Proving It Clean, on Site
Most of a decommissioning programme is not finding contamination. It is showing that the ground you are standing on does not have any — sample after sample, and paying a laboratory to say so.
EXECUTIVE SUMMARY
On this programme the question was not where the contamination was. It was whether the ground was clean enough to stop. 54 samples were screened on site for the three hydrocarbon fractions, and 21 of them were also sent to an accredited laboratory. All 21 came back meeting criteria — and the screen had already called all 21 of them clean. Nothing was passed as clean that the laboratory later found over a limit. Four other samples the screen flagged were never submitted, so this case establishes clearance, not detection: there is no confirmed exceedance in the checked set to test the flagging side against.
AT A GLANCE
54 samples screened on site • 21 checked against an accredited laboratory
1.THE CHALLENGE
Hydrocarbon limits are written against what an accredited laboratory recovers by solvent extraction. A vapour meter measures something else — what evaporates into the air above the sample — and the two drift further apart the heavier the hydrocarbon gets. On diesel and lubricating-oil impacts, most of the mass never reaches the air at all. On a decommissioning programme that gap carries a second cost: proving ground is clean takes a great many samples, and a field number nobody trusts means paying for every one of them twice.
2.THE APPROACH
Petro runs the same kind of solvent extraction in the field and reports F1, F2 and F3 separately, each against its own criterion, in the same units the laboratory uses. There is no correlation to build first and no proxy to convert. The crew screened 54 samples on site and used those results to decide which 21 were worth sending for accredited analysis.
3. THE DATA
Every one of the 21 checked samples sat well inside its limits on all three fractions, and the laboratory agreed. The highest screening result anywhere in the programme was 616 mg/kg of F3 against a 1,300 mg/kg shallow criterion — less than half the limit. 6 of the 54 samples came from below three metres and were scored against the higher deep criteria.
WHERE THIS RESULT APPLIES
- Decommissioning programmes that have to prove ground is
- clean
- Deciding which samples are worth a laboratory analysis
- Sites where the criterion is written against extractables, not
- volatiles
- Endpoint confirmation while the crew is still mobilised
4.THE VERIFICATION
The screen decided which samples were worth paying for. 54 locations were characterised on site, 21 went to the laboratory to carry the regulatory record, and the four reading over a limit were separated out rather than averaged in. Clearance was called while the crew was still mobilised.
5.THE DECISION
The screen decided which samples were worth paying for. 54 locations were characterised on site, 21 went to the laboratory to carry the regulatory record, and the four reading over a limit were separated out rather than averaged in. Clearance was called while the crew was still mobilised.
6.THE VALUE
Coverage is the value, not a headline accuracy figure: 54 samples' worth of information for 21 analyses, every clearance confirmed, each fraction resolved against its own criterion. What it does not buy is a claim about finding contamination — that needs a site with some.
BASIS & LIMITS — WHAT THIS CASE DOES AND DOES NOT ESTABLISH
- No sample in the checked set exceeded any criterion, and the four the screen read over a limit were not submitted. This case establishes clearance
- only: the flagging side is unverified and no effectiveness rating is claimed.
- Screening values read consistently higher than the laboratory on all three fractions. With 13 of 21 F1 and 10 of 21 F2 laboratory results below detection,
- no numeric-agreement conclusion is drawn.
- TRIUM was on site for initial training only; sample locations and laboratory submissions were selected by others. The extraction chemistry is proprietary,
- and field screening supports rather than replaces accredited laboratory analysis.
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