Sampling Site Dispute After a Positive Coliform
A utility serving 3,200 connections collects its routine bacteriological samples on a fixed schedule. One sample from a residential tap returned positive for total coliform and negative for E. coli. The sample was collected by a new operator who ran the tap for 30 seconds, used a bottle with a preserved label, and left the sample in a vehicle for six hours before delivery to the lab. The lab reports the result as valid. The state requires repeat samples within 24 hours at the original site and at upstream and downstream locations. The homeowner reports recent plumbing work. The utility's sampling plan lists that address as a routine site.
1. Evaluate the validity of the positive result and what the operator should do first.
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Several factors could invalidate the sample: a 30-second flush may be insufficient for a residential tap, especially after plumbing work, and a six-hour hold in a vehicle may exceed holding time and temperature requirements. However, the operator cannot unilaterally discard a reported positive. The first step is to review the sampling record against the plan and the lab's requirements, document the discrepancies, and contact the lab and the state about whether the result stands. Regardless, the operator should proceed with the required repeat sampling within 24 hours, because the compliance clock does not pause for a validity review.
2. Describe the repeat sampling strategy and how to interpret possible outcomes.
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Collect repeats at the original site and at upstream and downstream locations within 24 hours, using proper flush time, aseptic technique, and prompt delivery with temperature control. If all repeats are negative, the utility may be able to close out the event depending on state rules, but the original site should be investigated for a plumbing or sampling cause. If any repeat is positive, additional repeats and possibly a triggered assessment are required. The homeowner's recent plumbing work makes a plumbing-related cause plausible, but that does not excuse the utility from the repeat protocol.
3. What corrective actions should follow once the event is resolved?
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The utility should retrain the new operator on flush time, bottle handling, hold time, and temperature control, and audit recent samples for similar issues. The sampling plan should be reviewed to confirm site suitability and to ensure the site is representative. If plumbing was the cause, the utility should document it and consider whether the site should remain a routine location. All records, correspondence with the lab and state, and corrective actions should be retained.