Disinfection Byproduct (DBP) Sampling Guide

Disinfection byproducts form when chlorine or chloramines react with organic matter and bromide in source water. The two regulated DBP groups are Total Trihalomethanes (TTHM, MCL 80 μg/L) and Haloacetic Acids Five (HAA5, MCL 60 μg/L). Under Stage 2 DBPR, each monitoring location must meet these limits independently using locational running annual averages rather than system-wide averages, meaning a single high-DBP location cannot be masked by low results elsewhere.

All community water systems and non-transient non-community systems that add chlorine or chloramines must monitor. Your state primacy agency determines your specific monitoring schedule, frequency, and required sampling locations. Stage 2 requires at least one location per 500 service connections. Most systems sample quarterly, though groundwater systems under 500 connections may sample annually. Samples must be collected during the designated calendar quarter; samples outside that period do not count toward compliance.

TTHM and HAA5 samples require separate pre-prepared bottles from a certified laboratory — never substitute bottles or rinse them. TTHM bottles are 40 mL glass VOA vials with PTFE-lined caps, preserved with sodium thiosulfate or ascorbic acid, and must be filled completely with no air space to prevent volatile loss. HAA5 samples are preserved with ammonium chloride or ascorbic acid to quench the chlorine residual and stop further DBP formation. Keep bottles at ambient temperature before sampling; refrigerate only after collection to avoid condensation that dilutes the preservative.

Sample from the same fixed tap locations in your distribution system, typically high water-age areas (far from the treatment plant or pump station) or zones with high disinfectant demand, as identified in your approved monitoring plan. Do not sample at entry points or from dead-end flushing points. Deviating from approved tap locations without state approval may invalidate the sample.