When rain falls upon the land and flows into streams and reservoirs, it picks up contaminants that are present within the environment. Those contaminants can either be Biological or Chemical. Both types of these contaminants can make the water unsafe (cause disease) or undesirable (objectionable) for human consumption. Water Purification is necessary to remove these contaminants so that your drinking water is both potable (safe) and palatable (pleasing) for you to consume.
The U.S. Environmental Protection Agency regulates the amounts of contaminants that are deemed safe for human consumption. Currently, water quality parameters for drinking water regulates 86 contaminants and set Maximum Contaminant Levels (MCL's) for each individual contaminant.