Some sources of drinking water are of such high quality that not much treatment is needed. Usually adding small amounts of chlorine is sufficient to kill any harmful bacteria or other microorganisms. Other water sources, especially large rivers, have higher levels of pollution. Such sources require more to bring the water up to the needed level of quality. River water usually contains fine sediment particles in suspension. The water can be passed through filtration materials, like sand, to remove the fine sediment. Filtering the water also tends to remove bacteria.

Most people in the United States get their water from municipal (city and town) water systems. Most people in rural areas, and some also in suburbs, get their water from their own wells, which tap shallow or deep groundwater. The water that is supplied from municipal water systems comes mainly from three sources: streams and rivers; natural lakes or artificial reservoirs; and ground water, pumped from large wells. Lakes and reservoirs that are located in unpopulated areas far from cities and towns usually have the highest-quality water.

Naturally occurring substances also affect water quality. Even raindrops are not pure water. As they fall, they pick up tiny dust particles and also harmful substances like acid that are in the atmosphere. When the rainwater comes in contact with soil and rock material, some of that material dissolves in the water. Substances that are dissolved in water are called solutes. The concentration of natural solutes depends mainly on two factors: the composition of the soil and rock material, and how long the water is in contact with that material.

Microorganisms that cause illnesses also affect water quality. Bacteria are single-celled organisms that cannot be seen with the human eye except through a microscope. Some kinds of bacteria are the most dangerous microorganisms. They usually get into water supplies when untreated sewage mixes with the water supply. It is not only a matter of human wastes from leaky sewer pipes. Dog and cat droppings are also deposited on land inside and outside the city.

Some of the substances that affect water quality are called pollutants. Pollutants are mostly substances that get into water by human activities. The number of toxic chemicals that are produced and used by humans is enormous. Many of these toxic chemicals are used in ways that cause them to be added to surface water or ground water. Here are two major examples:

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.

You probably take water for granted because it is so common, but water is a very unusual substance. Its most spectacular property is that ice floats in water. You probably think that's no big deal, but water is almost the only substance in the universe for which the solid floats in the liquid! Water is very unusual in several other ways as well. For example, the heat capacity of water is higher than just about any other substance. The heat capacity of a substance is the amount of heat you need to add to a mass of material to raise its temperature by a given amount.

Most of the materials beneath the Earth's surface are porous. That means that they contain tiny open spaces as well as solids, just like a sponge. The porosity of a material is the percentage of open pore space it contains. Loosely packed sand and gravel can have porosities as high as 25 percent. Solid rock is much less porous. Many rocks have a porosity of only a small fraction of a percent.

Water exists at the Earth's surface as liquid, solid, and vapor. It is forever changing from one of those three states to another. You can easily observe how water changes from liquid to solid by freezing and from solid to liquid by melting. Water also changes from liquid to vapor by evaporation and from vapor to liquid by condensation. Condensation is how clouds and raindrops form. Water can even change from vapor directly into solid; that is how snowflakes are formed, high in the atmosphere.

Many substances that are hazardous to human health can enter water supplies. Chemical waste from factories is sometimes dumped into rivers and lakes, or directly into the ground. Pesticides (chemicals that kill insects) applied to farmland enter surface water and groundwater, often in large quantities. Leaks from underground storage tanks for liquids like gasoline go directly into groundwater. Salt put on icy roads in winter pollutes water also, although it is not as hazardous to health.