Well Water After a Flood: Testing and Disinfecting a Private Well

Quick answer: if floodwater reached or covered your private well, assume the water is unsafe. Stop drinking it, use bottled or properly treated water, and stay away from the pump and wiring until they are checked. After the water recedes, the usual sequence recommended by the CDC and EPA is to inspect and repair the well, disinfect it with shock chlorination, flush it, and have it tested for bacteria by a certified laboratory before drinking from it again. Your local or state health department can tell you the exact steps and where to send samples.

City water systems are run by utilities that test and treat the water and announce problems. Private wells are different. The U.S. Environmental Protection Agency points out that it does not regulate private wells, so the owner is responsible for keeping the water safe. After a flood, that responsibility becomes urgent.

Floodwater can carry sewage, animal waste, fuel, farm runoff and debris. When it reaches a wellhead, those contaminants can get into the well and the aquifer around it. This guide walks through what to do, step by step, from the moment the water rises until the well is cleared for use again. It is part of our emergency water guide, which covers storage, treatment and filters for every kind of household.

Signs Your Well May Be Contaminated

Some situations make contamination likely enough that the CDC and EPA advise treating the water as unsafe without waiting for a test:

• Floodwater covered the wellhead or rose above the top of the casing
• The well cap is missing, cracked or loose
• Water pooled around the wellhead and soaked into the ground near the casing
• The well is shallow, dug or bored rather than drilled, since these are more easily contaminated
• The water looks cloudy, muddy or discolored, or smells or tastes different than usual
• A nearby septic system, livestock area or fuel tank was flooded

Clear water is not proof of safety. Bacteria, viruses and many chemicals cannot be seen, smelled or tasted. When in doubt, stick with safe water until a lab result says otherwise.

What to Do Right Away

Stop using the water for drinking and cooking

Switch to bottled water or water you have stored. If neither is available, treat the water first. The CDC recommends bringing clear water to a rolling boil for one minute, or three minutes at elevations above 6,500 feet. Boiling kills disease-causing organisms but does not remove chemicals such as fuel or pesticides. If chemical contamination is possible, boiling is not enough, and bottled water is the safer choice.

Water you treat or store should also be used for brushing teeth, washing dishes, making ice, preparing baby formula and washing produce. Our comparison of purification tablets, bleach and boiling explains how each method works and when to use it.

Stay away from the pump while it is flooded

The CDC warns that touching a flooded well pump or its wiring can cause electric shock. Do not turn the pump on or off while it or its controls are under water or wet. Once the water drops, have a licensed electrician or well contractor inspect the pump, pressure switch and wiring before you restore power. Submerged components often need to be cleaned, dried or replaced.

Keep your household stocked

A typical recovery takes days and can stretch longer if testing must be repeated. Ready.gov and the American Red Cross recommend at least one gallon of water per person per day for drinking and basic hygiene. Our guide on how much water to store per person turns that into a household total.

Inspect and Repair the Well

After floodwaters recede, the next step is a careful look at the well itself. Many health departments recommend calling a licensed well contractor, especially if the well was submerged or damaged.

Points to check include:

• Damage to the casing, cap or seal, including cracks or gaps
• Erosion or a depression in the soil around the casing that lets surface water drain down
• Debris, sediment or mud inside the well
• Damage to the pump, pressure tank, electrical controls and any treatment equipment
• Whether the wellhead now sits below the surrounding ground level

The EPA’s guidance for private well owners stresses that the top of the casing should extend above ground and that the ground should slope away from the well so water drains away rather than toward it. If the flood changed the grading, fixing it is part of protecting the well from the next storm.

Water treatment equipment

Filters, softeners, ultraviolet units and reverse osmosis systems can be contaminated as well. Follow each manufacturer’s instructions for sanitizing them and replacing cartridges or media. Many owners bypass treatment equipment during disinfection so the chlorine does not damage it, then sanitize and reconnect it afterward.

How Shock Chlorination Works

Shock chlorination is the standard way to disinfect a well after a flood. The CDC, the EPA and state health departments all describe the same basic idea: add a strong chlorine solution to the well, circulate it through the plumbing, let it sit long enough to kill organisms, then flush it out.

The exact amount of chlorine depends on the depth and diameter of the well and the amount of water standing in it. That is why the CDC and most states publish worksheets or tables for owners to follow. Use the instructions from your state or local health department rather than a generic recipe, and use plain, unscented household bleach or a chlorine product labeled for disinfecting water. Scented, splash-less or color-safe bleaches contain additives that should not go into drinking water.

The general sequence

Every state’s instructions differ slightly, but most follow these broad steps:

1. Run water from an outside tap until it clears of mud and sediment, if the pump is safe to operate.
2. Bypass or remove treatment equipment and filters as the instructions direct.
3. Calculate the amount of chlorine for your well using your health department’s table.
4. Mix the chlorine with water and pour it into the well, often through the vent or after removing the cap.
5. Recirculate the water by running a hose from a tap back into the well until you smell chlorine at the hose.
6. Open each faucet inside and outside the house, one at a time, until you smell chlorine, then close it. Flush toilets as well.
7. Let the chlorinated water stand in the well and pipes for the contact time your instructions specify, often overnight.
8. Flush the system until the chlorine smell is gone.

Chlorine at these concentrations is strong. Wear eye protection and gloves, keep the area ventilated, and never mix chlorine bleach with ammonia or other cleaners, since the combination releases toxic gas.

Where to drain chlorinated water

Large volumes of heavily chlorinated water can harm plants, streams and septic systems. Many state guides recommend draining it onto a gravel driveway or an open area away from gardens and waterways, and avoiding sending large amounts into a septic system, where the chlorine can kill the bacteria the system needs. Follow your state’s guidance on disposal.

Testing the Water Before You Drink It

Disinfection is not the finish line. The CDC and EPA advise testing the water after shock chlorination to confirm it is safe. The usual test looks for total coliform bacteria and E. coli, which indicate whether disease-causing organisms could be present.

How to get a test

Your local or state health department can tell you how to get a sample kit and where to send it. Many offer free or low-cost testing after a declared disaster. The EPA also maintains information on state-certified drinking water laboratories. Use the sterile bottle the lab provides and follow the sampling instructions exactly, since a careless sample can give a false result.

Timing matters. Most instructions say to wait until the chlorine has been fully flushed out before sampling, because leftover chlorine can produce a misleading result. Ask the lab or health department when to collect the sample and whether a second test is needed.

Reading the results

A result showing no coliform bacteria is what you want. If the test finds coliform bacteria or E. coli, do not drink the water. The usual response is to repeat the shock chlorination and test again. Repeated positive results can point to a damaged well, a cracked casing or a contaminated aquifer, and that calls for a well contractor and advice from the health department about longer-term treatment or a new water source.

Chemical Contamination and Septic Systems

Shock chlorination targets microorganisms. It does not remove fuel, oil, pesticides, fertilizer or industrial chemicals. If your well was near a flooded fuel tank, a farm, a business that stores chemicals, or if you notice a fuel smell or oily sheen, tell your health department. They can advise which chemical tests to run, and in the meantime bottled water is the safe choice for drinking and cooking.

Nitrate is a specific concern in farming areas. Floodwater can carry fertilizer and manure into wells, and boiling water concentrates nitrate rather than removing it. Infants are most at risk from high nitrate levels, so families with babies should use bottled water for formula until a nitrate test comes back acceptable.

Septic systems after a flood

A flooded septic system and a private well often sit on the same property, and one can affect the other. The EPA’s guidance on septic systems after a flood advises not using the system while the soil around the drainfield is still saturated, and having it inspected and pumped if needed by a professional before normal use resumes. Reduce water use in the house while the ground dries. Leaking or backed-up sewage near the well is another reason to test the water and to call the health department.

Getting Ready Before the Next Flood

Wells in flood-prone areas benefit from a few steps taken long before the next storm.

• Keep a record of the well’s depth, diameter and construction, which you will need to calculate chlorine amounts
• Make sure the casing extends above ground and has a sanitary, watertight cap
• Keep the ground sloped away from the wellhead
• Store fuel, chemicals and fertilizer well away from the well
• Note your health department’s phone number and testing instructions in your emergency plan
• Keep a supply of stored drinking water for the days when the well is out of service
• Test the water regularly, even in normal times; the CDC recommends testing private wells at least once a year for total coliform bacteria, nitrates and other contaminants of local concern

When a flood warning is issued, the National Weather Service and local officials will advise on evacuation. Never wade or drive into floodwater to reach equipment. Our guide on flood safety explains why even shallow moving water is dangerous.

Frequently Asked Questions

Can I drink my well water after a flood if it looks clear?

Not until it has been tested. Bacteria, viruses and many chemicals are invisible. The CDC and EPA advise using bottled or treated water until a certified lab confirms the well is safe.

Does boiling well water make it safe after a flood?

Boiling clear water at a rolling boil for one minute, or three minutes above 6,500 feet, kills disease-causing organisms. Boiling does not remove fuel, pesticides or other chemicals and can concentrate nitrate, so use bottled water if chemical contamination is possible.

How much bleach do I need to shock a well?

It depends on the depth and diameter of the well and the volume of water in it. Use the worksheet or table from your state or local health department, and use plain, unscented bleach or a product labeled for disinfecting water.

Where can I get my well water tested after a flood?

Start with your local or state health department, which can provide sample kits and point you to a certified laboratory. Many offer free or reduced-cost testing after declared disasters.

What if the test still shows bacteria after disinfection?

Do not drink the water. Repeat the shock chlorination and test again. Ongoing positive results suggest damage to the well or a contaminated aquifer, so call a licensed well contractor and your health department.Sources: CDC (Private Water Systems after an Emergency; Making Water Safe in an Emergency), U.S. Environmental Protection Agency (Private Drinking Water Wells; What to Do After a Flood; Septic Systems: What to Do after the Flood), FEMA and Ready.gov (Floods; Water), American Red Cross (Flood Safety), National Weather Service (Turn Around Don’t Drown). This article is general information. Follow your local or state health department’s instructions for disinfecting and testing your well.