To maintain quality control, monitoring occurs at selected sites in order to validate the assumptions of the model and to ensure that it remains fit for purpose.
In 2022/23, 22 freshwater sites and 62 coastal sites were sampled either weekly or fortnightly during the summer bathing season (November to March inclusive). 7 coastal sites were also monitored for their suitability for shellfish gathering over this time. 16 coastal sites were monitored fortnightly during the winter period (April to October), reflecting significant year-round use at these sites.
Water sampling is carried out in accordance with the Ministry for the Environment (MfE)/Ministry of Health (MoH) (2003) microbiological water quality guidelines for marine and freshwater recreational areas. This involves the analysis of water samples for E. coli (fresh waters), enterococci (coastal waters) or faecal coliform (recreational shellfish waters) indicator bacteria.
A two-tier sentinel site monitoring framework (implemented December 2018) targets monitoring efforts at known ‘sentinel’ sites, allowing for flexibility to increase surveillance and responsiveness in responding to current health risks.
Tier-1 Sentinel Sites
Classed as sites where toxic algae has been known to historically and predictively 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 Whaitua where a threshold exceedance was observed. This benchmark of 15% is regionally specific for Greater Wellington, based on analysis of historical toxic algae coverage across sites in relation to the probability of heightened risk alerts.
Action guidelines for the Wellington Region
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).