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Otago Regional Council staff will start sampling and testing the water in a few weeks’ time.
Strategy, policy and science general manager Gwyneth Elsum said water quality often changed when conditions changed.
‘‘Even the sites that typically have excellent water quality can see bacteria spike as a result of heavy rainfall washing contaminants from urban and rural land into waterways,’’ she said.
People should avoid swimming for two or three days after heavy rainfall, she said.
Land Air Water Aotearoa (Lawa) has a section on its website that answers the question, ‘‘Can I swim here?’’.
Cyanobacteria, or blue-green algae, can also cause water quality concerns when algal blooms are present.
The regional council recommends that people learn how to identify potentially toxic algae in lakes and rivers and avoid contact with water if they think the algae is there — a general rule of thumb is that if lake water is looking green, stay out of it.
For rivers, toxic algae can grow on rocks and sometimes washes up along the water’s edge.
In such cases, dogs should be kept away from the water’s edge.
Lawa has information about long-term grades on its website, based on five years of data.
Six sites in Otago have poor long-term grades, meaning they have elevated bacteria concentrations 5% or more of the time.
A poor grade can indicate a higher risk of illness to swimmers.
The sites in Otago with poor long-term grades are: Lake Hayes at Mill Creek Shallows, Otokia Creek at Brighton Beach, Taieri at Outram, Taieri at Waipiata, Manuherekia River at Shaky Bridge and Kakanui Estuary.
Under new national guidelines, action plans must be developed for all poor-graded freshwater swimming sites.
The sites with a ‘‘fair” grade, indicating a smaller risk to swimmers, are: Lake Wakatipu at Frankton Bay, Lake Waihola Jetty, Tomahawk Beach (east and west), Macandrew Bay, Waikouaiti River at Bucklands and Pounawea Estuary at the Catlins.
The 14 other summer recreation monitoring sites in Otago have good or excellent quality, or insufficient data samples for a long-term grade.
Weekly testing can sometimes show spots with a long-term poor grade are all right for swimming, reflecting some limitations in the long-term data.
The regional council said weekly sampling results provided strong indicators of whether spots were safe to swim at.