Structure was used for heating, water supply to 5 acres of nearby
office and storage buildings. What pollution clean up is mandatoryin
Illinois and what parties must pay for it?

Charity5,

This is actually a near-impossible question to answer without knowing
the specifics, because there are so many different requirements for
different types of pollution problems.  But there are a couple of
general principles to be aware of.  (1) there is generally a
responsibility to fess up -- if you are aware of a pollution problem,
you probably have an obligation to tell this to the prospective buyer
of the property  (2) many laws embrace the "polluter pays" principle
-- he who made the mess pays to clean it up.  (3) some laws also
embrace "strict liability" -- whoever is in possession of the property
has the responsibility for clean up, whether he polluted it or not
(but if you can find the previous owner who *did* pollute it, they can
be held responsible as well.

If you want to tell us more details of your situation, it may be
possible to provide a detailed answer.