Some areas now have hazardous household waste disposal services at the
county or state level. Here in NJ, Mon cty has just such a facility
and they take antifreeze, solvents, oil, paint, herbicides, etc. I'd
look in the county govt listings and call the closest agency, maybe
health dept, if nothing else.

If you can figure out what it is, it may not be harmful. There are
antifreezes for winterizing water systems that are non-toxic and I
would think there is a good chance that is the type that would have
been used, not car antifreeze. But unless you are sure, better to
dispose of it properly.

You could also try to take it to a car service type place, but they may
be unwilling to take some unknown substance.