Over summer, we coordinate water quality sampling at over 80 freshwater and coastal sites in the Greater Wellington Region. We monitor from mid-November to 31 March. We use the bacterial contamination history of each site, and recent or forecasted rain to predict whether a site is safe to swim at.
For toxic algae, high priority sites have been identified from historical toxic algae monitoring records. These are now routinely checked for toxic algae, depending on local river conditions.
When there is an alert or warning at a particular site, it is added to the interactive map on the Land, Air, Water Aotearoa (LAWA) website.
In our region, we use a "risk model" to calculate where it is safe to swim, rather than just using the latest physical monitoring results at a particular swimming site.
We use this method because it provides more timely information about the suitability for swimming at popular swimming sites. The latest monitoring results are only updated once a week or fortnight, but the overall swimming risk rating is updated every hour. This means that our region's predictions not only uses our sampling data, but also take into account changes in water quality due to recent and forecast rain, and the "long term grade" of the site.
When we know about any sewage spills and infrastructure issues, we add these warnings to the interactive map as well. If water quality at any location becomes a public health risk, we will issue a public health alert through the media and local authorities.
Long term grades
Our monitoring data is also used to calculate long term microbiological water quality grades, or the "microbiological history" of each site. This is done in accordance with national guidelines.
This assessment generates a rating on the map which should gives you a good idea of water quality at your favourite swimming spot on any given day.
If your local swimming spot has a poor "long term grade", this doesn't necessarily mean it's unsafe to swim in. It means that there is an increased risk of getting sick if you were to swim there, compared to a site that has a better grade.
Poor water quality causes
The cause of poor water quality depends on the land use in the surrounding area or upstream catchment.
In rural areas: The most common source of pollution is agricultural land use. In wet weather, excess rainwater flows over farms and into nearby streams and rivers, picking up manure and other contaminants along the way. In dry weather, stock defecating directly into streams or rivers can cause contamination of swimming spots. Septic tanks can also be a source of contamination in rural areas.
In urban areas: Storm water and sewer leaks or overflows are the main sources of contamination. During wet weather, rainwater from roofs, roads, car parks and other surfaces is piped directly into rivers, streams and the coast. Along its travels, this storm water picks up sediment, rubbish, animal faeces and other contaminants.
During very heavy or prolonged rainfall, sewage overflows can result in untreated sewage being discharged to rivers or the coast via the storm water system.