The Surface Water Treatment Rule requires that the turbidity of the water entering the distribution system be equal to or less than ______ in at least ____ of measurements each month.

• A. 0.3 ntu, 90%
• B. 0.5 ntu, 90%
• C. 0.3 ntu, 95%
• D. 0.5 ntu, 95%

Answer: (D)

Turbidity is a measure of how clear the water is and indicates how many suspended particles remain after treatment. The Surface Water Treatment Rule sets a performance standard for the water that enters the distribution system: its turbidity must be 0.5 NTU or less in at least 95% of monthly measurements. This keeps the water consistently clear enough so that disinfectants can work effectively and there’s less chance that particles shield any microorganisms from disinfection. The 95% requirement means most measurements meet the limit, with only a small portion allowed to exceed it in a given month. If turbidity rises above 0.5 NTU too often, operators adjust treatment processes (like coagulation, filtration, or sludge handling) to bring turbidity back down.