Bass:
I�m not an expert / biologist / chemist or lawyer, but it is my understanding that all of the approved stuff he might use to kill the fish would still leave the water safe for humans.
It would therefore be more of a civil issue as to your loss for value of fish, personal use / enjoyment, and real estate value. Frankly, I really honestly doubt that any governmental or private agency would get very involved in this situation. I think this would be fought out in civil court. That said, here are some links.
University of Illinois
http://www.farmdoc.uiuc.edu/legal/index.html University of Illinois Extension
http://www.extension.uiuc.edu/pubs.html Illinois Rural Policy Digest - Recreational Use of Land & Liability Risk (There are two topics on this link�s page that should be of great interest to any property owner who allows others unto their property � especially property owners in Illinois)
http://www.farmdoc.uiuc.edu/policy/ Illinois Department of Agriculture
http://www.agr.state.il.us/Environment/Pesticide/pestuses.html http://www.ag.state.il.us/environment/envdivision.html Illinois Environmental Protection Agency
http://www.epa.state.il.us/ Prairie Rivers Network - A private group affiliated with the National Wildlife Federation, which might or might not wish to get involved. This group is very active with water quality issues (from my perspective, to the point of being way overboard � pun intended).
http://www.prairierivers.org/ If the neighbor used non-approved stuff to kill the fish that could leave the area contaminated, that is another story. He could get in bigger trouble IF IT COULD BE SATISFACTORILY PROVEN THAT HE IS RESPONSIBLE FOR THE CONTAMINATION. That can be a pretty high standard.
Unfortunately and reluctantly, I would suggest some caution to anyone in approaching any �enforcement� type agency (including the prairie rivers network) for assistance. It may be like going to the police complaining that someone stole some of the drugs from your car.
Even though you didn�t cause the contamination, those parties just might want to see it cleaned up. As owner of 4/5th of the contaminated property, it is theoretically possible that you may be held responsible for 4/5th the cleanup cost � which you may or may not recover from your neighbor if successful in civil court. Ludicrous? Yes. Asinine? Yes. Unlikely? Yes. Impossible? No. :-(
Just FWIW (and it may be worth even less than it cost...)
Jeff