What should chlorine (Cl2) residuals be in the far reaches of a water system?

Chlorine residuals in the far reaches of a water system are critical to ensure that the water remains safe for consumption throughout the entire distribution system. The minimum residual concentration of chlorine is essential for maintaining effective disinfection, particularly in areas that may experience lower circulation and longer water retention times.

The correct answer indicates that a chlorine residual of 0.2 mg/L for free chlorine or 0.5 mg/L for combined chlorine is an acceptable level to ensure proper disinfection and the absence of potentially harmful microorganisms. These levels reflect a balance between ensuring sufficient residual chlorine for microbial control while minimizing any potential adverse effects on water quality or taste that higher levels might cause.

In contrast, other options may specify lower thresholds, which could result in inadequate residual chlorine, leaving portions of the distribution system vulnerable to pathogen regrowth, or higher thresholds, which could contribute to increased taste and odor issues that are commonly associated with chlorine. Therefore, the levels specified in option B are established standards that reflect best practices for maintaining water safety in distribution systems.