To most people not professionally involved in water quality issues, water is either drinkable (technically potable) or contains potentially harmful or toxic substances. However, the vast majority of surface water on the planet is neither potable nor toxic. This remains true even if we eliminate from consideration the more than 97% of the earth's water found in the oceans (sea water). Another general perception of water quality is that of a simple property that tells whether water is polluted or not. In fact, water quality is a very complex subject—in part because water is a complex medium—intrinsically tied to the ecology of the entire planet.
Interest by individuals and volunteer groups in making local water quality observations is high, and an understanding of the basic chemistry of many water quality parameters is an essential first step to making good measurements. Most citizens harbor great concern over the purity of their drinking water, but there is far more to water quality than water treatment for human consumption. In point of fact, purification of drinking water is really a different, although obviously related, subject altogether. Many people in the world live where community water purification is simply not a reality. For these people, water quality, even for drinking purposes, relates directly to the local stream, lake, or groundwater. Thus, at its heart, water quality is about preserving uses. Not only use of water as a consumable product, but all other uses such as wildlife habitat, irrigation, swimming, fishing, rafting, and boating—any or all of which can be adversely impacted by water quality degradation. Of course, industrial uses are also important, and industries are always interested in the quality and quantity of water available to them.
Statements to the effect that "uses must be preserved" are included within water quality regulations for the reason that they provide for broad interpretation of water quality results, while preserving the ultimate goal of the regulations. Technical measures of water quality—that is, the values obtained when making water quality measurements—are always subject to interpretation from multiple perspectives. Is it reasonable to expect a river to be pristine in a landscape that no longer is? If a river has always carried sediment, is it polluted even if the cause is not man induced? Can water quality be maintained when water quantity can not? The questions that arise from consideration of water quality relative to human uses of the water become more complex when consideration must be given to conditions required to sustain aquatic biota. Yet inherent in the concept of preserving uses is a mandate that waterways must be much more than conduits for a fluid we might want to drink or fill our swimming pool with.