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Dr Dave Fearnley is concerned about the ''dubious'' water quality in the lower Manuherikia and its potential effect on swimmers. Last summer, the Otago Regional Council's sampling showed faecal bacteria (Escherichia coli) levels reached an ''action/red'' status near Alexandra four times, which meant it could be a health risk for swimmers.
''Last summer, my nephew became unwell after swimming by the Shaky Bridge. How many people might have their holidays affected in a similar fashion this summer?'' Dr Fearnley asked.
''During the recent warm weather there's been 20, 30 or up to 50 people swimming in the Manuherikia River under the bridge, including lots of children. I wonder how many people get ill after swimming here in summer and put it down to something they've eaten, but it hasn't been food poisoning -it's been water poisoning.''
The regional council website has a section called ''Water Monitoring for Recreational Activities'' which outlines the results from bacteria count sampling.
The samples are taken weekly from the most popular swimming spots in Otago, from December through to the end of March. The results are compared to national guidelines set by the Ministry for the Environment and Ministry of Health and are classified in three categories - green (should be very safe for swimming), alert/amber (should be satisfactory for swimming) and action/red (could be a health risk for swimming).
Regional council director of environmental information and science John Threlfall said he would ''think twice'' about swimming in the Manuherikia near Alexandra in summer.
''If I was looking at that spot, I'd swim there with caution and in fact, I'd probably keep kids out of the water.
''But no waterway is 100% safe in terms of bacteria.''
He was surprised to find the bacteria level had reached ''action/red'' status four times last summer. Three were ''just over the line'' into a higher concentration of E. coli while the fourth was high.
It was very difficult to isolate the causes of the high bacteria count during low river flow.
''It could be farming practices, septic tanks, dead stock, ducks upstream - wild fowl can make people sick - and another source of bacteria during low flow is irrigation bywash, especially flood irrigation, which is common upstream,'' Dr Threlfall said.
The council was working towards cleaning up contamination from farm and septic tank discharges but would consider more intensive sampling upstream in the Manuherikia to check if there was a pattern and if the source could be isolated.
Results from sampling took four to five days to come through, he said. The most recent one showed the E. coli levels were at ''alert/amber'' status, but Dr Threlfall believed the results from last summer should be seen as an indicator of what might happen this summer.
He would discuss the matter with the Central Otago District Council and Public Health South. Dr Fearnley believed the council had a responsibility to warn people their health might be affected by swimming in the river.
''I wouldn't go so far as to say the ORC are negligent but I believe if it's at a level that's unacceptable for human health, they have a responsibility to advise the public, rather than relying on them to read it online.''
The logical answer was to improve water quality, but obviously that would not happen overnight, Dr Fearnley said.
''If people who become unwell after swimming were to report their illness to the ORC, the health impact of poor water quality might become better appreciated.''