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.

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

Category

Percentage of exceedances over 540
E.coli per 100 ml

Percentage of samples above 260
E.coli per 100 ml

Median:
E.coli per
100 ml

95th percentile:
E.coli per
100 ml

Narrative risk descriptor

What it means

How often the river exceeds the acceptable threshold for swimming

How often the river goes over the point where additional monitoring is needed at primary contact sites

The mid-point (ie, half the time E. coli is lower than this, half the time it is higher)

E. coli only rarely goes past this point (only 5% of the time)

Risk of Campylobacter infection (based on E.coli indicator)

A
(Blue)

<5%

<20%

≤130

≤540

For at least half the time, the estimated risk is <1 in 1000 (0.1% risk)
The predicted average infection risk is 1%*

B
(Green)

5-10%

20-30%

≤130

≤1000

For at least half the time, the estimated risk is <1 in 1000 (0.1% risk)
The predicted average infection risk is 2%*

C
(Yellow)

10-20%

20-34%

≤130

≤1200

For at least half the time, the estimated risk is <1 in 1000 (0.1% risk)
The predicted average infection risk is 3%*

D
(Orange)

20-30%

>34%

>130

>1200

20-30% of the time the estimated risk is ≥50 in 1000 (>5% risk)
The predicted average infection risk is >3%*

E
(Red)

>30%

>50%

>260

>1200

For more than 30% of the time the estimated risk is ≥50 in 1000 (>5% risk)
The predicted average infection risk is >7%*

* The predicted average infection risk is the overall average infection to swimmers based on a random exposure on a random day, ignoring any possibility of not swimming during high flows or when a surveillance advisory is in place (assuming that the E.coli concentration follows a lognormal distribution). Actual risk will generally be less if a person does not swim during high flows.

 

1 Attribute state should be determined by using a minimum of 60 samples over a maximum of 5 years, collected on a regular basis regardless of weather and flow conditions. However, where a sample has been missed due to adverse weather or error, attribute state may be determined using samples over a longer timeframe.

 

2 Attribute state must be determined by satisfying all numeric attribute states.

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 2. The Cyanobacteria categories (attribute states) in detail

Value

Human health for recreation

Freshwater Body Type

Lakes and lake fed rivers

Attribute

Cyanobacteria – Planktonic

Attribute Unit

Biovolume -  mm3/L (cubic millimetres per litre)

Attribute State

Numeric Attribute State

Narrative Attribute State

 

80th percentile*

 

A
(Blue)

≤0.5 mm3/L  biovolume equivalent for the combined total of all cyanobacteria 

Risk exposure from cyanobacteria is no different to that in natural conditions (from any contact with fresh water).

B
(Green)

>0.5 and ≤1.0 mm3/L biovolume equivalent  for the combined total of all cyanobacteria 

Low risk of health effects from exposure to cyanobacteria (from any contact with fresh water).

C
(Yellow)

>1.0 and ≤1.8 mm3/L biovolume equivalent of potentially toxic cyanobacteria OR
>1.0 and ≤10 mm3/L total biovolume of all cyanobacteria

Moderate risk of health effects from exposure to cyanobacteria (from any contact with fresh water).

National Bottom Line

1.8 mm3/L biovolume equivalent of potentially toxic cyanobacteria OR
10 mm3/L total biovolume of all cyanobacteria

D
(Orange/Red)

>1.8 mm3/L biovolume equivalent of potentially toxic cyanobacteria OR
>10 mm3/L total biovolume of all cyanobacteria

High health risks (eg, respiratory, irritation and allergy symptoms) exist from exposure to cyanobacteria (from any contact with fresh water).

* The 80th percentile must be calculated using a minimum of 12 samples collected over 3 years.
30 samples collected over 3 years is recommended.

 

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:
09/08/17