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ITS on FOUNDATION CONTAMINATED BY PRODUCT - CRUDE OILBy: ITS 
Date: Friday, January 13, 2017
Time: 2:51:45 PM
Well to answer your first question, I think you are already inspecting the foundation concrete annular ring. Your crude oil, which I presume is thick and out of the ground thick, will find the path of least resistance which is seeping out from under the steel bottom because of the floor lap plates. As the crude oil cools from its pour point temperature then it should thicken nicely. What has entered the ground will plug the soil or as we say - the soil sizing factor will determine the amount of soaking depth until is cools, then it will go no where.
If the concrete is in reasonable condition the crude oil will not effect the strength of the concrete ring follows what I consider a reasonably good design. You may end up core drilling the concrete ring to draw seepage if any and possibly place a slight vacuum on the soil, but this needs a professional opinion.
When repairing the floor of the AST, you may remove the plate or plates and access the soil, but I think you will find that very little saturation in the soil to any appreciable depth will be evident. Shovel it out. Sure you can get a hydrocarbon sniffer but that may not be practical or an application of good common sense until the entire tank farm is remediated if ever.
I am not sure my help represents you site conditions, but maybe someone else can assist too.