Gasoline contains many toxins, one of the hardest to clean up that is used here is MTBE. It is many lakes, ponds and wells here. It was intended to help fuel burn better and limit smog. If I come across a gas spill I put speedi-dry on it. It's dried chips of clay that absord it and then we walk away or the town pays a company to get rid of the clay bits after they absord the gas. If its a huge spill I get a highway truck full of sand. Sand and dirt drink it in and then an environmental company comes and hauls away the sand. The state or town pay the environmental company or the person who spilled it does.
In your case, the dirt has already drank in the toxins and now its headed for your ground water/well. If on a well you need it tested now. Everytime your chickens scratch they may be getting another dose in the lungs. Calling the officials or a private environmental company is the legal way to go but, could get expensive. Even though you didnt do it, you may get billed.
If you didn't follow the rules and report it you could get in big trouble for illegal dumping etc... however that said.... someone might just have a big teenager in the family dig up the area and place the dirt in doubled up plastic trash bags. What you do with the trash bags then is the big question
Here is a brief explanation of how deadly gasoline:
http://www.envtox.ucdavis.edu/cehs/TOXINS/gasoline.htm
"There are several known toxins in gasoline, some of which are confirmed human carcinogens[aka cancer causing]. The most famous of these toxins are lead and benzene, and both are regulated. Lead alkyls also require ethylene dibromide and/or ethylene dichloride scavengers to be added to the gasoline, both of which are suspected human carcinogens. In 1993 an International Symposium on the Health Effects of Gasoline was held, and major review papers on the carcinogenic, neurotoxic, reproductive and developmental toxicity of gasoline, additives, and oxygenates were presented. Oxygenates, added to gasoline for the purpose of reducing carbon monoxide emissions, are also being evaluated to see if they are cancer-causing agents. Engines without catalysts will produce increased levels of toxic emissions such as formaldehyde and acrolein when using oxygenated fuels, and increased levels of toxic benzene when using highly aromatic fuels. "