Water Quality, Sampling, and Health Protection
What this domain tests
This domain tests whether you can collect a valid sample and interpret the result for health protection. Expect questions on bacterial sampling, nitrate, disinfection, and the difference between a sampling error and a real water-quality problem.
Sampling and health rules
- New wells are commonly tested for total coliform bacteria and nitrate. Coliform absence is the health target; any positive result requires investigation and corrective action.
- The nitrate maximum contaminant level is 10 mg/L as nitrogen. Infants are especially vulnerable to methemoglobinemia, so a result above the MCL calls for alternative water, not just retesting.
- Bacterial samples must come from a clean, cold, treated-or-untreated tap with the aerator removed, after flushing, in a sterile bottle. Do not sample through a hose, filter, softener, or pitcher.
- New or repaired wells are disinfected with a strong chlorine solution, commonly about 100 mg/L, held in contact for 12 to 24 hours, then flushed before sampling. Sampling too soon after chlorination gives a false negative.
Worked example
A sample taken the morning after shock chlorination shows no coliform, but a resample two weeks later is positive. The first result is not proof of a safe well; residual chlorine masked the bacteria. The correct response is to resample after the chlorine has dissipated, inspect the wellhead and grout seal, and disinfect again if the positive result is confirmed. The reasoning: sampling timing changes the answer.
Traps that cost marks
- Sampling immediately after disinfection and accepting the negative result.
- Using a non-sterile bottle or sampling from a filtered tap.
- Treating a nitrate result of 12 mg/L as a minor aesthetic issue.
- Failing to record the well identification and sample date.