The difference between the amount of residual chlorine found by testing at distant points of the distribution system is referred to as what?

The variation in residual chlorine concentration measured at different points in the distribution system is known as chlorine demand. This term reflects the amount of chlorine that is consumed by reactions with organic matter, ammonia, and other substances in the water, reducing the detectable levels of residual chlorine over distances. When residual chlorine levels are tested and show differing values, it indicates the consumption of chlorine in the system due to these factors, hence the term "demand." Understanding chlorine demand is crucial for ensuring that adequate disinfection is maintained throughout the distribution system, as it can impact water quality and safety for consumers. The other terms listed do not accurately describe this phenomenon; instead, they refer to different aspects of chlorine behavior in water systems.