I would be
VERY Hesitant to confront an individual over something as trivial (Your Choice) as a little trash dump (even a bag) on the water/road...Particularly on the Road. That perpetrator might just be low life enough to respond with a GUN. Things are a little different now than they were a few years ago. Just Sayin.
NB
PS: I would just go over and pick up the trash and be on my way..as others have suggested...