Coliform bacteria in your well: what to do next

What the result is telling you

Coliform bacteria live throughout the environment, and most kinds are harmless. A lab tests for them because they are easy to find. EPA calls total coliform a useful indicator of other germs in drinking water. Penn State Extension puts the meaning plainly: their presence shows "a contamination pathway exists" between a source of bacteria and your water.

The result comes in up to three parts:

On the lab report	What it means
Total coliform	Something from the surface is reaching the water. Not proof of illness, but a sign that germs could get in the same way.
Fecal coliform	Bacteria from the intestines of warm-blooded animals, people included.
E. coli	A fecal coliform, and the most serious result. Penn State says it means human or animal waste is entering the water. EPA's health goal for E. coli is zero.

If your report shows only total coliform, ask the lab for an E. coli test too, as Penn State recommends.

How the number is written. Many labs report "present" or "absent." Penn State notes that "present" only means at least one coliform in 100 mL of water. Others count colonies per 100 mL, which shows how bad the problem is. "TNTC" (too numerous to count, generally over 200) or "confluent" means the lab couldn't get a clean count, and Penn State says to send another sample. Whatever the count, any coliform at all fails the standard for drinking water.

It's common. In a Penn State survey of 450 Pennsylvania wells, about 35 percent had coliform bacteria and about 15 percent had E. coli.

What to do today

1. Stop drinking it unboiled. Minnesota's health department says to stop using the water for drinking and preparing food unless you boil it at a full rolling boil for a full minute, or use bottled water. CDC adds 3 minutes above 6,500 feet. Boiling kills germs but doesn't remove chemicals.
2. Know who's most at risk. Minnesota's health department says the germs these results point to can cause diarrhea, vomiting, cramps and fever. Infants, children, older people and anyone with a weakened immune system are most at risk.
3. Retest to confirm. Penn State suggests a second sample to confirm the first, especially if the lab used a present-or-absent test. Ask for a count this time.

Penn State notes that counts run highest after several weeks of rain and lowest in a dry, cold winter. In one of its studies, fewer than half of 38 wells that tested positive still had coliform when retested a year later in dry weather. A single positive after a wet spell can be a one-time event, which is why the retest matters.

Find how it's getting in

Penn State and Minnesota's health department point to the same places to look:

• The well cap. A loose cap lets in insects and runoff. Penn State suggests switching to a sanitary well cap. Minnesota's list includes missing bolts, damaged vent screens and loose gaskets.
• The casing. It should stand above the ground, with the ground sloped away from it, and have no cracks.
• Recent work or weather. A new pump, well repairs, unusually heavy rain, or a flood within 50 feet of the well can all bring bacteria in.
• Waste nearby. If E. coli turned up, Penn State says to check that the septic system is working and to move animal waste away from the well.
• Springs and shallow wells. Penn State says they are much more prone to coliform than wells deeper than 100 feet, though deep wells can be contaminated too.

Fix what you find before disinfecting, or the bacteria will come straight back. Minnesota's health department says to call a licensed well contractor about any damage.

Disinfect the well: shock chlorination

Shock chlorination puts a strong dose of chlorine through the well and every pipe in the house, then flushes it out. Penn State says that done properly, it "will kill all the bacteria existing in a well." It doesn't stop new bacteria from getting in, and it does nothing for nitrate or other chemicals.

Minnesota's health department publishes step-by-step instructions for owners, and says you can follow them or hire a licensed well contractor. Penn State recommends a well professional, and the work involves the pump's wiring and strong chlorine. If you do it yourself, follow your state's instructions in full. The outline:

• Bleach. Minnesota specifies unopened, unscented household bleach, 6 or 8.25 percent sodium hypochlorite, made within the last six months. Never pool or hot tub chlorine: Minnesota says they contain algicides and fungicides, and Pennsylvania warns against stabilized pool products.
• Bypass the equipment. Put softeners and filters on bypass and remove filter cartridges. CDC warns that the chlorine can damage a softener.
• How much. Minnesota aims for 50 to 200 parts per million, and no stronger: above 200 ppm, chlorine disinfects less well. Its amounts, for a well and the plumbing in an average home:

Feet of water in the well	2-inch casing	4-inch casing	6-inch casing
10	2 cups	2 cups	2 cups
50	2 cups	2 cups	3 cups
100	2 cups	3 cups	4 cups
300	3 cups	4 cups	10 cups

"Feet of water" is the well's depth less the depth to the water. If you don't know it, Minnesota says to use the total depth.

• Mix it through. Pour the diluted bleach into the well, then recirculate: run water from an outside faucet through a garden hose back into the well. Minnesota says to keep it going about 30 minutes after the hose first smells of chlorine. Then run every faucet, hot and cold, until each one smells of chlorine.
• Let it sit. Minnesota says at least two hours and preferably overnight; Pennsylvania says at least four, preferably overnight; CDC's instructions for wells after an emergency say at least 12. Overnight meets all three. Nobody uses the water meanwhile.
• Flush it out. Run the water from an outside faucet onto gravel or a driveway, never into the septic system, onto plants, or near a stream or pond. Chlorine kills fish at very low levels. Minnesota says it can take 30 minutes to a day or more, and to flush the water heater too. Run the indoor taps until no chlorine remains.

Retest, twice

Test only once the chlorine is gone, since chlorine in the sample would kill the bacteria the lab is looking for. The agencies differ on timing:

Who	First retest	Then
CDC	At least 7 to 10 days after disinfection	Again in 2 to 4 weeks, and in 3 to 4 months
Penn State Extension	10 to 14 days after	Again 2 to 3 months later
Minnesota Department of Health	Before drinking the water again	Again 2 to 4 weeks after disinfection

Minnesota says you can use the water without boiling once the well has been disinfected and no longer tests positive. If coliform turns up again, both Minnesota and CDC say to repeat the disinfection. Minnesota calls it "not unusual" to disinfect a system several times.

If it keeps coming back

Shock chlorination often doesn't last. In a Penn State study, shock chlorination with a sanitary well cap kept bacteria out for a year in only 15 percent of the wells. It worked best where counts were low, under 10 colonies per 100 mL, and there was no E. coli. Springs are harder still: Penn State says water runs through a spring box too fast for the chlorine to work.

When bacteria return, Nebraska Extension says the well needs continuous disinfection rather than more shock chlorination. Pennsylvania's environmental agency says to re-examine how the well is built and where it sits. EPA notes that some owners end up needing a new well.

The continuous options, per Penn State:

• UV light. Kills bacteria inside the unit as water flows through. It needs clear water, usually with a sediment filter ahead of it, and leaves no disinfectant in the pipes after it. Penn State doesn't recommend UV above 1,000 total coliform or 100 fecal coliform colonies per 100 mL. The bulb is replaced about once a year, and the electricity runs $2 to $4 a month. Look for a listing under NSF/ANSI 55 that says Class A, the class for water that may carry germs. The VIQUA PRO10 is one, and whole-house water treatment for well water explains where UV goes among sediment, iron and softening equipment.
• Continuous chlorination. A feed pump keeps about 0.3 to 0.5 mg/L of chlorine in the water, with about 30 minutes of contact time, more than a standard pressure tank gives. Penn State says it takes significant maintenance, and a stronger dose may need a carbon filter after it to take out the chlorine taste.
• Ozone and iodine. Penn State doesn't recommend either for a household: ozone costs much more than chlorine or UV, and iodine is now considered short-term only.

Test again after any treatment goes in, and keep testing at least once a year. Bacteria tests at public labs run about $10 to $41 (what a water test costs), and TapBuff lists the labs each state certifies.

On city water instead

A public water system tests for coliform under EPA's rules, and E. coli calls for EPA's most urgent kind of notice, due within 24 hours. That notice comes with its own instructions, and what a notice from your utility means explains each kind. To see your system's record, look it up: