For drinking water, any detectable coliform count is considered high. The EPA’s goal for both total coliforms and E. coli in drinking water is zero. Public water systems must keep coliform detections to fewer than 5% of monthly samples, and for private wells, a single positive result means the water is unsatisfactory for drinking. In recreational water like lakes and beaches, the thresholds are higher but still tightly regulated.
Understanding what your specific test result means depends on the type of water being tested, the type of coliform detected, and how the count compares to federal or state limits.
Total Coliforms, Fecal Coliforms, and E. Coli
Coliform bacteria are a broad group of organisms found naturally in soil, vegetation, and the intestines of warm-blooded animals. Not all coliforms are dangerous, but they serve as indicator organisms: their presence suggests that harmful pathogens could also be in the water. Testing distinguishes between three nested categories, each telling a different story about contamination.
Total coliforms are the largest group, including many species that live harmlessly in the environment. Finding total coliforms in a water sample usually points to an environmental source like soil or surface water seeping into the system. It does not necessarily mean fecal contamination, but it does signal a vulnerability in the water supply.
Fecal coliforms are a subset of total coliforms that exist primarily in the intestines and feces of humans and animals. Their presence in water strongly suggests recent fecal contamination, which raises the risk that disease-causing organisms are present.
E. coli is a further subset of fecal coliforms and the most specific indicator. Detecting E. coli in a water sample almost always confirms recent fecal contamination. While most E. coli strains are harmless, six types can cause diarrheal illness, and Shiga toxin-producing strains can trigger hemolytic uremic syndrome, a serious condition that can damage the kidneys.
Drinking Water Limits
The EPA has set the Maximum Contaminant Level Goal for both total coliforms and E. coli at zero in public drinking water. That zero target exists because waterborne disease outbreaks have occurred even at very low coliform levels. In practice, public water systems must keep total coliform detections below 5% of all samples collected each month. If they exceed that threshold, they’re required to conduct a system assessment to find and fix the source of contamination.
The World Health Organization follows the same principle: E. coli (also called thermotolerant coliforms) must not be detectable in any 100 mL sample of drinking water. There is no “acceptable low level” for treated drinking water anywhere in the world.
Private Well Test Results
Private well tests are typically reported as either “absent” (satisfactory) or “present” (unsatisfactory) for total coliforms and E. coli. Some labs also report a colony count, measured in colony-forming units per 100 mL (CFU/100 mL). Any detection of total coliforms means the sample is considered unsatisfactory for drinking. Any detection of E. coli makes it unsatisfactory and signals a more urgent problem.
If your well test comes back positive, you should stop using the water for drinking and cooking until the well is disinfected. Boiling water vigorously for one full minute will make it safe in the short term. The CDC recommends testing your well at least once a year for total coliform bacteria, nitrates, total dissolved solids, and pH. Spring is a good time, since snowmelt and heavy rain increase the risk of contamination.
Recreational Water Limits
For lakes, rivers, and beaches, the standards allow higher counts because you’re swimming in the water rather than drinking it. The EPA’s 2012 recreational water quality criteria use E. coli for freshwater and enterococci for both fresh and marine water.
For freshwater beaches, the geometric mean of E. coli should not exceed 126 CFU per 100 mL over a 30-day period. A single-sample “statistical threshold value” of 410 CFU per 100 mL marks the upper boundary. Beach advisories are typically posted when a single sample hits 235 CFU per 100 mL. Older standards used fecal coliforms with a limit of 200 CFU per 100 mL, and some states still reference that number.
To put these numbers in context: a pristine mountain stream might have fewer than 10 CFU per 100 mL, while urban waterways after a rainstorm can spike into the thousands or tens of thousands.
What Causes High Coliform Counts
Elevated coliform levels almost always trace back to surface contamination reaching the water source. For private wells, the most common culprits are faulty or aging septic systems, cracked well casings, and improperly sealed well caps that allow surface water to seep in. Agricultural runoff carrying animal manure is another frequent source, especially in rural areas after heavy rainfall.
Broader environmental sources include chemical fertilizers and wastewater irrigation, which can alter groundwater chemistry and introduce bacteria. Landfill seepage and industrial discharge also contribute in some regions. Even wildlife activity near a wellhead can introduce coliforms. Seasonal patterns matter: spring thaw and periods of heavy rain push surface bacteria into groundwater more readily, which is why testing in spring is particularly important.
Health Risks of Contaminated Water
Total coliforms alone don’t usually cause illness. They’re a warning sign that the water system has a pathway for contamination. The real health risk comes when fecal coliforms or E. coli are present, because they travel alongside genuinely dangerous pathogens like Giardia, Cryptosporidium, norovirus, and harmful E. coli strains.
Symptoms of waterborne illness from fecal contamination typically include diarrhea (which can be bloody or watery), severe stomach cramps, vomiting, and low fever. Most healthy adults recover within a week, but young children, elderly people, and those with weakened immune systems face higher risks. Shiga toxin-producing E. coli is the most concerning strain, as it can lead to hemolytic uremic syndrome, which requires hospitalization and can cause lasting kidney damage.
How to Fix a Contaminated Well
The standard fix for a well that tests positive for coliforms is shock chlorination, a one-time disinfection that flushes the entire system with a concentrated bleach solution. The process involves mixing household bleach (6% sodium hypochlorite) with about 10 gallons of water and pouring it into the well. The amount of bleach depends on well depth: roughly one quart for a 50-foot well, scaling up to about 3.5 quarts for a 300-foot well.
After pouring the solution in, you let it settle for three hours, then circulate the chlorinated water through every faucet, showerhead, and appliance in the house until you can smell chlorine at each one. The chlorinated water then sits in the system for 12 to 24 hours. After that, you flush it all out through outside faucets first, then inside fixtures, until the chlorine smell is gone. Wait at least one week after flushing before collecting a new water sample and sending it to a state-accredited lab for retesting.
If coliforms come back after shock chlorination, the problem is likely structural: a cracked casing, a failing seal, or a contamination source that needs to be physically addressed. Repeated positive results often call for a well inspection or, in some cases, installing a continuous disinfection system like UV treatment.