So after soil is remediated, if necessary, it just sits there exposed to the elements for a while in the hopes any lingering contamination dissipates?
Its not just the soil effected, it often runs into the groundwater as well. Usually they identify the worst hit spot, excavate what they can and then monitor the soil and groundwater until clean to government standards. The drill rigs you see are usually the environmental company, contracted out to that property, taking soil samples and/or installing new places to monitor the groundwater.
Often times these monitoring wells are found off property as well because, as someone mentioned, the contamination can spread. I have seen certain gas companies buy out surrounding properties and houses and kick the tenants out because it is just too contaminated.
Treatment is often just waiting for everything to air out and break down on its on, or they have some machines they can install to try and speed up the process. But I have seen properties be empty for almost 20 years and counting because the contamination is so complicated.
My moral of the story, I wouldn't buy property near an old gas station, auto body shop or laundromat.