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.

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)[4].

Many freshwater sites are occasionally unsuitable for swimming

In the 2018–19 bathing season, 119 beaches (40.6% of those surveyed that year) were unsafe to swim at on at least occasion during the season, as were 72 (75.0%) rivers and 13 (24.1%) lakes.

Figure 1: Percentage of sites that tested unsafe for swimming on at least one occasion, 2018–19

Coastal sites are suitable for swimming more often than freshwater sites

Coastal sites are generally less susceptible to faecal contamination as the action of the ocean current and tides normally disperses contaminant matter quickly. Between 2016–19, the overall bacterial risk at marine bathing sites was generally lower in contrast to freshwater sites across all regions, though fewer than half (161 out of the 352 unique sites monitored between 2016–19) were able to be confirmed as ‘generally safe to swim’. By contrast, most freshwater bathing sites were not suitable for swimming, with only around one in six sites nationwide (33 out of 252 unique sites) having FIB concentrations low enough to be considered ‘safe’. 

Figure 2: Overall bacterial risk, by bathing site type, 2016–17 to 2018–19

Suitability for swimming varies by region

The Hawke’s Bay region had the largest proportion of unsuitable beach sites (3 out of 22 monitored sites) while the Wellington region had the lowest (2 out of 64) (Chart 1). As for freshwater quality, the Manawatū-Whanganui region had the greatest proportion of unsafe sites (44 out of 49 sites). Furthermore, the Gisborne, Hawke’s Bay, Nelson, Tasman and Southland regions did not have a single freshwater bathing site that was generally safe to swim at. (Chart 2). Chart 3 offers an interactive view of the data presented in the static charts below.

Chart 1: Overall bacterial risk at marine bathing sites, 2016–17 to 2018–19

NatCoastalRBD

Chart 2: Overall bacterial risk at freshwater bathing sites, 2016–17 to 2018–19

NatFWRBD

Chart 3: Interactive map of overall bacterial risk at all bathing sites, 2016–17 to 2018–19

View larger map

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

Monitored marine (Figure 5) and freshwater (Figure 6) swimming sites in main and secondary urban areas had smaller proportions of ‘generally safe’ marine swimming sites.

Figure 5: Overall bacterial risk at marine bathing sites, by urban/rural category, 2016–17 to 2018–19

Figure 6: Overall bacterial risk at freshwater bathing sites, by urban/rural category, 2016/17 - 2018/19

Information about the data

Faecal Indicator Bacteria concentrations

Source: Land and Water Aotearoa: Recreational Bathing Dataset

Grading of sites

Two measurements of swim site quality are used in this indicator, based on data contained in Land and Water Aotearoa (LAWA)’s recreational bathing water quality raw dataset. Firstly, the regular monitoring results which are passed to LAWA by regional councils, 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.

Figure 7: Criteria for assigning grades to individual measurements of FIB concentration

LAWA Grades

Source: LAWA (2019c)[3]

Secondly, ‘overall bacterial risk’, which is calculated based on the 95th percentile value of the recorded FIB concentrations at a given swim site over the past three monitoring seasons. Overall risk is assigned as follows:

Figure 8: Criteria for determining overall bacterial risk

LAWA OBR

Source: LAWA (2019c)[3]

To receive a valid ‘overall risk’ grading, sites must have at least 30 sample results across the 2016/17 – 2018/19 period, with at least 10 results in each year. If a site has less than 10 sample results in one year, then a minimum of seven samples in that year is required, with at least 35 samples over the three-year period.

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 and Water Aotearoa. (2019a). Factsheet: Coastal and freshwater monitoring. Retrieved from https://www.lawa.org.nz/learn/factsheets/coastal-and-freshwater-recreation-monitoring/

2. Land and Water Aotearoa. (2019b). Recreational bathing water quality raw dataset. Retrieved from https://www.lawa.org.nz/download-data/ on 01/05/2020

3.Land and Water Aotearoa. (2019c). What do the swim icons mean? Retrieved from https://www.lawa.org.nz/learn/factsheets/what-do-the-swim-icons-mean/ on 02/11/2020

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

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