Water quality for swimming categories (attribute states) in detail

This page has information on attribute states for rivers and lakes.

Rivers

The table below shows the categories and the attribute states in the National Policy Statement for Freshwater Management and the metrics that define them.

There are a series of tests involved in establishing the categories (attribute states) for rivers from Blue to Red.  In New Zealand, there is a high correlation between the tests. If you meet one, it is highly likely that all tests will be met.

One test is the percentage of times that samples from a river exceed 540 E. coli per 100 ml. This was chosen as the main metric on the maps as it is easy to show graphically.  

The minimum acceptable state for swimming is 540 E. coli per 100ml, as set out in Appendix 2 of the National Policy Statement for Freshwater Management 2014 and the New Zealand Microbiological Water Quality Guidelines for Marine and Freshwater Recreational Areas 2003.

All swimmable categories (Blue, Green, Yellow) must also meet another test, which is to have a median of  ≤130 E. coli per 100 ml. This ensures the risk of infection people face when swimming is less than 1 in 1000 at least half of the time.

The 95th percentile test controls how high the E. coli spikes are allowed to be in a particular category.

The percentage of samples above 260 E. coli per 100 ml assesses swimming water quality categories. When a waterway measures over 260 E. coli per 100ml, regional councils are required to start daily monitoring to manage health risks.

Table 1. The E. coli swimming categories (attribute states) in detail

Category

Percentage of exceedances over 540:

E. Coli per 100 ml

Median:

E. Coli per

100 ml

95th percentile:

E. Coli per

100 ml

Percentage of samples above 260:

E. Coli per 100 ml

Blue

< 5 per cent

≤  130

≤ 540

< 20 per cent

Green

5-10 per cent

≤  130

≤ 1000

20-30 per cent

Yellow

10-20 per cent

≤  130

≤ 1200

20-34 per cent

Orange

20-30 per cent

>130

>1200

>34 per cent

Red

> 30 per cent

>260

>1200

>50 per cent

Water quality risk profile

The proposed changes to water quality standards are part of a package that will improve the overall quality of our rivers and lakes over time. The changes are consistent with World Health Organisation standards for safe swimming levels and are in line with the approach taken by the US and EU.

The table below sets out the risk profile for rivers of each of the categories.

Table 2. The risk profile of categories (attribute states) 

Category

Percentage of exceedances over 540:

E. Coli per 100 ml

Median:

E. Coli per

100 ml

Risk profile

Blue

< 5 per cent

≤  130

For at least half the time the estimated risk of Campylobacter infection is less than 1 in 1000.

Less than 5 per cent of the time, the estimated risk of Campylobacter infection is ≥50 in 1000

Green

5-10 per cent

≤  130

For at least half the time the estimated risk of Campylobacter infection is less than 1 in 1000.

 

5-10 per cent of the time the estimated risk of Campylobacter infection is ≥50 in 1000.

Yellow

10-20 per cent

≤  130

For at least half the time the estimated risk of Campylobacter infection is less than 1 in 1000.

 

10-20 per cent of the time, the estimated risk of Campylobacter infection is ≥50 in 1000.

Orange

20-30 per cent

>130

20-30 per cent of the time the estimated risk of Campylobacter infection is ≥50 in 1000.

Red

> 30 per cent

>260

For more than 30 per cent of the time the estimated risk of Campylobacter infection is ≥50 in 1000.

People should always check with their Regional Councils or the Land Air Water Aotearoa (LAWA) website for the latest information on whether it’s safe to swim at popular swimming spots.

More information on the swimming categories for E. coli in the Clean Water package

Lakes

The table below sets out the indicator involved in establishing an attribute state from Blue to Red for lakes.

The categories for toxic cyanobacteria (known as toxic algae) show the bio-volume of potentially toxic cyanobacteria present in each lake. We take a precautionary approach and assume that all cyanobacteria are toxic. Table 2 shows a definition of the categories, and the health risks for swimmers associated with each category.

These categories are based on Appendix 2 of the National Policy Statement for Freshwater Management 2014, and the 2009 Guidelines for Cyanobacteria in Recreational Fresh Waters.

There is insufficient data to model E. coli levels across all lakes. Where an individual lake has elevated levels of E. coli we have noted this on the swimming maps with a pop-up warning.

Table 3. Alignment of the lake categories on the maps with toxic algae (cyanobacteria) levels considered acceptable from a public health risk perspective

Category

80th percentile:

Biovolume of potentially toxic cyanobacteria

Risk of exposure from cyanobacteria based on the
National Policy Statement for Freshwater Management 2014

Blue

< 0.5 mm3/L biovolume

Low risk

Green

0.5-1.0 mm3/L biovolume

Low risk

Yellow

1.0-1.8 mm3/L biovolume

Low risk

Orange

1.8-3.0 mm3/L biovolume

Potential health risks

Red

> 3.0 mm3/L biovolume

Potential health risks

How water quality for swimming was modelled

Water quality for swimming was modelled using the approach outlined in Strategic assessment of New Zealand’s freshwaters for recreational use: a human health perspective.

Adjustments of model predictions

Adjustments were made to the predictions where the models didn’t match actual data at a site.

Adjustments were made based on:

  • expert opinion from freshwater scientists
  • fact-checking with regional councils
  • actual data at a monitoring site. River reaches were adjusted to account for data from monitoring sites. Changes were made at and upstream of the monitoring site if a category was incorrectly predicted (ie, a different result to the monitoring site)
  • the values in surrounding river reaches. If one reach was surrounded by reaches with higher or lower category, the reach was changed to the predominant category to account for model error (ie, the reach was only just over or under a category threshold).

Where to check for up-to-date information before going for a swim

At the monitoring site level, there is some statistical uncertainty associated with each of the categories. On any one day a site could be in a higher or lower category for swimming.

There are also other factors which can affect how suitable a waterway is for swimming, including flow, access, heavy rain events, and other weather conditions. These maps don’t provide advice on these.

For these reasons, always check regional council websites and the Land, Air, Water Aotearoa website for the latest conditions and warnings at popular swimming spots before you swim.

About the methods

The methods used to model river and lake water quality for swimming for these maps are outlined in the following report:

Strategic assessment of New Zealand’s freshwaters for recreational use: a human health perspective. Escherichia coli in rivers and planktonic cyanobacteria in lakes.

Peer review of the methods

This report was peer reviewed by the Cawthron Institute.

For the review and the authors response to the review see: Review of Strategic assessment of New Zealand’s freshwaters for recreational use: a human health perspective.

Reviewed:
01/03/17