One of Warkworth’s most popular swimming spots remains closed while Auckland Council tries to identify if current E. coli levels pose any threat to human health.
‘Public health warning’ signs were erected at the Wilson Road cement works swimming hole on November 9, after water there turned a rusty red colour.
Initial results in mid-November revealed that the colour change was due to an algal bloom, but also showed the presence of 540 E. coli cells per 100ml of water.
Council health enforcement principal specialist Darryl Thompson says most strains of E. coli are harmless, but when levels reach 550 cells per 100ml, it indicates pathogens such as salmonella or norovirus are likely to be present.
“Council policy is that if E. coli numbers reach 260 cells per 100ml then the water should be avoided and tests taken,” Mr Thompson says.
Results from a second test, taken on November 23, were being processed when Mahurangi Matters went to print.
“Regardless of whether the E. coli cell count has dropped to a reasonable level in the second test, the warning signs will remain up until we can get some consistency with results."
“We expect to be running one test every week until then.”
Mr Thompson says if levels remain high, Council will try to establish the source of the E. coli, but due to the costs involved, this will only happen at a later stage if necessary. The most likely cause would be waste from animals or humans.
Tests can also be run to identify any pathogens that might be present, but these are also expensive at upwards of $1000 per test.
Mr Thompson says Council has not run water tests at the cement works before, so it’s unclear if this is an ongoing issue or an isolated incident.
He says the E. coli could be connected to the algal bloom if it’s providing bacteria to feed off, but equally, they could be unrelated.
Meanwhile, oyster farmers in the Mahurangi Harbour will stop harvesting for an extended period of two months next year over concerns about contamination from norovirus.
Norovirus is spread via human waste, usually from boats. Anyone who eats an oyster that has contracted the norovirus can suffer from vomiting, diarrhoea and stomach cramp.
Jim Dollimore, who owns Biomarine, the largest farm in the harbour, says they have stopped harvesting on dates such as Labour Weekend when they know more boats will enter the harbour.
Although norovirus only remains in the oysters for up to a week, oyster farmers have decided to adopt a precautionary approach.
“To help make sure our oysters are safe to eat next year, we have decided to stop all harvesting in the Mahurangi Harbour over the busy period of January and February,” Mr Dollimore says.
“We can control water quality issues from things like run off, but we can’t monitor every boat that comes into the area.”
The challenge with the norovirus is that there are no tests that can identify it, so oyster farmers can’t harvest and monitor the water at the same time to avoid the issue.
The norovirus caused the closure of the harbour earlier this year and sparked a full Ministry of Primary Industries investigation.
“We can’t be sure, but we believe it came from a boat that was moored in the harbour,” Mr Dollimore says.
He says the closure will have an economic impact, as oysters are popular over the summer, but the local industry remains sustainable.
“The best oysters are harvested between July and November.
“This is the first time we’ve had the norovirus problem since operations started in the 1970s, so I don’t see it as a major issue going forward.”
Mr Dollimore would like to see more education for boaties on the subject and increased signage to reduce the risk of further closures in the future.