Post-Flood Drinking Water Safety for Private Water Systems

Flooding following heavy rainfall or rapid snow thaws can significantly impact water quality in private water systems, such as wells, springs, or cisterns. Floodwaters often carry a mix of pollutants, including bacteria, viruses, and other chemicals from various sources, including sewage, farms, household chemicals, industrial waste, etc. Some of these contaminants can pose human health risks.

Under flood conditions, contaminated surface water may enter directly into a well through a loosely fitting well cap or damaged well casing. Contaminated groundwater may also migrate underground to your well from nearby flooded wells, pits, or excavations. Floodwater can increase water flow into the spring and compromise the water quality. Cisterns can be contaminated by floodwater if not properly sealed at the top or if floodwater enters through openings, vents, or cracks.

Six Steps You Can Take

if your water supply was inundated during a flood or you suspect that it was impacted:

1. Use an alternative drinking water source

If your drinking water source was flooded or you suspect that water is contaminated by floodwater, do not use the water for drinking, cooking, mixing baby formula, making ice, dishwashing, or brushing your teeth until testing confirms the safety of your water. Switch to an alternative source known to be safe and free of contaminants, such as a community water supply or bottled water. 

2. Visually inspect your drinking water source to assess the impact

• For wells: Check for visible damage to the well casing, wellhead, and surrounding area. Look for cracks on the casing and wellhead, debris, sediment, or signs of erosion around the wellhead area.
• For springs: Check for visible damage to the spring box and look around the spring source for erosion, debris, or changes in flow rate.
• For cisterns: Examine for cracks or leaks on the cistern walls or the presence of sediment or debris inside the cistern. Note any changes in water levels that may indicate floodwater entry.

Water Quality: Look for any visible changes in the water, such as discoloration, cloudiness, floating debris and particulates, or unusual smell.

If you observe these signs, assume the flood impacted the drinking water source and take cautionary measures. Use an alternative safe water supply until testing confirms the safety of your water. 

3. Secure the system

If the water source is damaged, protect it from additional contamination by covering exposed parts and securing any damaged components. Contact a licensed contractor, inspector, or plumber to assess and repair any damages you cannot safely correct. 

4. Test water quality

Arrange for water testing through a Pennsylvania Department of Environmental Protection (PA DEP) certified laboratory.

Bacterial Testing: Arrange for testing for coliform bacteria and E.coli. These are indicator organisms, and their detection indicates that other disease-causing pathogens could be present in water. The drinking water standard for coliform bacteria and E. coli in drinking water is zero.

Chemical Testing: Water can also be tested for nitrates. Nitrates can become a concern if the drinking water source is impacted by an adjacent septic system, runoff from manure amended or fertilized agricultural fields, or sewage treatment plants. The drinking water standard for nitrate in drinking water is 10 mg/L.

5. Treat the drinking water source

If you suspect your source was impacted by the flood or your water test results indicate bacterial contamination, consider disinfecting it before use. 

Boiling for drinking or cooking:

Boiling is a practical method to kill pathogens in small amounts of water for drinking or cooking. To use this method:

1. Place the water in a clean pot or kettle
2. Heat the water until it reaches a boiling point. This is seen when the water is bubbling vigorously.
3. Allow the water to boil for 1 to 3 minutes.
4. Cool the water before storing it in clean containers with lids.

Boiling is only recommended when the primary concern is biological contamination, such as bacteria. Do not boil or use the water if chemical contamination is suspected. Boiling water as an emergency treatment method is not recommended when:  

• The water is full of sediments, highly turbid, or cloudy.
• There are visible signs of chemicals in your water. This can include heavy discoloration, oily sheen on the water, chemical smell, and gasoline-like or solvent-like smell.

Chlorine and iodine tablets:

Chlorine or iodine tablets can disinfect water in emergencies, especially when there is a need to make water safe for consumption quickly. Both methods are effective against a broad range of bacteria and other pathogens. These tablets are available from drug and sporting goods stores and should be used following manufacturer instructions. Chlorine and iodine tablets do not remove any chemical contaminants in water. Consider an alternative drinking water source or additional treatment if chemical contamination is suspected or you have doubts about water safety.

Shock chlorinating the drinking water source:

This often involves mixing unscented bleach with water and adding it to the water source to kill bacteria in the water source.

For Wells and Springs: Detailed instructions on shock chlorination are available from PA DEP: disinfection of private wells and springs. After disinfection, flush the well with clean water until the chlorine smell disappears. A well driller can also assist with shock chlorination if you are not confident in doing it yourself.

For Cisterns: When disinfecting cisterns affected by flooding, drain the cistern and remove all debris. Mix 1 cup of 5-9% unscented bleach with 10 gallons of water. If possible, use this solution to scrub the inside of the cistern. Rinse the cistern with clean water, drain and allow to refill from a water source. Detailed instructions on disinfecting cisterns are available from the US Centers for Disease Control and Prevention (CDC). Depending on the water source for your cistern, the water may also need to be disinfected and plumbing lines flushed.

Shock chlorination will kill pathogens or microbes such as bacteria in water but will not remove chemical contaminants such as nitrates, heavy metals, pesticides, gasoline, oils, and other chemicals. If you suspect the water is unsafe because of chemical contamination, consider installing other water treatment devices to remove the contaminant of concern. Use this Common Drinking Water Problems and Solutions to determine potential treatment solutions fact sheet to identify potential solutions for the contamination issue you are dealing with or talk to your local Extension Educator.

6. Re-test your drinking water source

After disinfection and flushing, retest the water to ensure it meets safety standards. Ensure that bacteria and chemical levels are within acceptable limits. Consider installing a point of entry or point of use water treatment devices if health-related biological or chemical water quality concerns exist. Use this Common Drinking Water Problems and Solutions to determine potential treatment solutions fact sheet to identify potential solutions for the contamination issue you are dealing with or talk to your local Extension Educator.

Long-term Considerations

Even after the immediate issues are addressed, private water system owners and users should remain vigilant to protect their water supply. For example, well owners should ensure that the casing extends 12 inches above the ground and is fitted with a watertight sanitary well cap, and the ground should be sloping away from the wellhead. Here are some resources for construction features to protect your private drinking water sources:

• Sanitary Well Construction Guidelines.
• Spring Development and Protection.
• Rainwater Cisterns: Design, Construction, and Treatment