In 2023/24, water samples were routinely collected at 22 freshwater sites and 62 coastal sites during the summer bathing season (November to March inclusive) in accordance with Ministry for the Environment (MfE)/Ministry of Health (MoH) (2003) microbiological water quality guidelines for marine and freshwater recreational areas.
Samples are analysed for faecal indicator bacteria including E. coli at freshwater sites, and enterococci at coastal sites. Additionally, 17 coastal sites were sampled fortnightly during the winter period (April to October), as there is significant year-round use at these sites.
To inform the suitability for shellfish gathering, 7 coastal sites are also analysed for faecal coliforms.
Toxic algae is monitored regularly at freshwater sites during the summer bathing season (November to March inclusive). During the winter period (April to October inclusive), monitoring occurs monthly.
Toxic algae monitoring follows a two-tier sentinel site network, with sites known to be high risk identified as Tier-1 Sentinel Sites. If algal coverage at Tier-1 sites breaches threshold levels, Tier-2 Sentinel Sites are also checked. See below for further details about the site network.
There are three observation-based methods for determining toxic algae coverage and informs the toxic algae risk. Monitoring procedures are based on those outlined in New Zealand Guidelines for Cyanobacteria in Recreational Fresh Waters Interim Guidelines:
Transect Assessment. Transects are completed by using an underwater viewer (bathyscape) at 20 spot observations to estimate toxic algae coverage on the riverbed. An overall percentage is calculated by averaging the observations.
Bankside Assessment. A bankside average is estimated considering coverage across all areas of the site. This helps assess areas that cannot be safely accessed during transects assessment
Detached mats. Detached toxic algae mats floating down the river or washed up on the riverbank pose a higher risk because they are more accessible to dogs and small children. Observations are completed to determine the presence of any detached mats at a site.
Tier-1 Sentinel Sites
Classed as sites where toxic algae has been known to historically bloom under optimal weather and river flow conditions.
Monitoring of Tier 1 sites occurs if there had been an accrual period of seven days. An accrual period is defined as the number of days between a freshwater flushing flow (i.e. 6x median flow). At this flushing flow, toxic algae mats are generally washed away and the potential health risk due to the algae is negligible.
Tier-2 Sentinel Sites
Known to bloom when cover at Tier-1 Sites reaches a critical threshold level of (i.e. >15%, defined for the Wellington region only).
Monitoring of Tier 2 sites occurs if a toxic algae coverage greater than 15% is observed at a Tier-1 Sites. Tier-2 site monitoring is restricted to sites within the same geographical area (i.e. Whaitua) where a threshold exceedance was observed.
Other Toxic Algae Sites Some freshwater recreational sites aren’t classed as Tier-1 or Tier-2 as part of the Toxic Algae Sentinel Site Network, however visual observations for toxic algae are still completed at these sites in conjunction with water sampling for faecal risk. If a significant toxic algae bloom is identified at Tier-1 and Tier-2 sites, these other sites may also be monitored to determine the extent of the bloom.
Greater Wellington has adopted a more conservative action level guideline of 20%, driven by the rapid growth rates of toxic algae experienced in the region and the subsequent need to manage risks to the public. This approach is consistent with the procedures set out in Brasell and Conwell 2018.
In addition to the routine transect assessment as set out in the Interim Guidelines, a bankside assessment was introduced to assess non-wadeable sections of a river site, and where the assessment from a transect method alone was insufficient to assess percentage toxic algae cover.
Full details of methods and guideline comparisons are described in full in the 2017/18 annual technical report (see Brasell and Conwell 2018).