Water quality is a term used to describe the chemical, physical, and biological characteristics of water, usually in respect to its suitability for a particular purpose. Although scientific measurements are used to define a water's quality, it's not a simple thing to say that "this water is good," or "this water is bad." After all, water that is perfectly good to wash a car with may not be good enough to serve as drinking water. To ensure your health, the U.S. Environmental Protection Agency (EPA) requires water suppliers to put annual drinking water quality reports into the hands of their customers.