Water Pollution

I chose water pollution to write about for our sustainability research paper. I chose the subject water sustainability because I am concerned with Montana's water systems. It may not seem as though they are polluted but not all pollutants are visible. There is a vast amount of pollutants that are human caused. I want to find out which pollutant causes the most harm to our water and if it can be avoided. Also, I want to find out how harmful the pollutants are to our bodies and if we should drink our tap water. I would like to research how Missoula's water contaminants can be "below legal limits, but above health guidelines". http://www.newwest.net/topic/article/toxins_in_missoula_theyre_on_tap/C559/ How is that even legal.