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Water quality trends at National River Water Quality Network sites for 1989–2007

Publication date:  August 2010
Publication reference number:  CR 113

As part of the Ministry's national environmental reporting programme, this report summarises water quality trends at National River Water Quality Network sites between 1989 and 2007.

The report updates an analysis of trends in water quality between 1989 and 2003 which was summarised in Environment New Zealand 2007.

Important note: This second edition report replaces the original publication dated March 2009. This edition uses a newer version of the trends software and a higher number of decimal places, which has improved the accuracy of the report and removed some false zero trends reported in the original. While this has led to small changes in trend strength for some water quality variables, trend directions are still the same, and the overall picture remains one of water quality deterioration.

Key findings

  • The report shows that the 1989 to 2007 trends in water quality are similar to those reported in the Ministry's state of the environment report, Environment New Zealand 2007 (ENZ07), with trends all in the same direction.
  • There were strong increasing trends in four nutrients (total phosphorus, dissolved reactive phosphorus, oxidised nitrogen and total nitrogen). There are strong correlations between these trends and the percentage of catchment in pasture, which indicates deteriorating water quality is mainly attributable to expansion and intensification of pastoral agriculture.
  • Temperature and dissolved oxygen showed few environmentally meaningful or significant trends at the 77 sites. However, there is a slight increasing national trend in temperature, which is close to being significant.
  • There was a significant improvement in the visual clarity of rivers at the national scale which is better for fish and insect life, and also aesthetically. However, rivers surrounded by more pasture generally had poorer clarity.
  • In recent years there have been rapid changes in some aspects of water quality in some rivers, with both improvements (eg, increasing clarity and decreasing nutrients in the Manawatu River) and deteriorations (eg, increasing nutrients in the Waimakariri River). This information will be useful as an early warning system for regional councils.
  • Overall, some aspects of water quality have deteriorated in rivers over the past 20 years mainly as a result of farming (pastoral land cover) and environmental gains in terms of reduced ‘point’ pollution of waters in New Zealand are being overshadowed by increasing ‘diffuse’ pollution This is consistent with previous reporting from the same monitoring network.