My dear old dad was in the motor trade for several decades and sold petrol as well as the workshop stuff. Over that period we had several underground tanks which corroded out and made holes.
Never, I repeat never, did the tanks "lose" fuel, always they dipped higher and, in fact, we used that as an indicator of a problem. Pretty messy when they are dug up - puts you out of the petrol business for a few days.
Try a bit of logic - what is the SG of fuel, petrol and avtur? Yes, it "floats" on the incoming water and tanks almost invariably corrode out the bottom. Condensation always settles on the bottom, hence the tanks go to holes at the lower extremities.
Therefore I wonder about the provenance of this story about fumes, groundwater contamination at al. I highly doubt it's a tank problem, maybe piping leaks if there is, in fact, a problem at all.