Some answers.......
The distance could vary widely and is hugely influenced by the geological and hydrogeological conditions. It could be minimal (eg <5m) where low permeability London Clay or similar strata is present, to several 10's of metres or 100's of metres in highly permeable gravels with a shallow water table. Certainly, 20-75m would not be a major surprise in the 'wrong' ground conditions.
It would depend on soil types. Gravels chalks, sands and you will have a widespread plumes may be in excess of a kilometre. If clay soil it will be localised 10 20 - metres..........
Depends on geology, clay 50m, sand and gravels 100 - 150m perhaps. Worth a window sample hole on the site of concern at the boundary nearest to the station though for water and soil contamination
Of course the holding establishment has little to do with site sensitivity, the type of aquifer, fuels stored, age and volume of tanks etc are. FYI when screening Council assets for transfer purposes we generally use 110m unless there is a history of leaks reported to the Petroleum Officer, however for Part 2A you will have a different priorities based on the receptor.
If you are to inspect one with other buildings in proximity or there is work nearby try to avoid rotary air flush in a fractured or porous aquifer as it can redirect vapours. There was an incident in South West Wales a few years ago when petroleum vapours were forced into someones property, which caused some problems.
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