How do I treat a dug well contaminated with coliform?

Water

How do I treat a dug well contaminated with coliform? —Jake Svanda, via e-mail

Unfortunately, coliform contamination can indicate an ongoing problem with your well. Treating it will disinfect your water for now, but it will probably get contaminated again. Even a little bacteria (a “safe” level of five total coliform per 100 ml) could mean that your system isn’t entirely secure, warns Andrew Barton, the manager of the Safe Water Program for the Grey Bruce Health Unit. “It’s like the Check Engine light in your car. Something’s not quite right.”

So inspect the well for defects—damage to the well cap, a crack in the casing —then call your local health unit for advice. Some problems you can fix yourself; others require an expert.

But let’s assume the contamination is a one-time thing (maybe some decayed vegetation got into the water). To disinfect the water, Theresa Warren, a public health inspector with the County of Lambton Community Health Services Department, recommends the shock chlorination method. This sounds complicated and a little violent, but it simply involves putting a lot of chlorine directly into your well water. (Unscented household bleach works fine. The amount depends on the well’s dimensions, regardless of the type of well—ask your public health inspector.) Once you’ve added the bleach, run the taps inside until you smell chlorine. The well is shocked! Turn off the taps and leave the system alone for 12 hours so the chlorine has enough time to kill the bacteria.

To remove the bleach, pump the well water out through a hose attached to a tap (inside or out, but away from the septic system) until you can’t smell the chlorine anymore.
Wait three or four days, and test your water. To be safe, Warren recommends you then test it twice more. If you get three clean samples over three weeks, you and your well can go back to your previous healthy relationship.