Flood water is not the same as normal river water.  Floods contains potential contaminants that we are generally able to keep out of our rivers and streams. 

  • Bacteria and viruses from inundated septic systems can cause infection and illness.
  • Other materials like household cleaners and chemicals, lawn care products, and pesticides can cause skin irritation, rash and allergic reaction.

People should make all reasonable efforts to avoid coming in to contact with the flood water. Those who must work in flood-inundated areas should cover their skin with some type of water-impermeable (rubber) boots and gloves and wash exposed areas with soap and water and rinse thoroughly immediately after exposure. The Health Department also recommends residents ensure their tetanus shots are up to date. The department’s walk-in clinic offers vaccinations on a sliding fee scale. Clinic hours and additional information are available online



If you are on a private well, and the well head is surrounded by water, assume that the well water is contaminated.  Until the floodwaters recede, and you can test and disinfect your well, you should use bottled or boiled water for drinking.

Even if your wellhead is not underwater, floodwater and high groundwater can cause contamination by traveling along the well casing or getting in through the distribution system. Testing for bacteria is a good indicator of whether floodwater has contaminated your well. Several water labs in the area that can run this test, including a lab at the Health Department (tests here are $15 each). Results take about two days to receive. If initial test results show that your well does not have bacteria, be aware that rising water and changing flood conditions may cause the well to be contaminated after you test.

There are a few methods for killing bacteria and viruses in contaminated or potentially contaminated water:

  • Boil water for five minutes and store in a clean container. The flat taste can be eliminated by shaking the water in a bottle or pouring it from one container to another.
  • Mix 5 drops of household bleach with 1 quart of water (or 20 drops per gallon) and let stand for at least five minutes (preferably 30 minutes to an hour) or longer before drinking. Bleach should be unscented and free of detergents or additives.

Because bacteria and viruses are not the only possible contaminants in wells affected by flooding, bottled water is the safest for drinking water. 

If you are a Missoula Water customer, the flooding situation has not affected the quality of your drinking water. The wells in the Missoula Water system are carefully and frequently monitored. If a Missoula Water well becomes vulnerable to potential contamination because of its proximity to a flooded area, Missoula Water has the ability to inactivate the well and provide customers with drinking water from another protected well. Call 406-552-6700 with questions. Also the Seeley Lake water treatment system is not affected, and Seeley Lake's public water is safe to drink. The Seeley Lake Sewer and Water District can be reached at 406-677-2559. 


Septic systems are designed to get rid of wastewater, but also to treat the sewage before it enters groundwater.  When the drainfield is flooded, the sewage will not be treated.  When your system is flooded or saturated with groundwater, stop using it.  Otherwise, it will be a source of pollution.

Flooded or saturated systems can back up into the house.  If floodwater comes up through your drains or fixtures, it is probably coming through your septic system and can be contaminated with disease-causing organisms.  You can try to prevent your system from backing into your house by plugging any drains that are lower than the level of flooding.

Do not have your tank pumped when it is flooded or the ground around it is saturated, unless your tank was designed not to “float” during high water.  Even though tanks are made of concrete, when they are empty, under the right water and soil conditions, they can float up and cause damage to your system.

If you have floodwater or groundwater in your house or basement, do not pump it into your house plumbing or drains.  This will send the water to your septic system and could cause it to fail - even if the septic system is located in an area that isn’t flooded.

If sewage has backed up into the house, clean and disinfect the area.  The EPA recommends a chlorine solution of half a cup of bleach for each gallon of water to disinfect the area completely.

More information about septic systems during a flood:

The City of Missoula’s Wastewater Treatment Plant is not in danger of flooding. It is well above the maximum flood level. The plant continues to treat waste coming into it through the public sanitary sewer system. Residents should not pump flood water into the sanitary sewer. The wastewater treatment plant can be reached at 406-552-6600.



As a result of flooding, hazardous chemicals can also make their way into soils, gardens, and groundwater (drinking water).  As a precaution, put all household hazardous chemicals such as gasoline, pesticides, fertilizers, oil-based paints and solvents in plastic bins and remove from your property or place at highest level possible in your garage or yard. 

 If you are ready to dispose of chemicals, more info can be found here: