The correct answer is "forget it ever existed, and destroy all evidence that it was ever there, and deny, deny, deny. Move if necessary."
Washington state law says you have to either pull it out or fill it with solid material, or, if it is small, just clean it out, cap the intake, and leave it in place. Some local jurisdictions, including City of Seattle, don't allow that last option. And of course you have to clean up any leaks that have occurred, which means removal of all the soil that has been contaminated, which in a worst-case scenario means your entire lot down as far as the center of the earth.
Making sure your permits are in order, the work is performed correctly, testing is properly done, and everything is documented the exact right way is complicated. Most people hire a contractor, like Seattle Tank Services (not a recommendation, I just googled them up).
An even better answer would be to travel back in time to when it was being put in and STOP THEM.