The source of the contamination in Kuressaare is still not known, and the municipality on Saaremaa has ordered expert studies to attempt to change that.

Whereas contamination incidents earlier in the year were thought to have been caused by untreated sewage traces which had found their way into the water supply, this time it is thought that ground water pollution in the water catchment area is to blame.

Aivar Sõrm, board member of Kuressaare Veevärk, which provides the town's water, said: "The cause of the pollution is the contamination in the water catchment area, but as of today, we don't know whether this entered the catchment via longer routes, thanks to heavy rainfall at the end of August, or via a shorter route from the ground on up. This is currently a matter for debate, and the analysis being carried out will make it clear what we are dealing with, one way or another."

The Health Board (Terviseamet) has also taken additional water samples in order to obtain a picture of what happened; possible sources could also include agriculture, though the acting head of the local Environmental Board (Keskonnaamet) denied that chemical or natural fertilizer was to blame here.

The use of chlorine has reportedly helped with drinking water quality; E-coli was detected in one further location within Kuressaare city limits on Monday, but the downside is that the tap water will continue to exude a chlorine taste for several months.

Ain Saaremäel, board member of the Kuressaare waterworks, said there was nothing to be done more.

Samples taken at boreholes on Monday showed both E.coli and E.coli-like bacteria in a water supply which starts at that point its 7km journey to Kuressaare itself.

The Health Board had advised residents to boil water before drinking, or better still use bottled water, while the municipality had put in place water tanks in the town for obtaining clean water in that way, and provided bottled water to schools.

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