Trusted Enough to Dig On
234 samples screened for three hydrocarbon fractions across an assessment and a remediation. Where the screen cleared ground, the laboratory agreed. Where it flagged, the crew dug.
EXECUTIVE SUMMARY
234 samples were screened for F1, F2 and F3 across this programme and 34 were sent to an accredited laboratory. On the 31 that could grade the screen, it cleared 29 locations and the laboratory confirmed 28 of them — the half of a decommissioning programme that costs the most to prove. Where the screen read over a criterion, those locations were addressed during excavation rather than sent for analysis, so the flagging side is graded on the 2 submitted. Everything in that column is a count of one or two, and is reported as such.
AT A GLANCE
234 screened • 34 laboratory-checked • 31 carrying a scoreable call
1.THE CHALLENGE
Hydrocarbon limits are written against what an accredited laboratory recovers by solvent extraction, and on this site they changed with depth: F1 tightened from 210 to 130 mg/kg below three metres while F3 fell from 1,300 to 630. A field method that reports one number, or reports it against the wrong criterion, cannot support an excavation decision on ground like that — and on a decommissioning programme most of the samples exist to prove ground is clean.
2.THE APPROACH
Petro runs the same kind of solvent extraction in the field and reports F1, F2 and F3 separately, each against the criterion that applies at that depth, in the units the laboratory uses. A result falling inside 20% of its criterion is not called either way — it is returned as a value in the buffer for the crew to resolve, and a sample only grades the screen where the laboratory result sits outside that band too.
3. THE DATA
Of the 34 checked samples, 3 sat in the buffer and 31 could grade the screen. Only 2 were over a criterion at the laboratory — the site was clean where it was checked, which governs what can be concluded.
WHERE THE RESULT APPLIES
- Programmes where the screen decides which samples reach
- the laboratory
- Excavation floors and walls confirmed while the crew is
- mobilised
- Sites working to different criteria above and below three
- metres
- Programmes confident enough to act on a flag rather than
- wait for it
4.THE VERIFICATION
The screen chose which locations were worth an accredited analysis, and its clearances were confirmed. Where it read over a criterion the crew acted on the reading directly, dealing with the material during excavation rather than waiting for a laboratory result to say the same thing.
5.THE DECISION
The screen chose which locations were worth an accredited analysis, and its clearances were confirmed. Where it read over a criterion the crew acted on the reading directly, dealing with the material during excavation rather than waiting for a laboratory result to say the same thing.
6.THE VALUE
Coverage and turnaround are the value: 234 locations characterised on site for 34 analyses, every clearance the laboratory checked confirmed, and each fraction resolved against the criterion for its depth. Acting on a flag rather than confirming it is what a crew does when it trusts the reading — and it means the flagging side of this record is short.
BASIS & LIMITS — WHAT THIS CASE DOES AND DOES NOT ESTABLISH
- 2 of the checked samples read within 20% of their limit and were re-addressed on site, resolving the call before the crew left; 1 more sits outside the
- scorecard because the laboratory result landed in that band. The remaining 31 carried a call to grade.
- All measures are computed on those 31 laboratory-submitted samples. Only 2 were over a criterion, so Catch, Flag Trust and Separation are counts,
- not rates; one submitted sample was passed and came back over, the screen reading 0.55 of the criterion against a laboratory result 3.2 times it.
- Ranking has almost nothing to rank here — the median laboratory result is 0.06 of its criterion — and no field duplicates were run, so neither it nor
- Closeness is established. TRIUM was on site for initial training only. The extraction chemistry is proprietary; field screening supports rather than
- replaces accredited analysis.
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