The Westborough Water Department issued a drinking water warning Sunday night after E. coli bacteria was detected in one out of three samples collected on Aug. 16.

The department said there is no emergency, but warned that residents with severely compromised immune systems, infants and some elderly individuals may be at increased risk for getting sick.

Officials said the contamination was found in a sample from Westborough’s Fay Mountain water storage tank, which has been taken offline until the department determines the cause of contamination and conducts additional tests to ensure the situation is resolved.

Bacterial contamination can occur when increased run-off enters the drinking water source, which often happens after heavy rain. Breaks in pipes, contamination in the storage tank itself and failures in the water treatment process can also be potential causes, the department’s warning said.