Local authorities in Avesta recommend that residents boil water used for drinking and cooking as a precaution.
The advice applies to residents in central Avesta and Skogsbo. It remains in place at least until Monday.
The recommendation was issued on Thursday following concerns that groundwater may have entered the system during maintenance work. No evidence indicates the water is unsafe.
Samples have been collected, though results are pending. The town has around 16,000 inhabitants.