Suitability for swimming

This section provides the latest statistics on the water quality at recreational freshwater (rivers and lake) and marine swimming sites. Suitability for swimming is measured in terms of the concentration of faecal indicator bacteria at a swimming site.

While the presence of a small amount of bacteria (typically measured in terms of the number of bacteria per 100ml of water) may pose little to no danger to swimmers, higher concentrations may pose risk to children, the elderly or people with compromised immune systems. Concentrations of bacteria may, and often do, rise to levels were swimming is not recommended for anybody.

Documents

Metadata: Faecal indicator bacteria at recreational bathing sites Download report PDF
Factsheet: Faecal bacteria at recreational bathing sites (December 2021) View interactive report Download report PDF

Faecal contamination and human health

Faecal indicator bacteria (FIB) occur in the gut of warm-blooded animals – including humans. They may be introduced to the environment through animal or bird excrement, stock effluent, wastewater discharge, and run-off from contaminated soil. The presence of FIB in recreational water may impact human health by causing gastrointestinal illnesses, as well as infections of ears, eyes, nasal cavity, skin, and the upper respiratory tract.

Testing for the presence of FIB as a measure of suitability for recreation is a common practice internationally, as it is difficult to test for the full range of pathogens that may be present in water. Bacteria like E. coli (at freshwater sites) and Enterococci (at marine sites) are used as indicators as their presence implies that other microorganisms such as Campylobacter, Cryptosporidium, or Giardia may also be present (McBride & Soller, 2017)[3].

Many freshwater sites are occasionally unsuitable for swimming

At the national level, 47 river sites (18.8% of all those monitored) were found to be unsuitable for swimming on 20.0% or more of the occasions they were surveyed in the 2019–20 bathing season, along with 11 beach sites (3.6%) and two freshwater lakes (3.4%) (Figure 1).

Figure 1: Percentage of sites that tested unsafe for swimming at least 20% of the time, 2019/20

 

Coastal sites are suitable for swimming more often than freshwater sites

Between 2015–20, the long-term bacterial risk at marine bathing sites was generally low across all regions, with just 9.4% of the 309 monitored sites receiving a 'poor' grade, making them unsuitable for swimming (Figure 2/left). Beaches tend to have lower concentrations of FIB as contaminants are more rapidly diluted by currents and the larger volume of water.

In contrast to marine bathing sites, more than half of all freshwater bathing sites were unsuitable for swimming, with 54.8% of all monitored river and lake sites receiving a 'poor' grade between 2015–20 (Figure 2/right).

Figure 2: Overall bacterial risk, by bathing site type, 2015/16 to 2019/20

 

Suitability for swimming varies by region

The Hawke's Bay and Tasman regions had the equal highest proportion of unsuitable beach sites,  with a quarter of monitored sites in each region graded poor. The Southland region had the greatest proportion of unsafe sites, with all seven of its freshwater bathing sites being graded 'poor'.

Chart 1: Overall bacterial risk at marine bathing sites, 2015/16 to 2019/20

 FIB grades (marine)

Chart 2: Overall bacterial risk at freshwater bathing sites, 2015/16 to 2019/20

FIB grades (freshwater) 

Swimming sites in urban areas are less likely to be safe to swim

Monitored marine swimming sites in main and secondary urban areas had roughly half as many sites that received ‘excellent’ grades compared to those in minor urban or rural areas. No freshwater bathing sites in the main or secondary urban areas of New Zealand received an 'excellent' grade (Figures 6a & 6b). Overall, urban areas also had a greater share of 'poor' graded sites than rural areas. Secondary urban areas were under-monitored compared to the other areas, more than a third of sites in such areas were not monitored often enough to derive a valid long-term risk grade.

Figure 3: Overall bacterial risk at marine bathing sites, by urban/rural category, 2015/16 to 2019/20

 Figure 4: Overall bacterial risk at freshwater bathing sites, by urban/rural category, 2015/16 to 2019/20

 

Information about the data

Faecal Indicator Bacteria concentrations

Source: Land, Air, Water Aotearoa: Recreational Bathing Dataset

Grading of sites

Two measurements of swim site quality are presented in this fact sheet. Firstly, the regular monitoring results, which are passed to LAWA by regional councils and are based on regular field sampling at each site. A grade is assigned to every measurement based on the concentration of FIB at the time of measurement. Sampling is usually conducted at least once per week during the summer bathing season (the last week in October to the end of March). Grades are assigned to each measurement as below:

Grade

Criteria (E. coli )

Criteria (Enterococci)

Green

The site was safe to swim at the time of measurement.

Equal to or less than 260 E. coli per 100ml

Equal to or less than 140 Enterococci per 100ml

Amber

The site was generally safe at the time of measurement, but caution would be advised for children, the elderly, or those with compromised health

More than 260 E. coli per 100ml

More than 140 Enterococci per 100ml

Red

The site was not safe to swim at the time of measurement.

More than 550 E. coli per 100ml

More than 280 Enterococci per 100ml

 

Secondly, 'long-term bacterial risk' is calculated based on the value of all recorded FIB concentrations at a given swim site over the past five monitoring seasons. The overall risk is determined according to these criteria:

Grade

Criteria (E. coli )

Criteria (Enterococci)

Excellent

95th percentile value of E.coli /100ml:

0–130

 

Estimated risk of Campylobacter infection is <0.1%, 95% of the time.

95th percentile value of Enterococci /100ml:

0–40

 

Estimated risk of contracting an illness is <1% during the summer bathing period

Good

95th percentile value of E.coli /100ml:

>130–260

 

Estimated risk of Campylobacter infection is >0.1–1%, 95% of the time.

95th percentile value of Enterococci /100ml:

>40–200

 

Estimated risk of contracting an illness is <5% during the summer bathing period

Fair

95th percentile value of E.coli /100ml:

 >260–500

 

Estimated risk of Campylobacter infection is 1%–5%, 95% of the time.

95th percentile value of Enterococci /100ml:

>200–500

 

Estimated risk of contracting an illness is >5%–10% during the summer bathing period

Poor

95th percentile value of E.coli /100ml:

>500

 

Estimated risk of Campylobacter infection is >5%, 95% of the time.

95th percentile value of Enterococci /100ml:

>500

 

Estimated risk of contracting an illness is >5%–10% during the summer bathing period

To receive a valid 'overall risk' grade, a site must have at least 50 sample results across the past five monitoring seasons (2015/16 – 2019/20) and must have been 'recently' monitored – i.e. it must have data recorded for the most recent two bathing seasons. Therefore, a site with more than 50 total measurements since 2015 but unmonitored in the 2019/20 swim season would be graded 'insufficient data'.

Samples taken as part of follow-up tests prompted by elevated FIB levels were excluded from the assessment. Where sites were monitored for both enterococci and E.coli, measurements of each FIB type were assessed separately, and the worse of the two resulting grades was assigned as the site's long-term grade.

 Data availability

As the Auckland region does not supply the results of water quality sampling to LAWA, and instead only provides modelled data based on in-field measurements, the region has been excluded from both the recreational bathing dataset, and so all analyses in this factsheet as field measurements and predicted data are not comparable. The Wellington region also uses modelled data, but also provides LAWA with field measurements for inclusion in the recreational bathing dataset.

References

1. Land, Air, Water Aotearoa. 2021. Factsheet: Coastal and freshwater monitoring. Retrieved from https://www.lawa.org.nz/learn/factsheets/coastal-and-freshwater-recreation-monitoring/ on 20/10/2021

2. Land, Air, Water Aotearoa. 2020. Recreational bathing water quality raw dataset. Retrieved from https://www.lawa.org.nz/download-data/ on 01/05/2021

3. McBride G, Soller J. 2017. Technical Background for 2017 MfE ‘Clean Water’ Swimmability Proposals for Rivers. NIWA

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