Water Quality and Laboratory Analysis
Sampling is half the exam
Regulatory compliance depends on a sample that is collected, preserved, transported, and analyzed correctly. Know the chain of custody, holding times, container type, preservation (cooling to ≤6 °C, acidification for metals, dechlorination for bacteriological bottles), and required volume. Bacteriological methods include membrane filtration, presence/absence, and enzyme-substrate tests; results are reported as counts or MPN. Chlorine residual is measured by DPD colorimetric or amperometric titration, and the difference between free and total chlorine is combined chlorine. Turbidity, pH, alkalinity, hardness, conductivity, nitrate, lead and copper, and disinfection byproducts (TTHM and HAA5) each have their own sample location, frequency, and reporting basis.
Traps
Site selection is a favorite: lead and copper are first-draw samples at high-risk plumbing, while bacteriological samples come from representative points in the distribution system, not from a dead-end hydrant or a customer's softener. Watch units: 1 mg/L = 1,000 µg/L, and TTHM/HAA5 results are usually µg/L. Total chlorine includes combined chlorine, so free chlorine is always lower or equal. Temperature, pH, and nitrification all change chlorine decay. Quality control is testable: blanks, duplicates, spikes, calibration checks, and method detection limits each answer a different question.
Worked example
Four quarterly TTHM results at one location are 70, 84, 78, and 88 µg/L. The locational running annual average is (70 + 84 + 78 + 88) ÷ 4 = 80 µg/L, right at the 80 µg/L MCL. The system is not compliant by margin - the next quarter must fall below 80 to pull the average down, so operational changes should start now.