Residents with private wells are being urged to have their water supply tested as soon as possible.
Lambton Public Health said there's the risk of harmful bacteria developing after up to 75 millimetres, three inches, of rain fell over the weekend across the county.
Public Health Inspector Vicky MacTavish said there are some precautions to take in the meantime.
"We would ask the community to use bottled water for daily use," said MacTavish. "That includes drinking water, making infant formula or juices, cooking, making ice, washing fruits and vegetables, or brushing your teeth. Or, you could boil the water for at least one minute before using."
Once floodwaters have receded, the well should be disinfected and tested at least three times, at one week intervals, before the water is used for drinking.
Free water sample kits can be picked up and dropped off at Lambton Public Health in Pt. Edward, CEE Hospital in Petrolia, or at the North Lambton Community Health Centre in Forest.
The samples must be brought back within 24 hours of being taken.
Lambton Public Health recommends testing well water three or four times per year as a precaution.