Water Quality, Disinfection, and Sampling
Rules and residuals
Every sample answers a regulatory question, so know which rule drives it. The Revised Total Coliform Rule sets routine and repeat sampling; a total-coliform-positive repeat triggers additional repeats, assessment, and public notice. Chemical rules set maximum contaminant levels: nitrate 10 mg/L as N, arsenic 10 µg/L, lead action level 0.015 mg/L, copper 1.3 mg/L, TTHM 80 µg/L, and HAA5 60 µg/L as running annual averages. Secondary standards (iron, manganese, sulfate, chloride) are aesthetic, not health-based - a favorite trap.
Disinfection is judged by CT: residual concentration × contact time, corrected for temperature and, with free chlorine, for pH. Warmer water or lower pH means a given CT achieves more inactivation, so the required CT drops. Chlorine demand consumes dose before residual appears; only what remains reaches the tap. Measure free and total chlorine by DPD, and remember that bacteriological samples are chilled, held briefly, and dechlorinated when taken from a chlorinated tap.
Sampling traps: sampling at a leaking or swivel tap, running the tap too long or not long enough, missing the 24-hour repeat window, and skipping chain-of-custody on legal or special samples.
Rule of thumb: CT = free chlorine residual (mg/L) × contact time (minutes). A clearwell providing 30 minutes at 1.0 mg/L free chlorine yields CT = 30 mg·min/L before temperature and pH corrections are applied.