Tracking bugs at the beach

Auckland beachgoers will be able to go online to check the level of harmful bacteria expected in the water. Photo / Greg Bowker

Auckland Council is putting final touches to a beach forecast model to let people find out online the level of harmful bacteria in the water for three days ahead.

At present the Safeswim site can tell you only what happened on the day a water sample was taken - usually a Tuesday or Wednesday - and you use it to infer the water is still safe until the weekend.

"With the model, you can jump online on a Friday afternoon and see that the beach is safe now and on Saturday and Sunday as well," says council senior marine scientist Dr Jarrod Walker.

The model will cover eight sites on Waitemata Harbour beaches from Pt Chevalier to St Heliers. Its information could be available by the Easter holidays.

The council recommends people avoid swimming for 48 hours after heavy rain, when stormwater carries bacteria to the beach.

The beauty of the model is that it works as the rain is falling. "The model can predict within the next hour and change the warning - almost on the fly," said Dr Walker.

The council's environmental science manager, Dr Martin Neale, said the model would predict bacteria levels based on rainfall projections, tidal cycles, water movements in the Waitemata Harbour and the known water quality of contributing catchments.

"This work remains experimental but we have tested the model over the last 12 months by comparing it to Safeswim and other monitoring data.

"The model has performed exceptionally well and we are in the process of securing funding for its continued development and roll-out across the whole of the Waitemata and beyond."

Dr Neale said the advantage of the model was that it would forecast water quality for three days ahead whereas the results from water samples taken for laboratory testing typically described the water quality as it was two days before.