Partner Blog

Information of use to those who serve private well owners.

FAQ: "Shock Chlorination Didn't Work"


I have shock chlorinated my well several times, but my sample results still come back positive for bacteria. What can I do?


Reoccurring bacteria problems mean there is a source of bacteria somehow connected to your well at the surface. It could be that your well is in a vulnerable geologic setting, meaning the groundwater itself is being contaminated from the surface and getting into your well. The other likelihood is that your well has a breach or was poorly constructed and allows near surface water into your well.  If your well is shallow, then the water coming into your well is from near the surface and more likely to be contaminated if there is a source nearby (usually livestock or septic). If so, this will always be an issue for your well. As an example, in New York there are many areas where a commonly used bedrock aquifer is at or near the surface, and because of this many shallow wells in this aquifer setting are vulnerable to surface contamination. Another vulnerable geologic setting is in karst areas, where you find caves, caverns, and sinkholes, all of which are conduits for surface contamination.

If you have a dug or bored well, they are made to allow water to seep into the well bore from the surrounding area over most of the depth of the well. They are typically more susceptible to surface contamination because of where water is getting into the well. If you have a deeper well, with casing to a considerable depth, it could also be that the well wasn’t properly constructed, maybe it wasn’t grouted properly so water can run down along the outside of the casing, or there are holes in the casing allowing water into the well from near the surface.

There are really only two solutions to this issue, properly construct a well into an aquifer that is not influenced by the surface (so in the NY example, it would have to be a different, deeper aquifer if one exists), or add continuous treatment to treat for the bacteria. The most common treatment is either a continuous chlorination or ultraviolet disinfection. If you are considering either of these alternatives, contact your health department for advice. Both have maintenance needs and you should understand the responsibilities of adding treatment.