Warnings over bathing at three of four sites in north Down were lifted on Friday after fresh tests showed levels of the E. coli bacteria to be acceptable.
Water close to four beaches, Crawfordsburn, Brompton Donaghadee and Ballyholme, were all re-tested with only the last remaining still with a warning on Friday afternoon. It was due to be further tested later in the day.
Annika McClements, a principal scientific officer at the Department of Agriculture, Environment and Rural Affairs, said the two main sources of E. coli and other similar bacteria is farm run-off and storm drain overflows.
"So during heavy rainfall periods there would be a combination of agricultural run-off from land and storm overflows to rivers and the sea," Ms McClements told the BBC.
"E. Coli is an indicator organism for a number of quite nasty bacteria all of which can cause stomach upsets. Sometimes it can take three or four days after ingesting them for symptoms to appear."
While most of the many strains of e-coli are harmless, at least one is dangerous while detection of any could signal other illness-inducing pathogens are present. Warnings were issued about the four beaches on Thursday.
Alliance North Down MLA Connie Egan described as deeply concerning the bathing spots across north Down failed water quality testing.
Ms Egan said: “Whilst it’s reassuring that we have the mechanisms in place to regularly test for water quality and the safeguards to protect people from harm, this is incredibly concerning.
“It’s a real shame to see any of them out of action for any length of time, but for a number to be affected at once is very worrying indeed."
