WOH DBP Sample Siting Plan Guide

Under Stage 2 DBPR (40 CFR Part 141, Subpart V), every community water system and non-transient non-community water system that adds a disinfectant must identify and document approved monitoring locations for Total Trihalomethanes (TTHM) and Haloacetic Acids (HAA5). Stage 2 requires compliance at each individual location using a Locational Running Annual Average (LRAA), not a system-wide average. This means a high-DBP location cannot be offset by lower results elsewhere—each designated monitoring point must independently meet MCLs of 0.080 mg/L for TTHM and 0.060 mg/L for HAA5.

Most systems completed a one-time Initial Distribution System Evaluation (IDSE) to identify high-DBP locations. The IDSE used four pathways: Standard Monitoring (quarterly sampling at candidate sites), System-Specific Study (hydraulic modeling or water quality studies), 40/30 Certification (if all prior results were below thresholds), or Very Small System Waivers. Once the state approved IDSE locations, those sites are fixed and documented in the DBP Sample Siting Plan unless the state approves a change.

Monitoring locations must be at customer cold-water taps in the distribution system—not at entry points, treatment plants, pump stations, or storage tank outlets. Priority locations are those with high water age (far from treatment, at main ends, or in low-demand zones), high disinfectant demand (older pipes, high organic loading), and taps that collectively represent different pressure zones and water quality conditions. Taps downstream of point-of-use treatment devices like activated carbon filters or softeners do not qualify.

To change an approved monitoring location, submit a written request to your state primacy agency with justification—such as water quality data or system changes—explaining why the existing location no longer works and proposing a replacement. Continue sampling at the current location until the state approves the substitution. The written DBP Sample Siting Plan must document system identification and sufficient detail for the state to verify monitoring is occurring at the correct approved locations.