The complex questions of whether it is safe to swim in the river

I am often asked (by parents mostly) whether the water in a river I am sampling in is 'safe'.  This is a good question, but it does not have a simple answer.  Once I was asked 'is it safe' about a minute after bumping in a dead rat, floating placidly down the river (I was in chest high waders).  Well, the answer depends on the relative position compared to the rat, and how long ago the rat came down the river.  Another thing we are discovering is how much metal pollution is in Flemish rivers, and how diverse the metal pollution is.  I can see one would not want to get exposure to the bacteria carried by a dead rat, but what if I said there is a lot of arsenic in the water?  what if I mentioned lead?  

All in all I never feel I can answer the question 'is it safe'.  I can just give one piece of advice: do you best not to drink the water.  If this seems a bit sad, well, it is.