Although the Waikato Regional Council does long-term monthly monitoring of water at 125 outdoor recreational sites in summer to check E. coli levels, it says its modelling information shows the levels can be confidently inferred by taking samples at only 16 of these sites.
Next summer the council will trial modelling at these 16 reference sites to estimate E. coli levels across the remainder of the region for unmonitored rivers.
It says other regional councils – including Wellington and Auckland – are already using a mix of monitored and modelled results.
Proposed changes to the monitoring programmes and a summary of the 2021-2022 summer recreational monitoring in fresh water, estuaries and open-coast beaches were presented at the regional council's environmental performance committee meeting last week.
Monitoring manager Ed Brown said the changes, including modelling information to include more river sites and greater use of education via web platforms and signs, would better empower people to decide where and when to go swimming.
Brown said the council would also extend reporting to a greater number of lakes where the risk of harmful algae blooms was often high.
"With these lakes, actual sampling may give a false impression that it's okay to swim there as the monitoring may give a green light for swimming but, in actual fact, the situation at these lakes changes quite dramatically, say when the wind changes direction causing algae to accumulate on the opposite shore.
The council will look to create permanent alerts for high-risk lakes on the "Can I swim here" Lawa (Land, Air, Water, Aotearoa) page using available information.
The council has implemented recreational water quality monitoring programmes during the summer months since 1985.
The programmes include 34 monitoring locations at popular open-coast, estuary, river and lake sites.
During the 2021/22 season, faecal indicator bacteria concentrations and/or cyanobacteria counts exceeded guideline values on several occasions.
Open beaches were mostly suitable for swimming, with two incidences of the guidelines for enterococci being exceeded - at Buffalo Beach and Port Waikato Beach.
Five estuary sites were monitored in the past summer for enterococci and E. coli bacteria at sites in Kāwhia, Maraetai Bay, Raglan, Tairua and Whangamatā. There were five cases of the enterococci guideline being exceeded - four at the Tairua Pepe Stream and one in Kāwhia; and four above the E. coli guideline - one at a Raglan motor camp and three at Tairua Pepe Stream.
Nine river sites were monitored weekly for E. coli from December to February (excluding Waikato River hydro-lakes), with 22 samples found to be above the recreational guideline. All rivers sampled were affected except for the Waikato River at Horahora domain, where all results were suitable for swimming.
Water quality in lakes is measured for E. coli (Lake Taupō and shallow lakes) and cyanobacteria bio volume (Lake Taupō, shallow and hydro-lakes). There were three exceedances of the recreational guideline for cyanobacteria in Lake Ohakuri and one in Lake Maraetai.