If a particular section of the distribution system suddenly has a drop in chlorine residual, what most likely has occurred?

• A. loss of chlorine feed at the plant
• B. closed valve in the area
• C. cross-connection
• D. broken main in the area

Answer: (C)

A sudden, localized drop in chlorine residual means water in that section is being diluted by an extra source that hasn’t been disinfected or is less disinfected. In a properly treated system, chlorine should persist downstream, so a drop confined to one area points to a cross-connection where non-potable or poorly chlorinated water is entering the distribution pipe, often through backflow during pressure changes.

If the plant lost chlorine feed, the entire system would show lower residual, not just one section. A closed valve could isolate a segment and cause stagnation, but the typical sign would be flow or pressure issues rather than a straightforward residual drop from mixing. A broken main would cause a pressure loss and leaks, with broader water quality impacts rather than a localized dilution pattern.

So the most likely scenario explaining a localized chlorine residual drop is a cross-connection allowing dilution from an unchlorinated or lower-chlorine source.