is estimated to occur in catchments that likely exceed the proposed DIN bottom-line by more than 60%.
  • In catchments where the DIN bottom-line is breached, on average 83% of the pastoral nitrogen leaching comes from dairy.

As these values use modelled data, and because measured data also has some uncertainty, the values should not be considered absolute but rather providing useful context for change required to achieve a key component of ecosystem health.

Please direct any queries to acanning@ept.org.nz

Dr Adam Canning

The Environmental Protection Trust (EPT) was founded in 2018 by Dr Adam Canning and Professor Russell Death with the aim of providing the public an avenue for independent assessments of environmental impacts, particularly to support community groups who often lack funding for peer-review or assessment of local plans or proposals,  as well as to support research and education on freshwater issues.

EPT is also a commonly used acronym for the insect orders Ephemeroptera, Plecoptera and Trichoptera, otherwise known as mayflies, stoneflies and caddisflies. These are small invertebrates that live in rivers and are the main food supply for our freshwater fish.

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