Monitoring, Sampling, and Laboratory Analysis
Monitoring plans
Every utility works from an approved monitoring plan: what to sample, where, how often, container, preservation, holding time, and reporting. Samples must be representative; grab samples show a moment, composites show an average. Chain of custody, unique sample IDs, and prompt delivery keep results defensible.
Field versus lab
Analyze chlorine residual and pH in the field because they change in transit. Turbidity may be continuous or grab. Bacteriological samples need sterile bottles, aseptic technique, and cool storage; do not freeze. Metals samples are acidified; ammonia and nitrate have short holding times. Know the difference between method detection limit, reporting limit, and the regulatory limit.
QA/QC
Calibrate instruments with traceable standards, run blanks, duplicates, matrix spikes, and check standards. Control charts reveal drift. Report only within the calibration range; dilute and reanalyze samples above range.
Traps
The common traps are holding time and preservation errors, collecting a coliform sample from a leaking or dirty tap, and reporting a diluted result without the dilution factor. Another is confusing precision with accuracy. Do not assume a result below the reporting limit is zero.
Worked example
You dilute 10 mL of sample to 100 mL to bring it into range, and the instrument reads 4.0 mg/L. The dilution factor is 100/10 = 10, so the original concentration is 4.0 x 10 = 40 mg/L. Record the dilution on the bench sheet.