Hi Adam,
There isn't a heirarchy of these numbers, although some may be considered authoritative while others might not. The law doesn't require you to use any particular set of screening values, so you have to decide which is most appropriate for the situation you're looking at. If you're carrying out a risk assessment that is for a Part IIA determination, soil screening values would come under the banner of "relevant information" as defined in the statutory guidance. For a planning application, local authorities are likely to expect the same "relevant information" to be used.
You should look at the source data first (tox, phys-chem, exposure assumptions) and decide whether you agree or disagree with its derivation. If the screening values are the result of an exposure model then you should look at the model algorithms and decide if they're appropriate for the situation you're looking at.
I suppose my point is that one number isn't necessarily better or worse than any other except in the eyes of the law.
Regards,
Jeremy
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