River water quality: Escherichia coli

  • Image, River water quality: bacteria (Escherichia coli).

    Escherichia coli (E.coli) is a group of bacteria commonly found in the intestines of warm-blooded animals, including people. E.coli in fresh water can indicate the presence of pathogens (disease-causing organisms) from animal or human faeces. The pathogens can cause illness for anyone who ingests them. Campylobacter is one of the most common pathogens associated with animal and human faeces but is difficult to measure. We use E.coli concentrations to infer Campylobacter infection risk in waterways. We report on the states and trends in E.coli in fresh water.

    We classified River water quality: Escherichia coli as a case study.

    Key findings

    The highest E.coli median concentrations were at sites in the urban land-cover class, followed by sites in the pastoral, exotic forest, or native land-cover class over the 2009–13 period.

    • Model-based predictions of E.coli concentrations showed 85.4 percent of river length had very low risk of infection (less than 0.1 percent risk) from occasional immersion, with 0.01 percent (less than 2km) of river length exceeding the national bottom line (greater than 5.0 percent risk of infection) for the 2009–13 period.
    • E.coli concentrations improved at 20.5 percent of sites in the pastoral land-cover class, with 14.2 percent of pastoral sites worsening from 2004–13.
    • E.coli concentrations were indeterminate at 65.3 percent of sites in the pastoral land-cover class, meaning we have insufficient data to determine trend direction for these sites over the 2004–13 period.

    Figure 1

    Note: Sites are classified by dominant land cover in the upstream catchment . The ends of each ‘box’ in the box-plot are the upper and lower quartiles (25 percent of the sites are either higher or lower than these values). The top and bottom ‘whiskers’ represent the highest and lowest value. The middle line of the box represents the median (middle) data point (half the sites are above and half below this value). The blue line (at 1,000 E.coli per 100mL) represents the national bottom line for wading or boating (National Policy Statement for Freshwater Management 2014).

    Figure 2

    Note: The National Objectives Framework in the National Policy Statement for Freshwater Management 2014 provides bands for secondary contact, as measured by E.coli. Bands A through D represent different states, with A being the best state and D the worst. The national bottom line is the boundary between bands C and D, where there is a high risk of infection (greater than 5 percent) from contact with water during activities with occasional immersion (such as wading and boating) above this level.

    Figure 3

    Note: Sites are classified by land cover class. The land-cover class assigned to a site is based on the land cover in the upstream catchment that is presumed to dominate conditions in surface water.

    Figure 4

    18 April 2019: The interactive map will be available again when it has been updated.

    Definition and methodology

    Escherichia coli (E.coli) concentrations are measured from laboratory analysis of water samples. The level is expressed as the number of E.coli per 100mL (E.coli/100 mL).

    Regional councils monitor river water quality to manage environmental impacts. These sites tend to be in catchments dominated by agricultural land use. Rivers in low-lying and hilly areas in the North and South islands are well represented, while mountainous areas in the South Island and parts of the central North Island are not.

    For this analysis, NIWA used E.coli data from up to 486 sites monitored by them and regional councils with consistent time periods and comparable methods (Larned et al, 2015).

    States of E.coli concentrations were calculated at 486 sites using a five-year median for each site for the 2009–13 period. Ten-year trends (2004 –13) were calculated at 396 sites. Increasing and decreasing trends at sites are inferred with 95 percent confidence using the Relative Seasonal Sen Slope Estimator. Indeterminate trends mean there is insufficient data to determine the trend direction over the time period looked at.

    Sites are classified by land cover using the River Environment Classification (Snelder & Biggs, 2002).

    If you want detailed regional-level information, we recommend you review the relevant regional council’s environmental reports.

    This is because although our data are sourced from regional councils, we adjust some datasets to ensure our reports are nationally consistent. The adjustments may include omitting information produced by non-comparable methods. As a result, our evaluations may differ from those produced by regional councils.

    NIWA modelled the current state of E.coli in rivers using data monitored by them and regional councils (Larned et al, 2016). Median E.coli for 2009–13 was predicted for all river segments using Random Forest modelling and predictors (explanatory variables) from the Freshwater Ecosystems of New Zealand database. The model performed well with low bias (R2 of 0.77).

    Data quality






     Freshwater quality, quantity and flows

    Modelled river water quality: Escherichia coli


    Case study


     Image, Medium accuracy.

    Monitored river water quality: Escherichia coli


    Case study


    Image, Medium accuracy.

    See Data quality information for more detail.


    Larned, S, Snelder, T, Unwin, & M (2017). Water quality in New Zealand rivers: Modelled water quality state. Prepared for the Ministry for the Environment. NIWA Client Report no. CHC2016-070. Retrieved from www.mfe.govt.nz.

    Larned, S, Snelder, T, Unwin, M, McBride, G, Verburg, P, & McMillan, H (2015). Analysis of water quality in New Zealand lakes and rivers: Data sources, data sets, assumptions, limitations, methods and results. Prepared for the Ministry for the Environment. NIWA Client Report no. CHC2015-033. Retrieved from https://data.mfe.govt.nz.

    Snelder, T & Biggs, B (2002). Multiscale river environment classification for water resources management, Journal of the American Water Resources Association, 38(5), 1225–1239.

    Archived pages

    See River water quality: bacteria (Escherichia coli) (archived April 2017).

    Updated 18 April 2019

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