Laboratory QC Failure on a Permit Parameter
Your monthly discharge monitoring report is due in 3 days. The BOD5 result for the composite is 38 mg/L against a 30 mg/L limit. The lab's QC for that run shows the duplicate pair at 38 and 21 mg/L (relative percent difference about 58%), and the method blank is clean. The seed control is within range. The sample was a 24-hour flow-proportioned composite collected with a refrigerated sampler; the sampler log shows the refrigeration failed for about 4 hours on the collection day. The lab analyst says the 38 is 'probably real' and wants to report it. The permit requires reporting of the analytical result and any known sampling or analytical deficiency.
1. Assess the validity of the 38 mg/L result and what the QC data tell you.
Read the worked answer
The duplicate RPD of about 58% is far outside typical BOD precision (commonly about 20% or better at this level), which signals the result is not defensible as a precise value. The clean method blank and in-range seed control rule out contamination and seed failure, so the problem is likely sample integrity - the 4-hour refrigeration failure allowed biological activity that can either consume oxygen (raising BOD) or, in a sealed sample, deplete DO and distort the result. The analyst's 'probably real' is not a basis for reporting a number that fails its own QC. The correct position: the result is qualified as estimated/invalid pending re-analysis or resampling, and the deficiency must be disclosed.
2. What do you do in the 3 days before the report is due, and how do you report the parameter?
Read the worked answer
First, check whether a valid backup exists - a second composite bottle or a hold sample - and re-analyze if holding time allows; BOD5 holding is limited, so a fresh sample may be the only valid path. If no valid result can be obtained before the deadline, report the result with a clear data qualifier and a narrative describing the refrigeration failure, the duplicate RPD, and the corrective action, rather than silently reporting 38 as a compliant or noncompliant number. Check the permit's provisions for resampling and for reporting known deficiencies; many permits allow a follow-up sample within a set period. Notify the supervisor and, if required, the regulator before the deadline rather than after. The judgement point is that an honest qualified report with disclosure is defensible; a fabricated precise number is not.
3. What corrective actions prevent recurrence, and how do you verify the sampler is fit for use?
Read the worked answer
Corrective actions: add a temperature alarm or data logger to the sampler refrigerator with a daily check on the collection log; add a pre-deployment check of the refrigeration and a post-collection temperature reading recorded on the chain-of-custody; and add a duplicate/QC sample to the routine schedule so precision failures are caught before the report. Verify the sampler by checking the logger history, confirming the cooler holds 4°C over a full cycle, and documenting the check. Also review whether the composite was properly preserved and whether the 4-hour gap exceeded the method's holding requirements - if so, the sample was invalid regardless of the analytical result.