The contamination appears to stem from raw water at the intake rather than within the network, though officials are awaiting repeat samples to confirm the source.
The findings are preliminary, and an official analysis report is not yet available. The municipality has begun preventive chlorination and flushing of the mains.

Saaremaa Municipality recommends boiling water before drinking to ensure it is completely safe. Repeat samples are to be taken immediately.

"The widespread contamination of the water network with coliform bacteria points more to contamination of the raw water coming from the water intake than to contamination occurring within the water network, but it is difficult to assess the causes of the contamination before the results of the repeat samples are available. Kuressaare Veevärk has also informed the Health Board of the situation and coordinated further steps," the municipality said on its website.

Coliform bacteria are common environmental bacteria found, for example, in the external environment, including nutrient‑rich water, wastewater, soil and decomposing plant material. Their presence in water does not generally cause illness in humans, but as an indicator they can provide valuable information about the drinking water distribution network. "The presence of small numbers of coliform bacteria in drinking water primarily indicates a need to flush the pipelines. One way to ensure water purity is to add chlorine to drinking water. Small numbers of coliform bacteria are unlikely to pose a health risk. However, because vulnerable groups require greater protection under the precautionary principle, stricter‑than‑usual measures are applied when they are detected," the municipality's website went on.

E. coli is a species of bacteria that forms part of the natural microbiota of humans and animals. It can enter natural waters and wells, particularly during heavy rainfall or snowmelt, through organic matter washed off the ground. The presence of E. coli in water is primarily a signal that drinking water needs to be checked further and, if necessary, water treatment adjusted. Although most strains of E. coli are completely harmless, certain rare strains can pose a risk of illness. For this reason, every detection is treated with full attention and the necessary steps are immediately taken to restore water quality.

Following routine tests on July 30, Kuressaare Water Utility started chlorinating the water supply after preliminary lab results found E. coli and coliform bacteria at pipeline ends, though official results are pending. Residents in the Upa area are advised to boil drinking water as a precaution, and while the utility notes that coliforms can appear during warm weather due to stagnant water, the first E. coli detection in three years requires immediate action—though it does not indicate widespread contamination.

The source of another E. coli contamination in Saue Municipality near Tallinn remains unknown as authorities conduct joint investigations, though public water is unaffected while private borewell water in the suspected zone is unsafe. Nearly 40 wastewater systems have been inspected — many substandard — but no cause has been confirmed, and construction is also being probed. With spread hard to track, more samples are being taken, and fresh drinking water tanks are available to affected residents.

E.Coli outbreaks were also found in Kuressaare in 2023.

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