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Below is the first set of LAWA National River Water Quality 10-Year Trends, covering the period 2007- 2016. The trends are based on analysis of the comprehensive data that’s freely available on the LAWA website. Regular water quality monitoring by New Zealand’s Regional and Unitary Councils, supplemented with NIWA data, means there’s water quality information for nearly 1,500 freshwater sites. After you’ve learned more about the national picture, visit the regions to find out more on the water quality of your favourite sites.
Trends in river water quality have been assessed for sampling sites all around the country over the last 10 years (2007-2016). Suitable data was available for between 296 and 696 sites depending on which water quality parameter is being looked at. The analysis included sites where samples are collected monthly, bimonthly or quarterly.
There were trends (improving and degrading) in river water quality over the last 10 years at between 29% - 67% of sampling sites depending on which water quality parameter you look at (e.g. E. coli vs ammoniacal nitrogen) (Figure 1). At the other sites there is insufficient evidence to determine an improving or degrading trend over the last 10 years – i.e. indeterminate trends.
Improving trends for water quality were more common than degrading trends for all water quality parameters. Water clarity, ammoniacal nitrogen and total phosphorus concentrations showed 11, 8 and 16 times more sites with improving trends than degrading trends, respectively (Table 1, Figure 1).
While all parameters show there are more sites with improving trends than degrading trends, there are degrading trends for all parameters (Table 1, Figure 1).
Compared to a previous national water quality trend summary based on data from 2004-2013, the latest results were generally consistent but provide more positive signs relating to water quality improvements. Both found more improving trends than degrading trends for total phosphorus, dissolved reactive phosphorus, E. coli, ammoniacal nitrogen and water clarity. The latest results also show more improving trends than degrading trends for total oxidised nitrogen and total nitrogen, this was the opposite in the older dataset.
Figure 1. Percentage of sampling sites with improving or degrading trends over the last 10 years for a range of water quality parameters. There was insufficient data to determine an improving or degrading trend at indeterminate sites. DRP = dissolved reactive phosphorus, E. coli = faecal indicator bacteria, NH4 = ammoniacal nitrogen, TN = total nitrogen, TON = total oxidised nitrogen, TP = total phosphorus. The number of sites with suitable data for each parameter is shown at the top of the bar.
Table 1. The number of sites with improving or degrading trends over the last 10 years for a range of water quality parameters. There was insufficient data to determine an improving or degrading trend at indeterminate sites. DRP = dissolved reactive phosphorus, E. coli = faecal indicator bacteria, NH4 = ammoniacal nitrogen, TN = total nitrogen, TON = total oxidised nitrogen, TP = total phosphorus.
 The direction of any trends was identified by determining whether zero was included in the 95 % confidence interval range around the Seasonal Sen Slope Estimate for each site and parameter combination. If zero was within the confidence interval range we concluded that there is insufficient data to determine trend direction i.e. an indeterminate trend.
 Sites were excluded from the analysis if they had less than 90% of the data expected in any one year of the data record, or if more than 30% of the data was less than or greater than laboratory detection limits. Sites with suitable data were available from all Regional and Unitary Council regions and from the National River Water Quality Network run by NIWA.
 From Larned S, Snelder T, Unwin M, McBride G, Verberg P, McMillan H 2015. Analysis of water quality in New Zealand lakes and rivers. Prepared for Ministry for the Environment. NIWA Client Report No. CHC2015-033. The data analysis approach is similar between the LAWA trends and previous national trend summary report, but not identical. The differences relate to flow adjustment and the threshold for laboratory detection limits.