A new report on water quality trends in Waikato rivers paints a mixed picture, with improvements in some measures but serious declining trends when it comes to nitrogen content, clarity and turbidity.
The report from Waikato Regional Council water scientist Bill Vant, covering trends from 1993 to 2012, is expected to make a significant contribution towards discussions on a forthcoming regional plan change covering the health of the Waikato and Waipa rivers.
“Increases in nitrogen levels in our waterways have the potential to stimulate the growth of nuisance algae which can spread in lakes and rivers, hurt aquatic life and contribute to toxic algal blooms,” said Mr Vant.
“Increases in turbidity and decreases in clarity make waterways less attractive for recreation purposes and can also damage aquatic life.
“While the region can take comfort from either improvements or no declines in a range of water quality indicators, the significant deterioration trends around nitrogen, clarity and turbidity in many places are concerning and must be taken seriously.”
Mr Vant said it was puzzling exactly why turbidity and clarity trends showed deteriorations at many monitoring sites on both the Waikato River and other waterways generally, although soil erosion and channel bank erosion was a possible cause.
However, runoff and leaching of nitrogen from areas of pastoral farming probably accounted for much of the deteriorating trends when it came to nitrogen in waterways, said Mr Vant. Intriguingly, levels of chlorophyll, which can be seen as a proxy for algal abundance, were either better or stable in the Waikato River, despite the poorer situation with nitrogen levels.
Levels of E. coli bacteria, which can make people sick, were generally stable at most sites, albeit with some significant improvements and deteriorations in places as well. But, even though levels were mostly stable, E. coli levels still exceed national contact recreation guidelines in many rivers around the region.
“How we address these trends, and identify clearly what the data is telling us, will involve ongoing scientific discussion as part of the work involved in deciding on a plan change for the Waikato and Waipa rivers, and in subsequent catchments after that,” said Mr Vant.
“The council is very pleased at how its partner iwi and other stakeholders around the region, including the agriculture sector, are working together to address these issues.
“There is clear evidence in this report of actual and potential problems for our waterways and it is important that we act decisively to address those issues,” said Mr Vant.
Some of the key statistics include:
- 20 years worth of water quality data from 114 monitoring sites on the Waikato River and other waterways were analysed. Ten of the sites are on the Waikato River and the rest on waterways around the region.
- For the Waikato River, there was a significant deterioration in nitrogen and turbidity in more than 75 per cent of monitored sites, and in clarity in more than half.
- Nitrogen levels had also deteriorated significantly in more than half of sites on other waterways, and in more than 25 per cent of sites for clarity and turbidity.
- However, on the Waikato River there were significant improvements in ammonia and cholorphyll in a clear majority of sites, while total phosphorus was either significantly better or stable on all Waikato River sites.
- E. coli bacteria levels were stable at most sites, but with a number of significant improvements as well as a few significant deteriorations at others.
The full report is available at http://www.waikatoregion.govt.nz/tr201320