Water quality can be degraded by pollutants entering from a point discharge, such as a factory, or a nonpoint discharge, such as from farmland. Regardless of source, the water pollutants can be placed into three categories. Each pollutant category is important to understand because options for treatment or prevention vary depending on the source and the type of pollutant.
Biological pollutants such as bacteria, viruses and parasites can be treated chemically or by biological means. Other living organisms may be introduced or may occur naturally that will eliminate harmful infestations by competition in the food web or by depleting nutrients vital for the survival of the harmful life, such as causing an anoxic (oxygen poor) condition.
Harmful bacteria can be eliminated from groundwater by making sure the water remains in the ground for 30 days prior to being used by humans or animals. This treatment is very effective for bacteria such as E. coli. Viruses usually are eliminated in this way as well, or they are significantly reduced in number and viability.
Bacteria such as Staphylococcus and Giardia can be eliminated by chlorination or other chemical treatment of the water. Note that Giadia can survive for an hour even in properly chlorinated water.
Parasitic infections are usually the result of a complex growth cycle of a parasite. Just one of the stages in the growth cycle needs to be addressed in order to break the cycle. Simply eliminating flies from potable water areas can halt some parasites. Other rely on poorly drained areas, or areas populated by snails, and either of those situations can be addressed to eliminate parasites.
Chemical pollutants represent a challenge to treat in a natural environment because of the complex interaction of the natural chemical mineral and biological environment. These types of pollutants are best treated at the source before being released into a waterway or groundwater. Many industrial process discharges can be detrimental to water quality, and they are usually monitored closely by regulatory agencies. Treatment is recommended for industrial process water before discharge. Many past industrial practices released heavy metals such as chromium, arsenic and mercury into the environment. These metals are still present and active today.
Other types of chemical processes occur as a result of changes and modifications to the landscape or as a result of farming and landscaping practices. These impacts include overuse of pesticides and fertilizers, and acid waters from mining drainage.
Treatment options vary from simple dilution in the case of road salt runoff, to modification of application methods or farming methods in the case of fertilizer use. Acid rock drainage can be treated in a number of ways, such as by limestone channels or artificial wetlands before the water is released downstream into the ecosystem.
Physical pollutants such as sediment and heat are often released from a point source. Since sediment will naturally settle to the bottom of a still body of water, this type of pollution can be removed from water by use of a stilling pond or retention pond before release to a waterway. Heated water can be allowed to naturally cool before being released into a waterway.
Sediment loads can also be introduced by nonpoint sources such as farmlands and construction activities. Remediation often consists of changes to farming practices, such as no-till farming. Land disturbed by construction can be temporarily seeded or mulched before the final grading and stabilization. Construction areas are often surrounded by silt fence before the area is stabilized to prevent sediment-laden runoff to enter waterways.
All three of the types of water pollution can be acting together in a waterway, making treatment options difficult. It is vital to treat each component separately, and prevention is the best way to do that. An assessment of an entire watershed may be undertaken to fully understand the practices and land features that are contributing to poor water quality. A comprehensive plan can then be implemented over time to address individual and overall water quality goals. The National Resource Conservation Center has a series of programs to assist in assessment of the watershed and achieving these goals.