I believe it also measures pH. so combined salinity and pH compared to rainwater might enable you to differentiate, but I'm not 100% sure. Send me some lake water and I'll try it I have tried tasting it (not recommended) but I can't really tell. I've drank/inhaled enough toxins in my lifetime that I don't want any more.hmmm - might even work for us lake-dwellers if it's sensitive enough.
I expect that our lake water has more salts of various types that rainwater would ever have.