Coliform Bacteria in Well Water

What coliform bacteria are

There are three results you'll see on a lab report, and they mean different things.

• Total coliforms are a large group found in soil, surface water and on skin. Most are harmless. Penn State calls them "indicator organisms" because they signal a contamination pathway.
• Fecal coliforms live in the intestines of warm-blooded animals, so they point to sewage or animal waste.
• E. coli is a type of fecal coliform. A positive result means waste is getting into the water. EPA says E. coli and fecal coliforms are directly associated with fresh feces.

A positive total coliform result is a warning. A positive E. coli result is a problem you treat as urgent.

The limits: what EPA allows

For public water systems, EPA sets the health goal (MCLG, maximum contaminant level goal) for E. coli at zero. Under the older Total Coliform Rule, the MCLG for total coliforms was also zero. Penn State puts the practical drinking water standard the same way: no total coliforms, and fecal coliforms and E. coli completely absent.

Those rules apply to public systems, not to private wells. Nobody tests a private well for you. The owner is responsible, and zero is the right target for your well too.

How coliforms get into a well

Bacteria need a source and a path. The sources are surface water, septic systems and animal waste. The paths are usually at the top of the well:

• A cracked or loose well cap, or a cap without a sanitary seal
• A casing that ends below or at ground level, so surface water runs in
• Ground that slopes toward the well instead of away
• Surface water running down the outside of the casing
• Recent work on the well or pump, which can introduce bacteria

Springs and shallow wells are much more likely to be contaminated. Deep wells over 100 feet can still test positive if they're poorly built or surface water runs down the casing. Penn State also notes that bacteria levels rise after weeks of warm, wet weather and drop in cold, dry spells, so a summer test can tell a different story from a winter one.

Is it common? A Penn State survey of 450 private wells found coliforms in about 35% and E. coli in about 15%.

Signs and health effects

You can't see, smell or taste coliform bacteria. Cloudy water or a new odor can come with contamination, but clear water can still test positive.

EPA says microorganisms in drinking water can cause gastrointestinal illness and infections. Penn State lists stomach upset and flu-like symptoms such as fever, cramps and diarrhea, with children and older adults at most risk. Visitors can get sick from water the family drinks without trouble, because they haven't built up immunity. Since those symptoms look like many other illnesses, it's hard to know whether the water is the cause without testing it.

Testing your well

The CDC says to test your well at least once a year for total coliform bacteria, nitrates, total dissolved solids and pH. Test more often if the well floods, if land nearby is disturbed, after any well or pump repair, if the water's taste, color or smell changes, or if someone in the home is pregnant or a child moves in.

Use a state-certified laboratory. Your county or state health department can give you a list, and your local health department may test private wells for free. Penn State says a total coliform test typically costs $10 to $30. Our well water testing guide covers the full panel.

About home test kits: a lab sample with a sterile bottle and the lab's instructions is what counts. Penn State warns that a sample not collected exactly as directed can be contaminated and give a false result.

Reading the results

If total coliforms are present, ask the lab to test for fecal coliforms or E. coli.

Treatment options

1. 1

   Cause 1: Confirm and fix the wellhead

   Retest first with a certified lab to confirm the result. Then fix what lets bacteria in. Penn State's list: extend the casing above ground, slope the ground away from it, install a sanitary well cap, check the septic system and clear animal waste away from the well. See well caps for what a sanitary cap looks like.

2. 2

   Cause 2: Shock chlorinate the well

   Shock chlorination briefly adds a high chlorine dose to disinfect the well and plumbing. Penn State recommends a well professional do it, and recommends it after a positive test, after a new well and after any pump or well work. Check with your treatment company or the equipment manual about bypassing softeners and filters first, since strong chlorine can damage or exhaust some equipment. Our shock chlorination guide has the steps.

   Set your expectations. In a Penn State study, shock chlorination plus a sanitary cap fixed only about 15% of wells with coliforms, and it worked best when counts were under 10 colonies per 100 mL with no E. coli. Retest about 10 to 14 days after shocking, and again two to three months later.

3. 3

   Cause 3: Continuous disinfection

   If bacteria come back, you need treatment that runs all the time. EPA lists chlorine, ozone and UV light as disinfection methods for well water.

   • UV light adds no chemicals and is what we'd pick for most homes. Penn State says it needs clear water and a sediment filter ahead of it, and isn't recommended above 1,000 total coliforms or 100 fecal coliforms per 100 mL. The bulb needs replacing about yearly, and it leaves no disinfectant in the pipes.
   • Continuous chlorination injects chlorine with about 30 minutes of contact time, aiming for 0.3 to 0.5 mg/L residual chlorine. It needs regular upkeep and careful handling.
   • Ozone works, but Penn State says ozonation units are generally not recommended for disinfection, mainly because of cost.

   What about a regular water filter or reverse osmosis? We don't recommend counting on a carbon filter or an under-sink RO unit as your disinfection step. Use one of the methods above, and see well water filters for what filters do well.

   If nothing holds, EPA notes that a new water source or new well may be the answer for some wells.