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).
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
Chart 2: Overall bacterial risk at freshwater bathing sites, 2016/17 to 2018/19
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 3) and freshwater (Figure 4) swimming sites in main and secondary urban areas had smaller proportions of ‘generally safe’ marine swimming sites.
Figure 3: Overall bacterial risk at marine bathing sites, by urban/rural category, 2016/17 to 2018/19
Figure 4: Overall bacterial risk at freshwater bathing sites, by urban/rural category, 2016/17 to 2018/19
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 used in this indicator, based on data contained in Land, Air, 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
Source: LAWA (2019c)
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
Source: LAWA (2019c)
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 fewer 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.
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.
1. Land, Air, Water Aotearoa. 2019a. Factsheet: Coastal and freshwater monitoring. Retrieved from https://www.lawa.org.nz/learn/factsheets/coastal-and-freshwater-recreation-monitoring/
2. Land, Air, Water Aotearoa. 2019b. Recreational bathing water quality raw dataset. Retrieved from https://www.lawa.org.nz/download-data/ on 01/05/2020
3.Land, Air, 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