How Basic Can a Rock Well Disinfection Be and Still Work?

The Village of Pandora, Ohio, experienced persistent total coliform positive (E. coli negative) in one of its water supply wells after an August 2007 inundation due to flooding over Northwest Ohio. Backflow and other surface-associated causes were ruled out in a systematic way, with the well continuing to be coliform positive. Repeated flushing to waste did not clear up the problem. After conducting a downhole video inspection, it was decided to attempt well cleaning and disinfection before abandoning the well would be considered.  A rapid form of well cleaning and disinfection was selected to return the well to service as quickly as possible during a very cold January. The well performance was benchmarked by step testing, when additional coliform and water quality testing was performed. Tests showed positive for TC during the first two steps, but negative for the second two. Total sulfide was tested and determined to be over 100 mg/L, a serious challenge to chlorination effectiveness. The well was subsequently brushed and then airlift developed. A dosage for chlorination using sodium hypochlorite, with pH adjusted using dilute acetic acid, was calculated by the village’s consultants, who also instructed the contractor and village to obtain NaOCl that was intended for potable water use and to mix the solution at the surface. Naturally, the water operator returned with off-the-shelf bleach from Walmart and white vinegar and the contractor did not have any mixing equipment. To expedite, the consultants and contractors tried simply pouring in the bleach and vinegar in amounts that met the chlorine demand and volume requirements. After dosage and contact, the well was again airlift developed to a clear state. The pump was reinserted and the well step tested. Performance was improved and the well was TC-free, and remained that way.