I don't know about your state, but in Arizona an official closure of an underground storage tank (UST) system, whether by removal or filling with an inert substance, requires collection and laboratory analysis of soil samples beneath the USTs, dispensers, and product lines. Closure activities, environmental sampling, and reporting has to be done by State certified firms under supervision of state regulators and the local Fire Marshall.

What environmental testing did the county do? I'm thinking more testing may be required to "close" the system and if it was a "leaker" things can get expensive.