I know some guys who got into big trouble in the mid 90's for something similar. They were using a tub grinder to grind various yard waste from grass clippings to entire trees and dumping it into a hole on a local farm. Someone reported them to the state environmental people and they were given 14 days to clean it all up or face huge fines. I don't know the specifics of why it was illegal, but it was definitely illegal.