The state of our rivers
Measured trends in nitrate levels in New Zealand rivers since the National Water Quality Network began in 1989.
Our rivers are showing the benefits of efforts to clean up point-source pollution, but the effects of diffuse run-off from intensified agriculture are becoming increasingly apparent.
NIWA scientists from the National Centre for Water Resources recently completed an assessment of current state and recent trends in river water quality for the Ministry for the Environment. All data were sourced from NIWA’s National River Water Quality Network.
Lead author, and national centre leader, Dr Mike Scarsbrook says that the most recent data (from 2005) and a detailed trend analysis (for 1989–2003) show a clear correlation between the level of contamination and the extent of pastoral land cover in a river catchment.
The report analyses changes in the state of our rivers since the National River Water Quality Network was established in 1989. Key results include:
- Significant increasing trends of oxidised nitrogen in those rivers that already have high levels of this nutrient. This implies that our most enriched rivers have deteriorated over the last 17 years, probably as a result of land use intensification.
- A significant increasing trend for dissolved reactive phosphorus concentrations at the national scale for the period 1989–2003, but some evidence of decreasing concentrations in our most enriched rivers in the last few years. This pattern would be consistent with a reduction in fertiliser use, but may also reflect differing climatic conditions.
- National scale trends of decreasing concentrations of ammoniacal nitrogen and biochemical oxygen demand. Both of these patterns are consistent with improvements in the management of point source discharges to waterways, such as wastewater outfalls.
‘Resource management is shifting towards a greater emphasis on control of non–point source pollution associated with intensive agriculture,’ says Dr Mike Scarsbrook. ‘The information gained from the National River Water Quality Network supports this shift in emphasis.’
The full report is available at: www.mfe.govt.nz/publications/ser