Water in the atmosphere becomes contaminated even before it falls to the earth. The clouds of water droplets absorb gases such as sulfur dioxide. They carry particles of dust and collect air-borne bacteria. When water falls to the earth as rain, it dissolves or absorbs part of many compounds that lie on the earth's surface: pesticides, herbicides, fungicides, animal wastes, fertilizers, etc.

Surface drinking water sources such as lakes and rivers are subject to agricultural and industrial pollution, run-off of mud and decayed vegetation, and animal and human wastes that provide an environment for the growth of algae, bacteria and viruses. Ground water sources such as wells and springs that percolate through the sand, rock and clay formations are more likely to provide lower amounts of suspended matter, color and bacterial contamination.

However, ground water is subject to come into contact with and absorb undesirable inorganic minerals such as iron, manganese, and hydrogen sulfide gas, as well as inorganic chemicals such as lead, sodium, cadmium, chromium, mercury, copper, barium and arsenic. Both surface and ground water surfaces are becoming increasingly contaminated with many contaminants.

Even some of the products used to treat water, such as chlorine, have been linked to many adverse health effects on humans, such as cancer and miscarriages.