The term "water quality" is used to describe how good a water source is for human use. The idea of water quality would be easy to deal with if all water sources were either "good" or "not good". The real world is more complicated than that. There are all degrees of "goodness" of water. That is because many substances can affect water quality, and their concentrations can range from very low to very high. The quality of water required also depends on its intended use. For example, the quality of water that is meant for drinking ("potable" water) is different from the quality of water that can be used irrigation of fields.