State and Trends in the National River Water Quality Network (1989-2005)

This assessment of current state and recent trends in river water quality at the national scale is based on data from the National River Water Quality Network. The Network includes 77 sites distributed across the country, at which river flow is measured continuously and water quality parameters are measured monthly.

Executive Summary

This report assesses the current state and recent trends in river water quality at the national scale. The National River Water Quality Network (NRWQN) is the source of all data.

The NRWQN includes 77 sites distributed nationally at which river flow is measured continuously and at which 14 physical/chemical parameters are measured monthly. All laboratory analyses are carried out at NIWA's Water Quality Laboratory in Hamilton. Data from January 1989 through until December 2005 were used.

An assessment of current state, using data from the 2005 calendar year, indicates strong associations between nutrient concentrations and percent pastoral land cover at the national scale. Median concentrations of all nutrient species and levels of the faecal indicator bacteria E. coli are positively correlated with extent of pastoral land cover.

Summaries of changes in annual state over time (1989-2005) highlight significant increasing trends of oxidised nitrogen (NO_x-N) in those rivers that already have high levels of this nutrient. This result implies that our most enriched rivers have deteriorated over the last 17 years, probably as a result of land-use intensification. Levels of dissolved reactive P show a somewhat different pattern. While there is a significant increasing trend for dissolved reactive P concentrations at the 80th percentile, there is not a similar trend in the 95th percentile. Dissolved reactive P concentrations at the 95th percentile actually show a non-linear response over time, with concentrations in our most enriched rivers peaking in the late 1990s and showing a decreasing trend since.

Detailed trend analysis for the period 1989-2003 highlights national-scale trends of decreasing concentrations of ammoniacal nitrogen and biochemical oxygen demand. Both of these patterns are consistent with improvements in our management of point source discharges to waterways. At the same time, increasing trends are observed in rivers around the country with respect to dissolved and total phosphorus and total nitrogen. There are positive correlations between trend magnitude and the extent of pastoral land cover in the catchment.

The picture of water quality in New Zealand rivers that emerges from the analyses and summaries presented in this report is consistent with a continuing shift in relative importance from point source to non-point source pollution as key anthropogenic pressures on surface waters. Resource management is shifting towards a greater emphasis on control of non-point source pollution associated with intensive agriculture. Information gained from the NRWQN supports this shift in emphasis.