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Trends in river water quality in the Waikato region, 1993-2017

TR 2018/30

Report: TR 2018/30

Author: Bill Vant

About this report

River water quality generally changes over time. Ongoing increases or decreases over a period of years or decades are called “trends”; these will either amount to improvements or deteriorations in water quality. The water quality of more than 100 river sites in the Waikato region has been monitored for more than 25 years, so we are therefore well-placed to assess trends over that period. This is the fifth in a series of trend reports that have been prepared by WRC every five years.

This latest report shows a mixed picture of changes in water quality over the past 25 years: many records of river water quality have been stable (i.e. no change in average conditions); some records have shown important improvements; and yet others have shown important deteriorations.

This report shows that we continue to see increases in nitrogen concentrations in many of the region’s river, although the rate of increase has tended to slow over the past 10 years. In the south-eastern part of the region, increases are likely to continue for some considerable time in the future due to storage of nitrogen-rich waters in the large underground “aquifers” or natural reservoirs of groundwater found there.

By contrast, a pleasing—if somewhat unexpected—finding is that since 1998, concentrations of the faecal microbe E. coli have declined at about one-quarter of the sites where it has been monitored. The average rate of improvement was a relatively-high 4% per year.


Read or download the report

Trends in river water quality in the Waikato region, 1993-2017 [PDF, 2 MB]


1 Introduction
2 Methods
2.1 Datasets analysed
2.2 Statistical analyses—general approach
2.3 Seasonal Kendall trend slope
2.4 Slope direction probability
2.5 Flow adjustment
2.6 Flow records
3 Results and discussion
3.1 Waikato River
3.2 Other rivers and streams
4 Conclusions