Well Water Safety: How to Prevent Contamination and What to Do After a Flood

Key points

• About 15% of Americans get their drinking water from a private well, and the EPA says those wells are not regulated by the federal government under the Safe Drinking Water Act. Testing is the owner's job.

• The CDC says to test at least once a year for total coliform bacteria, nitrate, total dissolved solids, and pH. Add local risks like arsenic, lead, radon, and pesticides based on where you live.

• The EPA limit for nitrate is 10 mg/L. Above that, infants under 6 months old can get seriously ill from blue baby syndrome.

• If your well floods, do not drink the water and do not switch the pump back on until someone qualified inspects the wiring. Boil water at a rolling boil for 1 minute until you have a clean test.

• After shock chlorination, the CDC says wait 7 to 10 days to sample, then test again at 2 to 4 weeks and again at 3 to 4 months.

If a public utility supplies your water, somebody tests it and mails you a report. If you own a private well, nobody does. Well water contamination usually gives no warning at all: no taste, no smell, no color change. You find out by testing, or you find out when somebody gets sick.

The EPA estimates about 15% of Americans, more than 23 million households, drink from a private well. It also cites a US Geological Survey study that found roughly 1 in 5 private wells held at least one contaminant above a human health benchmark.

It is late August, the busy stretch of hurricane and flood season. Floodwater is exactly what turns a fine well into a contaminated one, so this is the right week to get your test schedule straight and learn the flood steps before you need them. It is also the driest stretch of the year in a lot of places, and if the well is drawing down, the fastest way to ease the demand is outdoor watering, since drip irrigation and mulch cut outdoor water use more than any other change you can make.

Why is private well water not regulated by the government?

Because the Safe Drinking Water Act was written for public systems, not for you. The EPA states plainly on its private wells page that the quality and safety of drinking water from private domestic wells are not regulated by the federal government under the Act, and not by most state governments either. That has 3 practical consequences:

• Nobody samples your water. A city system has a mandatory monitoring schedule. Your well has whatever schedule you keep.

• Nobody mails you a notice. Public systems must tell customers when they exceed a limit. Nothing generates that letter for a private well.

• Nobody enforces a standard. The EPA's limits still show what a safe number looks like, but for your well they are a yardstick, not a law.

Penn State Extension puts it bluntly for its own state in its best management practices guide: Pennsylvania has no state regulations requiring private well maintenance or testing. Rules vary state to state, and plenty of states are in the same position. The part most guides skip is that a well drilled to code in 1974 is still legal now, even if the casing has corroded through since.

How often should you test a private well and what should you test for?

At minimum, once a year. The CDC's well testing guidance says to test your well water at least once every year for four things: total coliform bacteria, nitrate, total dissolved solids, and pH.

Those 4 are the baseline. What you add depends on your geology, your neighbors, and your land use. The CDC also lists fecal coliform and E. coli, volatile organic compounds such as benzene and trichloroethylene, lead, arsenic, mercury, radium, and pesticides or herbicides, depending on where the well sits and what state you live in.

The EPA's radionuclides in private wells page recommends testing every 3 years, because uranium, radium, and radon all dissolve and can accumulate in a well. Your local health department knows which of these actually matter in your county, so call them before you buy a panel.

The CDC also lists triggers to test off-schedule: any change in taste, color, or smell, flooding or land disturbance or nearby waste disposal, repairs anywhere in the well system, a pregnancy or a new child in the house, and any notice from local authorities about well water problems nearby. The same logic applies to stored water, which I walked through in how to test water quality in storage tanks.

What do private well water test results actually mean?

Numbers on a lab sheet only help if you know the thresholds. These come from the EPA's National Primary Drinking Water Regulations, the same table public systems are held to.

• Nitrate: 10 mg/L measured as nitrogen. The EPA's wording is that infants below 6 months old who drink water above this level could become seriously ill and, if untreated, may die, with shortness of breath and blue baby syndrome as symptoms. The condition is methemoglobinemia. This is the most important number on the report if there is a baby, a pregnancy, or formula mixing in the house.

• Nitrite: 1 mg/L, with the same infant warning.

• Total coliform: for your well, the answer you want is none detected. Coliform is not itself the threat. It is the flag that says something else may be getting in.

• Arsenic: 0.010 mg/L. Long-term exposure is linked to skin damage, circulatory problems, and increased cancer risk.

• Fluoride: 4.0 mg/L, above which bone pain and mottled teeth in children become the concern. Uranium: 30 micrograms per liter. Copper: 1.3 mg/L action level.

Lead is worth a note, because the EPA states it two different ways. The primary standards table lists lead as a treatment technique with an action level of 0.010 mg/L, while the EPA's Lead and Copper Rule page describes an action level of 15 ppb. Those are not the same figure, so check both pages and talk to your health department rather than trusting one number online. In a private well, lead usually comes from the plumbing and fittings rather than the aquifer.

One honest limit: a bacteria test tells you about the day you filled the bottle, not about next month. And a carbon filter under the sink does not fix nitrate or bacteria, a mismatch I covered in what activated carbon water filters actually remove.

How far should a well be from a septic system or livestock?

Farther than most people assume. NC State Extension's well construction and protection guide calls for at least 100 feet between a well and any septic system or drainfield, with a 50 foot minimum for single family homes on small lots, plus at least 50 feet from watertight sewer lines (25 feet on small lots).

Mississippi State University Extension's wellhead protection publication lists its state's spacing this way:

• 50 feet from a septic tank, aerobic treatment plant, sewage holding tank, animal holding pen with a concrete floor, or non-potable well.

• 100 feet from a subsurface injection field, the edge of surface discharge effluent, pesticide or fertilizer storage, and manure storage.

Setbacks are only half of it. The wellhead itself matters just as much, and this is the part you can fix in an afternoon:

• Casing height. At least 12 inches of casing above the land surface. A casing cut flush with the dirt is a funnel.

• The pad and the grade. A concrete pad extending at least 2 feet out from the casing in all directions, sloped to shed water, with the ground graded to send surface water away from the well.

• A sanitary well cap. Penn State recommends swapping a standard cap for a sanitary one, which keeps insects and small mammals out.

• A chemical free zone. Penn State recommends limiting fertilizer and pesticide use within 100 feet of the well. NC State suggests mixing chemicals several hundred feet away, preferably downslope.

• A real inspection. Penn State recommends a professional inspect the well at least every 10 years. Between visits, check the casing yourself once a year for cracks, holes, or corrosion.

What should you do if your well floods?

Stop using the water, and leave the pump alone until someone qualified has looked at it. Those are the first 2 moves, in that order. The CDC's guidance on disinfecting wells after an emergency is more specific than most state handouts.

1. Do not drink or cook with the water. Assume a flooded well is contaminated until a lab says otherwise. Use bottled water, or boil at a rolling boil for 1 minute, until you have a clean test result.

2. Turn off electricity to the well area before you clear debris. Floods move downed power lines and sharp metal into places you would not expect.

3. Get the wiring inspected before power goes back on. The CDC says to have a qualified electrician or well contractor check it first. Switching a flooded pump back on turns a water problem into an electrical one.

4. Never enter the well pit. Hazardous gas collects in pits. This is not a place to look around in.

5. Flush before you disinfect. Run the outside faucet nearest the well until it runs clear. That keeps the worst of it out of your indoor plumbing.

6. Shock chlorinate, then test twice on a schedule. Both steps are below.

One warning overrides everything above: if the water smells like fuel or chemicals, do not drink it and do not try to fix it yourself. The CDC states that water contaminated with fuel or toxic chemicals will not be made safe by boiling or disinfection. Call your health department. For drinking water while the well is out of service, the methods in how to purify water in an emergency work with no power, and the broader storm plan is in the hurricane preparedness checklist.

How do you shock chlorinate a well correctly?

Shock chlorination means putting a measured dose of unscented household bleach into the well, circulating it through the plumbing, letting it sit, then flushing it out. Here is the CDC's sequence, with the numbers that matter.

1. Bypass the water softener. Close the valves to any softener connected to interior plumbing so you do not push chlorine through the resin.

2. Measure the bleach against your well's depth and casing diameter using the CDC's tables. As one worked example from its guidance, a drilled well with water standing at 20 feet and a 4 inch casing takes about 6 and a half tablespoons of unscented household bleach at 5% to 9% concentration. Do not eyeball the dose.

3. Dilute it first. Mix the measured bleach into 3 to 5 gallons of water in a clean bucket.

4. Pour it in through a funnel, keeping it away from the electrical connections.

5. Circulate. Attach a clean hose to an outside faucet and run water back into the well to mix it, rinsing the inside of the casing for 5 to 10 minutes.

6. Pull it through the house. Open every indoor faucet until you smell a strong chlorine odor at each one, then shut them off.

7. Wait at least 12 hours. The CDC calls for a minimum of 12 hours of contact time in the well and plumbing. Overnight is the easy way to hit it.

8. Flush it out. Drain the chlorinated water onto a driveway or bare ground with no vegetation, not into a septic field or garden, until the odor is gone. Then run the indoor faucets clear.

Wear goggles or a face shield the whole time. This is a chemical job, not a household chore.

Why do you have to retest after shock chlorination?

Because chlorine kills what is in the well right now. It does not seal the crack that let the contamination in. If a bad casing or a failing septic system is the real cause, the bacteria come back, and the only way you find out is by sampling again weeks later. The CDC's schedule after disinfection is the step people skip:

• Wait 7 to 10 days after disinfecting to sample, so residual chlorine does not mask the result.

• Test for total coliform and E. coli or fecal coliform. A safe result means no coliform present.

• Follow up with another test at 2 to 4 weeks, and a third at 3 to 4 months.

• After that, keep testing at least twice a year.

• If bacteria show up again, repeat the whole disinfection process and contact your local health department if it keeps coming back.

A single clean test 8 days after you dumped bleach in the well is the most misleading result in well ownership. The second and third tests tell you whether the problem is fixed or just paused.

When should you call a professional instead of handling it yourself?

Testing and routine shock chlorination are homeowner jobs. This list is not.

• Anything electrical after a flood. Have a qualified electrician or well contractor inspect the wiring before power is restored. Do not skip this to save a service call.

• Fuel or chemical odor in the water. Boiling and disinfection do not fix it. Call your local health department and stop using the well.

• Repeat bacteria hits. Two failed tests after a proper disinfection means a structural problem. That is a licensed well contractor's work.

• Visible casing damage, a cracked pad, a casing below grade, or a well pit. That is contractor work, and a pit is a confined space you should never enter.

• Arsenic, radon, uranium, or persistent nitrate above the limit. Treatment is engineered to your specific water chemistry, not bought off a shelf.

• Any well more than 10 years past its last professional inspection.

On the medical side: if a baby under 6 months has been drinking formula mixed with water from a well that tested above 10 mg/L nitrate, call your pediatrician right away. Blue or gray skin around the lips, hands, or feet plus trouble breathing is an emergency. Call 911. For a suspected chemical exposure, Poison Control is free and staffed 24 hours a day at 1-800-222-1222.

Frequently asked questions

Can you tell if well water is contaminated by looking at it?

No. The contaminants that matter most, including E. coli, nitrate, arsenic, and radon, are invisible, tasteless, and odorless at the levels that cause harm. A change in taste, color, or smell is a reason to test, but clear water is not evidence of safe water.

How much nitrate in well water is dangerous?

The EPA's limit is 10 mg/L measured as nitrogen. Above that, the EPA warns infants under 6 months old could become seriously ill and, if untreated, may die, with blue baby syndrome as a symptom. If your result is over 10 mg/L, stop mixing infant formula with that water and call your health department. Boiling makes nitrate worse, not better, because water evaporates and the nitrate concentrates.

Is it safe to shower with water from a flooded well?

Not until it has been disinfected and tested. The CDC's guidance is to avoid a flooded well for drinking or household use until it is flushed, disinfected, and shown clean by a lab. Shower mist and skin contact are both routes for waterborne germs, and small children swallow bath water.

How long does it take to make a flooded well safe again?

Realistically 2 weeks or more. Disinfection needs at least 12 hours of contact time plus flushing, and the CDC says to wait 7 to 10 days before sampling. Plan on another water source for at least that long.

Does a home water filter make well water safe to drink?

It depends on the contaminant, and most filters do less than people assume. A standard carbon filter targets chlorine, taste, odor, and some organic chemicals. It is not a substitute for disinfection after a flood, and it does not remove nitrate. Match the treatment to the test result, and check the unit is certified for that specific contaminant.

A private well is a good thing to own. It just comes with a job attached, and that job is mostly a calendar reminder and a sample bottle once a year. Do that, keep the wellhead sealed and the ground sloped away, and know the flood steps before you need them.

You never know, but you can always be ready.

Sources: EPA, Private Drinking Water Wells, CDC, Guidelines for Testing Well Water, CDC, How to Disinfect Wells After an Emergency, EPA, National Primary Drinking Water Regulations, EPA, Lead and Copper Rule, EPA, Natural Radionuclides in Private Wells, NC State Extension, Your Water Supply: Well Construction and Protection, Mississippi State University Extension, Wellhead Protection, Penn State Extension, Best Management Practices for Private Water Wells.