Water Quality Monitoring, Sampling, and Laboratory Analysis
Monitoring questions test which rule drives each sample, how the sample is collected and preserved, and how results are compared with a limit.
Rule-driven monitoring
- Revised Total Coliform Rule: routine samples in 100-mL bottles containing sodium thiosulfate, held no more than 30 hours at 10 °C or cooler, with chlorine residual measured and recorded at collection. Positives trigger repeat samples and, where required, a Level 1 or Level 2 assessment.
- Lead and Copper Rule: first-draw 1-liter samples after at least six hours without use at high-risk sites; action levels are 0.015 mg/L lead and 1.3 mg/L copper, judged at the 90th percentile.
- Chemical monitoring: nitrate 10 mg/L as N, nitrite 1 mg/L, arsenic 10 µg/L, plus SOC, VOC, radionuclide, and DBP schedules.
- Laboratory support: calibration, blanks, duplicates, matrix spikes, chain of custody, and a certified laboratory.
Worked example
Twenty lead samples are sorted lowest to highest. The 90th percentile is the value in position 0.9 × 20 = 18, counting from the lowest. If that value is 0.018 mg/L, it exceeds the 0.015 mg/L action level - and an exceedance is confirmed only when more than 10 percent of sites are above the level.
Traps
Exceeding the coliform holding time or shipping warm samples. Pre-flushing a lead and copper sample. Recording chlorine residual hours after collection rather than at the tap. Comparing a single DBP result with the MCL instead of the running annual average.