I feel for you Sal, But as bad as it is I would consider yourself lucky stream-wise. I believe the majority of the diesel will have stayed above and even what made it ahead of the booms should dilute. 50 gallons is bad but not catastrophic. It sounds like the stream has enough friends to make sure it gets the TLC it'll need to recover. It is a shame about the study. As bad as it is, I'd have taken 50 gal. of diesel over what got dumped into Sinnemahoning-Portage creek last year.
I loved the reply to the comment about the square sticking out too far..."its been there for 120 years...I hope this guy has good liability insurance. He's gonna need it and a lawyer.