Point Source
Point Source
Point source pollution is contamination that enters the environment through any discernible, confined, and discrete conveyance, such as a smokestack, pipe, ditch, tunnel, or conduit. Point source pollution remains a major cause of pollution to both air and water. Point sources are differentiated from non-point sources, which are those that spread out over a large area and have no specific outlet or discharge point. Point source pollution in the United States is regulated by the Environmental Protection Agency (EPA).
Point Sources of Water Pollution
Point sources of water pollution include municipal sewage treatment plant discharges and industrial plant discharges. Municipal sewage treatment plant point sources can contribute pollution in the form of oxygen-depleting nutrients and in the form of pathogens that cause serious health hazards in drinking water and swimming areas. Industrial point sources can contribute pollution in the form of toxic chemicals and heavy metals. Examples of non-point source water pollution include agricultural and urban runoff, and runoff from mining, and construction sites.
The Clean Water Act (CWA), passed by Congress in 1972, provides the basic structure for regulating the discharge of pollutants from point sources to waters of the United States. The CWA gives the EPA the authority to establish effluent limits. Effluent is the outflow from a municipal or industrial treatment plant. The CWA also requires the acquisition of a National Pollution Discharge Elimination System (NPDES) permit prior to the discharge of pollutants. States may be authorized to implement CWA programs, but the EPA retains oversight responsibilities.
The EPA manages effluent limits for point sources in two ways: through technology-based controls and through water quality-based controls. Industry-wide effluent limits are established on a technology basis. These are minimum standards based on available treatment technology and pollution prevention measures. Effluent limits are also established on a water-quality basis. Water quality-based criteria are scientifically defensible standards that ensure protection of designated uses of a receiving water. Either standard may be superceded by the more stringent standard, as determined by the control authority.
Municipal point sources are the result of community sewage treatment systems. At the sewage treatment plant, wastewater is treated to remove solid and organic matter, disinfected to kill bacteria and viruses, and then often discharged to a surface water. Not all solids and organic matter are removed during treatment, resulting in degraded receiving water quality, due to a reduction in dissolved oxygen . Nutrients such as phosphorus that are not removed during treatment can cause overgrowth of algae and other organisms, also leading to lower dissolved oxygen. Many toxic substances can pass through conventional municipal treatment systems. Improperly treated sewage can be released as a result of upsets to the treatment process or as a result of operator error.
During heavy rain, discharges from sewage treatment systems can be a serious problem. In many municipalities, storm-water runoff is combined with municipal sewage in a common system. The increased water volume leads to reduced treatment. Combined sewer overflows occur when water flow exceeds treatment plant capacity, resulting in untreated sewage being discharged directly to rivers, lakes, or the ocean.
Industrial point sources are the result of industries using water in their production processes, and then treating the water prior to discharge. Some of the industries requiring process waters include pulp and paper mills, food processors, electronic equipment manufacturers, rare metal manufacturers, textile manufacturers, pharmaceutical manufacturers, forest product producers, leather tanners, and chemical manufacturers.
The National Pretreatment Program is charged with controlling the 126 priority pollutants from industries that discharge into sewer systems. These pollutants fall into two categories: metals and toxic organics. The metals include lead, mercury, chromium, and cadmium. The toxic organics include solvents, pesticides, dioxins, and polychlorinated biphenyls (PCBs).
Unlike municipal treatment methods, which are similar across the country, industrial treatment methods are industry-specific. For example, electro-plating wastewater may require cyanide removal through oxidization. In general, physical processes may be used to remove solids and biological processes to remove organics. Chemical treatment, such as precipitation and neutralization, is also widely used.
The National Water Quality Inventory: 2000 Report is compiled based on the water quality reports required to be submitted to the EPA by states every two years. The report identifies "impaired" waters: water that cannot support its designated use, such as fishing or swimming, due to contamination. According to the report, municipal point sources contributed to 37 percent and industrial discharges contributed to 26 percent of reported water-quality problems in the impaired portion of estuaries. Municipal point sources were the leading cause of contamination in 21 percent of the impaired ocean shorelines, and industrial discharges were the leading cause in 17 percent. Municipal point sources were a leading source of contamination in 10 percent of the impaired river miles and 12 percent of the impaired lake acres . These figures are improved over the percentages recorded in the 1992 Report when municipal point sources were a leading contamination source in 15 percent of the impaired river miles and 21 percent of the impaired lake acres.
The NPDES permit program can be credited with achieving significant improvements to the water quality of the United States. Immediately following passage of the CWA, efforts focused mainly on regulating traditional point sources, such as municipal sewage plants and industrial facilities. In the late 1980s, efforts to address "wet weather point sources," such as urban storm sewer systems, began. Currently, there is a greater focus on nonpoint source pollution. The EPA is moving away from a source-by-source and pollutant-by-pollutant approach to a watershed-based approach. A watershed, or "placebased," approach is a process that emphasizes addressing all stressors within a hydrologically defined boundary or drainage basin. Equal emphasis is placed on protecting healthy waters and restoring impaired waters.
Point Sources of Air Pollution
Point sources of air pollution include stationary sources such as power plants, smelters, industrial and commercial boilers, wood and pulp processors, paper mills, industrial surface coating facilities, refinery and chemical processing operations, and petroleum storage tanks. Examples of nonpoint sources of air pollution include: on-road mobile sources such as cars and trucks; nonroad mobile sources such as construction and recreation equipment engines; and natural sources such as windstorms and fires. Exposure to air pollution is associated with adverse effects on human health including respiratory problems and lung diseases. Air pollution can also significantly affect ecosystems.
The Clean Air Act (CAA) was passed by Congress in 1970 and amended in 1990. Under the CAA, EPA sets limits on how much of a pollutant is allowed in the air anywhere in the United States. Each state is required to develop a state implementation plan (SIP) to explain how it will do its job
under the CAA. A permit must be obtained for large sources that release pollutants into the air. The permits require information on which pollutants are being released, how much pollutant is released, what steps are being taking to reduce pollution, and plans for monitoring.
The EPA has set national air quality standards for six principal air pollutants (also known as criteria pollutants): carbon monoxide (CO), lead (Pb), nitrogen dioxide (NO2), ozone (O3), particulate matter (PM), and sulfur dioxide (SO2). CO, Pb, NO2, and SO2 result from direct emissions from a variety of sources, including point sources. PM can result from direct emissions or can form when emissions and other gases react in the atmosphere. Ozone is not emitted directly, but forms when nitrogen oxides (NOx) and volatile organic compounds (VOCs) react in the presence of sunlight. The EPA refers to chemicals that cause serious health and environmental impacts as hazardous air pollutants (HAPs) or air toxics. Currently, 189 air toxics have been identified, including chemicals such as benzene, chloroform, and mercury.
The EPA tracks air pollution in two ways: (1) emissions form all sources going back thirty years and (2) air quality measured from monitoring stations around the country going back twenty years. The EPA summarizes its most recent evaluations in the report Latest Findings on National Air Quality: 2000 Status and Trends. Since 1970, the total emissions for the six criteria pollutants have been reduced 29 percent. National air quality levels measured at monitoring stations across the country have also shown improvements over the past twenty years for all six criteria pollutants. Over 160 million tons of pollution (from both point sources and non-point sources) are emitted into the air each year in the United States.
In 2000 Status and Trends, the EPA reports an increasing focus on tracking and controlling ground-level ozone and fine particles, key components of smog and haze. Progress has been slowest for ground-level ozone. In some regions of the United States, ozone levels have actually increased in the past ten years. The ozone increase correlates to the increase in NOx emissions from power plants and other sources. NOx emissions also contribute to acid rain, haze and particulate matter. Sulfates, formed mainly from coal-fired power plant emissions, are the main source of particles in the eastern United States. The emissions also contribute to the formation of acid rain. The EPA's emissions trading program successfully reduced these air pollutants, resulting in improved visibility in the eastern United States.
While point source pollution is declining in the United States, it remains a global environmental concern. According to the UN report Global Environment Outlook 2000, rapid urbanization and industrialization in many developing countries is creating high levels of air and water pollution.
see also Air Pollution; CWA; CAA; Cuyahoga River; Disasters; Donora, Pennsylvania; National Pollutant Discharge Elimination System (NPDES); Nonpoint Source Pollution; Thermal Pollution; Toxic Release Inventory; Wastewater Treatment; Water Pollution.
Bibliography
U.S. Environmental Protection Agency. (2000). National Water Quality Inventory: 2000 Report. Washington, D.C.: U.S. Government Printing Office.
U.S. Environmental Protection Agency, Office of Wastewater Management. (1999). Introduction to the National Pretreatment Program. Washington, D.C.: U.S. Government Printing Office.
Vigil, Kenneth M. (1996). Clean Water: The Citizen's Complete Guide to Water Quality and Water Pollution Control. Portland, OR: Columbia Cascade Publishing Company.
internet resources
U.S. Environmental Protection Agency. National Pollutant Discharge Elimination System. Available from http://www.epa.gov/npdes.
U.S. Environmental Protection Agency, Office of Science and Technology. Available from http://www.epa.gov/OST.
United Nations Environment Programme. Global Environment Outlook 2000. Available from http://www.unep.org/geo2000.
Denise M. Leduc
Point Source
Point Source
A point source is a concentration of something that appears to be very small. In mathematics, for instance, a point source (a singularity, or a point with no dimensions) is often modeled as an approximation for a real source (with actual dimensions) in order to simplify the analysis. Such approximations are used in virtually all field of science. To describe a point source, the field of environmental science will be used as an example.
A point source, within environmental science, is a situation where large quantities of pollutants are emitted from a single, discrete source, such as a smokestack, a sewage or thermal outfall into a water body, or a volcano. (Although the source is real in the environment, its effects can be modeled as a point source.) If the emissions from a point source are large, the environment will be characterized by strong but continuous gradients of ecological stress, distributed more-orless concentrically around the source, and diminishing exponentially with increasing distance. The stress results in damages to organisms, but because tolerance differs among species, the net result is a continuous gradient of change in the ecological community and in ecological processes, such as productivity and nutrient cycling.
This ecological phenomenon has been well studied around a number of point sources of ecological stress. For example, the structure of terrestrial vegetation has been examined along transects originating at a large smelter located at Sudbury, Ontario. This smelter is a point source of great emissions of toxic sulfur dioxide and metals. The immediate vicinity of the smelter is characterized by severe ecological degradation, because only a few species can tolerate the toxic stress. However, at increasing distances from the smelter the intensity of the toxic stresses decreases rapidly. Consequently, there is a progressive survival and/or
invasion of sundry plant species at greater distances from the smelter, depending on their specific tolerances of the toxic environment at various distances. Farther than about 18.6 mi (30 km) from the smelter the toxicity associated with its point-source emissions no longer has a measurable influence on the vegetation, and there is a mature forest, characteristic of the regional unpolluted, landscape.
Often, species that are most tolerant of the toxic stresses close to a point source are uncommon or absent in the surrounding, non-polluted habitats. Usually, only a few tolerant species are present close to point sources of intense ecological stress, occurring as a sparse, low-growing community. At greater distances shrubs may dominate the plant community, and still further away relatively tolerant species of tree may maintain an open forest. Eventually, beyond the distance of measurable ecological responses to the toxic stress, a reference forest occurs. However, it is important to recognize that these ecological changes are continuous, as are the gradients of environmental stress associated with the point source. This syndrome of degradation of vegetation along transects from smelters and other large point sources has been characterized as a peeling of the vegetation.
In addition to changes in ecological communities along environmental gradients associated with point sources, there are also predictable changes in ecological functions, such as productivity, nutrient cycling, and litter decomposition.
See also Non-point source; Stress, ecological.
Point Source
Point source
A point source is a situation where large quantities of pollutants are emitted from a single, discrete source, such as a smokestack, a sewage or thermal outfall into a waterbody, or a volcano . If the emissions from a point source are large, the environment will be characterized by strong but continuous gradients of ecological stress, distributed more-or-less concentrically around the source, and diminishing exponentially with increasing distance. The stress results in damages to organisms, but because tolerance differs among species , the net result is a continuous gradient of change in the ecological community and in ecological processes, such as productivity and nutrient cycling.
This ecological phenomenon has been well studied around a number of point sources of ecological stress.
For example, the structure of terrestrial vegetation has been examined along transects originating at a large smelter located at Sudbury, Ontario. This smelter is a point source of great emissions of toxic sulfur dioxide and metals. The immediate vicinity of the smelter is characterized by severe ecological degradation, because only a few species can tolerate the toxic stress. However, at increasing distances from the smelter the intensity of the toxic stresses decreases rapidly. Consequently, there is a progressive survival and/or invasion of sundry plant species at greater distances from the smelter, depending on their specific tolerances of the toxic environment at various distances. Farther than about 18.6 mi (30 km) from the smelter the toxicity associated with its point-source emissions no longer has a measurable influence on the vegetation, and there is a mature forest, characteristic of the regional unpolluted, landscape.
Often, species that are most tolerant of the toxic stresses close to a point source are uncommon or absent in the surrounding, non-polluted habitats. Usually, only a few tolerant species are present close to point sources of intense ecological stress, occurring as a sparse, lowgrowing community. At greater distances shrubs may dominate the plant community, and still further away relatively tolerant species of tree may maintain an open forest. Eventually, beyond the distance of measurable ecological responses to the toxic stress, a reference forest occurs. However, it is important to recognize that these ecological changes are continuous, as are the gradients of environmental stress associated with the point source. This syndrome of degradation of vegetation along transects from smelters and other large point sources has been characterized as a peeling of the vegetation.
In addition to changes in ecological communities along environmental gradients associated with point sources, there are also predictable changes in ecological functions, such as productivity, nutrient cycling, and litter decomposition .
See also Non-point source; Stress, ecological.
Point Source
Point source
A localized, discrete, and fixed contaminant emission source, such as a smokestack or waste discharge pipe. Since point sources are usually easily identified, their discharges of pollution can be monitored readily. Point sources are distinguished from nonpoint sources and mobile (or vehicular) sources and historically were the first to receive emission controls. Some point sources release exceptionally large quantities of contaminants. For example, a smelter in Sudbury, Ontario , was for many years the largest single source of sulfur dioxide in North America, emitting several million tons of pollutants annually through a smokestack over 1,247 ft (380 m) tall.