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.

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].

While the presence of a small amount of FIB (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 a risk to children, the elderly, or people with compromised immune systems.

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:

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:

To receive a valid 'overall risk' grade, a site must have at least 50 sample results across the past five monitoring seasons (2017/18 – 2021/22) 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 2017 but unmonitored in the 2021/22 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 water quality sampling results to LAWA, the region has been excluded from the recreational bathing data set and, consequently, from all analyses in this Surveillance report, as field measurements and predicted data are not comparable. Coastal data from the Bay of Plenty region was not made available to LAWA for the 2021/22 monitoring period and as such has been excluded from the analyses for this report.