Why More Isn’t Always Better When You’re Sanitizing A Water Well

by Pure Water Annie

Pure Water Gazette Technical Writer Pure Water Annie clears the water on the troublesome issue of “shocking” a water well.

When a residential water well is “shocked” with chlorine to rid it of bacterial contaminants,  it is usually assumed that just dumping some bleach down the well will do the job.  This article will show you why quantity matters when it comes to adding chlorine to a well and why it is important to follow one of the many good  instruction sheets on well sanitation or to use a chlorine product especially designed for the task.

When chlorine is added to water, it produces hypochlorous acid (HOCl) and hychlorite ions (OCl-).  Hypochlorous acid is by far the most effective and quickest  chlorine ingredient for sanitizing.  It is 80 times as fast and efficient as OCl-.

What is often not considered is that hypochlorous acid is produced most abundantly at a relatively low pH.  Between pH 5 and 7, chlorine as hypochlorous acid acts mainly as a sanitizer–what you need for killing bacteria. As the pH goes up and the water becomes more alkaline, chlorine begins to act more as an oxidizer (what you need for precipitating iron or manganese).

The problem is that when you add calcium hypochlorite (chlorine pellets) or sodium hypochlorite (liquid bleach), you also raise the pH of the water. As the pH goes up, the chlorine loses its sanitizing power.  At pH 9, the chlorine is mainly an oxidizer and will not kill bacteria efficiently.

In a word, over-chlorinating  raises the pH to the point where chlorine does not kill bacteria.

HOCl predominates between pH levels of about 4 and 7.  After 7 it drops off rapidly.

Although acids such as muriatic acid or sodium bisulfate are sometimes used to keep pH low and thus enhance the sanitizing power of chlorine, for homeowners whose wells are in the normal pH range it makes most sense to simply avoid over-chlorinating by following the dosage and procedures put forth by experts in the field.