(Local) What was dumped into river?

On A-News last night a story was presented about someone videod dumping an unknown substance into the Detroit river. Apparently this happened east of Lauzon on riverside drive. A witness who was capturing the event, approached the guy and asked what he was dumping. No description of what it actually was, was given except that it turned the water black. So could this be oil? And thr question is, what could it do to the evironment and life in the water? All we know for sure is that the Environment agency are investigating the video.