Source of high bacteria levels in Kakanui River uncovered
February 14, 2013
Source of high bacteria levels in the Kakanui River uncovered
Otago Regional Council staff have pinpointed the previously elusive source of bacterial contamination of a popular North Otago swimming hole.
Up until recently, ORC staff and local farmers alike have been baffled as to what has been responsible for high concentrations of E.coli occurring at Clifton Falls on the upper Kakanui River, particularly during summer.
ORC staff have been concerned about the bacteria, as high levels have the potential to cause illness in recreational bathers.
ORC enlisted the help of local farmers, who provided access to their properties and the nearby river for close inspection. When still no source of bacteria was found, a helicopter was sent into the gorge to gain an aerial perspective of the problem.
The source - a large colony of nesting gulls - was found in rugged terrain, about 5 km above the Clifton Falls bridge.
Water quality samples were taken immediately above and below the colony, with widely divergent results Upstream of the colony, the bacteria concentrations were 214 E.coli/100ml, whereas immediately downstream, the concentration was far greater at 1300 E.coli/100ml .
ORC manager of resource science Matt Hickey said that according to Government water quality guidelines for recreational swimming areas, those with less than 260 E.coli/100m should be safe, whereas water with more than 550 E.coli/100ml could pose a health-risk.
Mr Hickey said six colonies of gulls were found in total, on steep rocky faces, where they clearly favoured the habitat for nesting.
While they had gone undetected up until now due to the inaccessible nature of the gorge, it was likely the gulls returned each year to breed in the same places.
“Unfortunately, these nesting gull colonies are likely to continue to cause high E.coli concentrations in the upper Kakanui River, particularly during the breeding season,” Mr Hickey said.
“Bird activity, river flow, or even whether it is a cloudy or sunny day, (as E.coli often died quickly in clear water when exposed to sunlight) will influence actual bacteria numbers at Clifton Falls bridge. With hindsight, it reflects the random nature of the historical bacteria results at this site.”
Mr Hickey said the E.coli concentrations reflected a large number of birds congregating in a small area and we are fortunate this situation was not common in Otago. Historically E.coli concentrations in the lower Kakanui River have been very low, despite the gull colonies being found upstream.