It is easy to dispose of waste by dumping it into a river or lake. In large or small amounts, dumped intentionally or accidentally, waste may be carried away by the current but it will never disappear. It will reappear downstream, sometimes in a changed form or just diluted.

Fresh water bodies have a great ability to break down some waste materials, whether chemically or through the routine actions of living organisms. In the latter case, energy from sunlight drives the process of photosynthesis in aquatic plants, producing oxygen that other organisms uses to break down some organic material, such as plant and animal waste. The decomposition produces carbon dioxide, nutrients and other substances needed by plants and animals living in the water. The purification cycle continues when these plants and animals die and bacteria decompose them, providing new generations of organisms with nourishment.

Unfortunately, the quantities of waste discarded into waterways in today's society by far exceed the self-cleaning capacity of most water bodies. The overload that results, called pollution, eventually puts the ecosystem out of balance and is of great environmental concern. Waterways are being polluted by municipal, agricultural and industrial wastes, including many toxic synthetic chemicals that cannot be broken down at all by natural processes. Even in tiny amounts, some of these substances can cause serious harm. Even every day activities like washing, eating, house-cleaning, tending the lawn and garden and driving can cause water pollution as the average person regularly uses hundreds of chemicals. In fact, of the almost 10 million chemicals known today, approximately 100,000 of them are used commercially.

Sources of Pollution

Water contaminants come from two categories of sources: point sources and distributed or non-point sources. Often water pollution is associated with images of point source pollution like oil spills or raw sewage and toxic chemicals spewing from pipes at industrial facilities and sewage treatment plants. Although point source discharges still produce some pollution, most of them are controlled by imposing specific permit conditions. Currently, less visible, non-point sources of pollution are more widespread and introduce vast quantities of pollutants into surface and ground waters. Non-point sources deliver pollutants to water-bodies in a dispersed manner rather than from a discrete pipe or other conveyance. Non-point sources include contaminated sediments and many land activities that generate polluted runoff and leaching, such as agriculture (pesticides/fertilizers), logging, and onsite sewage disposal. Less frequently cited sources include atmospheric deposition, in-place contaminants, and natural sources. Atmospheric deposition refers to contaminants entering waters from polluted air. In-place contaminants were generated by past activities, such as discontinued industrial discharges, logging, or one-time spills. In-place contaminants often reside in sediments but continue to release pollutants back into the water column. Natural sources refer to natural deposits of salts, gypsum, nutrients and metals in soils that leach into surface and ground waters.

Water Contaminant Point Sources

• Onsite septic systems
• Leaky tanks or pipelines containing petroleum products
• Leaks or spills of industrial chemicals at manufacturing facilities
• Underground injection wells containing industrial waste
• Municipal landfills
• Livestock wastes
• Leaky sewer lines
• Chemicals used at wood preservation facilities
• Mill tailings in mining areas
• Fly ash from coal-fired power plants
• Sludge disposal areas at petroleum refineries
• Land spreading of sewage or sewage sludge
• Road salt storage areas
• Wells for disposal of liquid wastes
• Runoff of salt and other chemicals from roads and highways
• Spills related to highway or railway accidents
• Coal tar at old coal gasification sites
• Asphalt production and equipment cleaning sites

Water Contaminant Non-Point Sources (Distributed Resources)

• Fertilizers on agricultural land
• Pesticides on agricultural land and forests
• Contaminants in rain, snow and dry atmospheric fallout