Is Coliform in Well Water Dangerous?

Pathogens in your drinking water can cause serious illnesses if they go undetected. That's why it's important to monitor your well water quality and have it tested regularly. Labs typically test for bacteria that fall into one of two broad categories: coliform and fecal coliform bacteria.

The presence of coliform in your well water could indicate a problem.

The presence of either group can mean different things.


Labs usually test for three or four categories of bacteria: total coliform, fecal coliform, or a specific species called E. coli. Total coliform encompasses a wide range of bacteria common in many different environments. Fecal coliform is a subset of total coliform common in the intestines and feces of humans and other mammals. E. coli is a species of fecal coliform abundant in human intestines and feces and therefore common in sewage.

Possible Dangers of Total Coliform

By itself, a high total coliform reading doesn't necessarily spell trouble, because many bacteria in this category are common in soil, so total coliform could be a sign of environmental contamination. If environmental contamination can enter your well water, however, it's possible that other more dangerous contaminants could do so as well, so it's important to find the source of the total coliform in your system. Moreover, since fecal coliform are a subset of total coliform, a positive test for total could possibly indicate sewage contamination.

Possible Dangers of Fecal Coliform and E. Coli

In general, fecal coliform and E. coli are not dangerous in and of themselves. Some strains of E. coli or other fecal coliforms can cause illness, but the vast majority do not. The presence of fecal coliform, however, strongly suggests that your water has been contaminated with effluent or feces, and the presence of E. coli is a sure sign of fecal contamination. Consequently, although these bacteria are not usually dangerous in and of themselves, they indicate your water is contaminated and thus extremely dangerous to drink.

What to Do If Fecal Coliform are Detected

If fecal coliform or E. coli are detected in your well water, do not drink it or use it for cooking. Use bottled water for drinking and cooking instead. Shock chlorination can kill the bacteria in your well, but if the source of the contamination is not found and repaired, the bacteria will return. Contact a local well professional and have your well inspected as soon as possible.