Thermal pollution

March 22, 2017


Thermal pollution

Natural Standard Monograph, Copyright © 2013 (www.naturalstandard.com). Commercial distribution prohibited. This monograph is intended for informational purposes only, and should not be interpreted as specific medical advice. You should consult with a qualified healthcare provider before making decisions about therapies and/or health conditions.

Related Terms

  • Agriculture, anti-fouling, aquatic ecosystems, aquatic organisms, biofouling, boating, boiling reactor, Clean Water Act, closed-loop system, cooling systems, dams, forestry, heat shock proteins, hydromodification, landscaping, lawn watering, National Pollutant Discharge Elimination System, nonpoint source pollution, NPDES, NPS, once-through system, point source, polluted runoff, power plants, pressurized reactor, reservoirs, riparian zones, sediment collection, silviculture, storm sewers, storm water runoff, thermal discharge, thermal effluents, thermal enrichments, thermal pollution, thermal shock, thermal waste, UHS, ultimate heat sink, urban runoff, water education, waste heat, water pollution, Water Quality Act, watershed management, Xeriscape™ landscaping.


  • Thermal pollution is the change in the water temperatures of lakes, rivers, and oceans caused by made-man industries or practices. These temperature changes may adversely affect ecosystems by contributing to the decline of wildlife populations and habitat destruction. Any practice that affects the equilibrium of an aquatic environment may alter the temperature of that environment and subsequently cause thermal pollution. There may be some positive effects, though, to thermal pollution, including the extension of fishing seasons and rebounding of some wildlife populations.

  • Thermal pollution may come in the form of warm or cold water being dumped into a lake, river, or ocean. Increased sediment buildup in a body of water affects its turbidity or cloudiness and may decrease its depth, both of which may cause a rise in water temperature. Increased sun exposure may also raise water temperature. Dams may change a river habitat into a lake habitat by creating a reservoir (man-made lake) behind the dam. The reservoir water temperature is often colder than the original stream or river.

  • The sources and causes of thermal pollution are varied, which makes it difficult to calculate the extent of the problem. Also, because the negative effects of thermal pollution may not directly affect human health, it is not as well known as other types of pollution. The nuclear power industry is tightly regulated; therefore, the impact of nuclear power plants on the environment, including its production of thermal pollution, usually in the form of warm water, is better documented. According to the U.S. Energy Information Administration, as of December 31, 2007, there were 104 nuclear power plants operating in the United States. The contribution of less regulated, but possibly more extensive thermal polluters is more difficult to ascertain. These include polluted runoff, or nonpoint source pollution, which may be caused by rainfall or snowmelt washing sediment and pollutants into surrounding bodies of water, and the removal of vegetation from the banks of rivers or coastal areas.

  • In 1972, the U.S. Environmental Protection Agency (EPA), under the auspices of the Clean Water Act, along with state regulatory agencies began a nationwide effort to reduce water pollution. The program first focused on point source polluters or identifiable, single sources of pollution in stationary locations. The impact of manufacturers, such as power plants, on water quality was first assessed. Scientists discovered changes in the water temperature of rivers, lakes, and other water sources in or near industrial facilities. In the case of power plants, the discharge of warm water, also called cooling water, from the facilities was found to dramatically affect the aquatic ecosystems. In addition to studying the effects of temperature changes on aquatic habitats, scientists were able to develop tools to predict temperature dispersion in bodies of water affected by thermal polluters. In the late 1980s, the EPA and local governments turned their attention to nonpoint source (NPS) pollution. NPS pollution does not originate from a single location or facility. A wide array of factors may contribute to NPS. For example, agricultural practices such as removing trees and topsoil may cause soil erosion. Wind may carry this sediment to nearby bodies of water. Sediment may affect the flow, turbidity, temperature, and the settling of particles (also called sedimentation) of the water and detrimentally impact the aquatic environment. NPS pollution is more difficult to identify and control but may contribute to thermal pollution.

  • Power plants are considered to be significant contributors to thermal pollution. They use water from nearby lakes, rivers, or oceans to cool equipment. The hot water is released back to its source and may increase water temperature by as much as 30 degrees. Power plants also practice anti-fouling, or the removal of aquatic organisms from their equipment, by flushing the equipment, which may change the water temperatures of nearby bodies of water. Hydroelectric power plants and dams may dramatically impact the flow of rivers and streams, which may, in turn, affect water temperature. They may also contribute to soil erosion and sedimentation of bodies of water.

  • Agricultural practices, forestry, hydromodification (physical changes to streams or channels), boating, landscaping, urban runoff, impervious surfaces (such as cement and asphalt), and other human activities may increase or decrease the temperature of nearby bodies of water. Agricultural practices are the leading source of the impairment of rivers, streams, lakes, ponds, and reservoirs.

  • The EPA along with state and local governments is involved in assessing and preventing thermal pollution. Because so many industries and practices may contribute to thermal pollution, many different approaches to reduce pollution have been adopted. Water pollution is regulated by federal statutes such as the Clean Water Act, which protects surface water; the Water Quality Act, which applies to polluted storm water; and the National Pollutant Discharge Elimination System, which regulates pollution from point sources. Also, state and local policies that support water conservation practices and the voluntary modification of behavior have been successful in reducing thermal pollution.

  • NPS pollution is widespread and a leading cause of impaired water quality, according to the EPA. Thermal pollution is not thought to directly affect human health, but it may have a dramatic impact on aquatic organisms, including plants, insects, microorganisms, and aquatic life.


  • General: Both point source pollution (from an identifiable source) and nonpoint pollution source (NPS, from many different sources and locations) contribute to thermal pollution. Major contributors include power plants (e.g., thermoelectric and nuclear power plants) and polluted runoff.

  • NPS pollution, also called polluted runoff, comes from many sources and is caused by rainwater or melting snow carrying pollutants and sediments produced by man-made or natural practices to rivers, lakes, or other bodies of water. NPS pollution contributes to thermal pollution by dumping warm water, cold water, or sediment, which can also affect water temperature, into nearby bodies of water. Water and sediment may be transported to bodies of water by the flooding areas of loose soil passing over impenetrable substances, such as concrete or pavement, or entering storm drains. Due to the diversity, extensiveness, and persistence of the problem, it is difficult to quantify NPS pollution. NPS pollution is widespread and a leading cause of impaired water quality, according to the U.S. Environmental Protection Agency (EPA).The effects of NPS may not be fully understood. Contributors to NPS pollution include agriculture, breeding and raising livestock, urban runoff, and tree harvesting.

  • Power plants: There are several types of power plants. Thermoelectric plants, such as natural gas, oil, coal-fired, and nuclear, convert chemical energy to electricity. Hydroelectric power plants convert mechanical energy to electricity. Natural gas, oil, and coal-fired power plants are significant contributors to greenhouse gases, such as carbon dioxide, sulfur dioxide, and nitrogen dioxides, to the environment. Greenhouse gases absorb and release radiation that may increase the planet's temperature and may adversely affect the environment. Hydroelectric and nuclear power plants' emissions emit fewer or no greenhouse gases. Many power plants, regardless of type, require water to operate. The negative impacts of these power plants on surrounding bodies of water may be significant and may include thermal pollution.

  • Power plants and waste heat: Power plants are usually built near lakes, rivers, or oceans and use nearby bodies of water to cool their equipment. Power plants produce waste heat during operation. To prevent equipment from overheating, this excess heat must be removed. The heat may be transferred to water, which is then removed from the plant. The water may be re-circulated back to the plant to be used again or returned to its original body of water as much as 30 degrees warmer. Nuclear power plants are typically 33% efficient, meaning for every unit of electricity they produce, two units of waste heat are also produced. The amount of water discharged by a power plant depends on the size and type of the fuel used, type of cooling system, plant's efficiency, type of service water system, and whether the plant is working under normal conditions or when the plant is shut down due to a serious problem with the functioning of its systems.

  • Heat production: There are two types of nuclear reactors: pressurized and boiling. In the pressurized reactor, water is passed through the core at a high pressure so that it will not boil. Both types of reactors use water to remove excess heat from the reactor core. This hot water is then used to create steam, which spins a turbine connected to a generator that produces electricity. The steam is condensed into water as it passes through metal tubes surrounded by cooler water taken from a nearby lake, river, or ocean. The condensed water returns to the reactor core. The cooling water is returned to the lake, river, or ocean up to 30 degrees warmer.

  • Cooling systems: Power plants use two major types of cooling systems to cool their buildings and equipment: once-through and closed loop. The environmental impact of each system is significantly different.

  • Once-through systems have been traditionally used because they are cheaper to operate. Once-through systems use water from a nearby body of water to remove waste heat from the plants' buildings and equipment. This heated water is then returned to the water source. Closed-loop systems reduce the amount of waste heat discharged into surrounding bodies of water. Heated water flows to a cooling tower (as opposed to its source) where excess heat is transferred to the air.

  • Closed-loop systems must compensate for the water lost in the cooling towers by pumping in additional water from nearby sources. They may still discharge water back into the environment to flush sediment and debris collecting in the cooling tower basins. Hybrid systems, which use both air and water, and dry systems, which use only air, have also been developed. According to the U.S. Department of Energy's National Energy Technology Laboratory, the application of technologies that reduce water consumption remains limited.

  • Thermal discharge systems: Warm water is discharged from power plants to surrounding bodies of water. The mechanism of discharge differs depending on whether the body of water is a lake, river, or ocean. In the case of a lake, a long canal may carry warm water from the power plant to the lake. The water cools as it is guided by a wall that circulates the water in a counterclockwise fashion around the lake. The cooled water is then pumped back into the power plant. Power plants located near the ocean may use underground pipes running from the plant to a point offshore where heated water can be discharged. This may be as much as mile from the plant site. Power plants near rivers may use similar systems, discharging heated water through pipes running along the riverbed. The National Pollutant Discharge Elimination System (NPDES) permit program regulates point source pollution such as thermal discharge, released from power plants.

  • Ultimate heat sink (UHS): In addition to cooling systems used to remove waste heat, power plants may use service water systems to cool their equipment. Once used, this water is returned to the ultimate heat sink (UHS), which may be a nearby lake, river, or ocean or a different body of water dedicated for this purpose. A UHS is important not only for the operation of service water systems but also in case of an accident at a nuclear power plant. During accidental conditions, when the reactor core is shut down, the UHS provides the plant's cooling water needs for 30 days. The UHS may be a man-made pond equipped with water sprays or a natural body of water.

  • Hydroelectric power plants and dams: A dam is a structure that blocks the flow of water and may be built on a river. Dams may serve many purposes, including water storage, irrigation, and flood prevention. A hydroelectric dam uses moving water to generate electricity. Of the 80,000 dams found throughout the United States, about 2,388 dams generate electricity. The presence and operation of electricity producing dams and dams serving other purposes may greatly affect water quality, flow, and turbidity (cloudiness). Large dams may disrupt the natural flow of rivers; in fact, they may change a river habitat to a lake habitat by creating a reservoir (man-made lake) behind the dam. The reservoir water temperature is often colder than the original stream or river, which can greatly impact the aquatic ecosystem. Dams may also cause increased sediment collection. Water may be released from the dam to flush out sediment, which may affect water flow, turbidity or cloudiness, the settling of particles (called sedimentation), and temperature downstream. Water from a lower reservoir may be pumped into a higher reservoir, changing the water temperatures of both.

  • Biofouling: Biofouling is the adherence of aquatic organisms, including barnacles, algae, seaweed, and bacteria, to structures built or operating in aquatic environments. Ships, power stations, and underwater pipes are examples of potentially affected structures. Biofouling may impair the functioning of industrial equipment. For example, organisms may adhere to cooling water circuits of power stations and disrupt water flow. Anti-fouling is the removal of these organisms from equipment with chemical agents or by flushing equipment with hot water. The warm water discharge from anti-fouling may cause thermal pollution and affect the water quality of nearby bodies of water.

  • Agriculture: Agricultural practices have a significant effect on water quality. The 2000 "National Water Quality Inventory" ranked agricultural NPS pollution as a leading contributor to the water impairment of rivers, lakes, estuaries, wetlands, and ground water. Agricultural practices may contribute to thermal pollution by dumping warm or cold water or sediment into nearby rivers, lakes, or other bodies of water.

  • The causes of agricultural NPS pollution include overgrazing, over-plowing, and improperly used pesticides or fertilizer. Sedimentation is the most prevalent form of agricultural water pollution. Soil may be washed off fields by over watering and dumped into surrounding bodies. Overgrazing and over-plowing also contribute to soil erosion and sedimentation. Improving the soil quality, reducing slopes, improving plant cover, planting field borders, and terracing can reduce soil erosion. Also, by diverting runoff, sediment can be trapped in filters or retained in designated ponds. Polluted runoff containing solid waste from poorly managed animal feeding facilities may also impair nearby bodies of water and contribute to thermal pollution. Diverting rainwater on roofs away from the facility and properly storing liquid manure may prevent pollution.

  • Urban runoff and storm sewers: Every day the practices of people living in urban areas may impair nearby bodies of water. Urban runoff is the second leading contributor to the pollution of estuaries, which are bodies of water with streams running through them and a connection to the ocean. Sediment and pollutants can be washed into nearby bodies of water by rainwater, melting snow, and other practices. These pollutants may increase water temperature and cause thermal pollution. Surfaces such as pavement, open spaces, and roofs may increase the velocity and volume of runoff, causing the erosion of stream banks and the deposition of sediment. Increased water flow, soil erosion, and sediment collection may affect water temperature. Higher flow may widen streams, making them shallower and increasing their temperature. Bridges and highways, construction sites, grading of urban areas, and lack of vegetation may contribute to thermal pollution in urban areas. During warmer months, impervious surfaces like pavement and concrete act as heat collectors, warming runoff as it passes over them. Stream temperatures in urban areas may increase by as much as 10%. Pollution management may also inadvertently contribute to thermal pollution. Runoff may be collected in detainment facilities to prevent it from polluting nearby bodies of water. During containment, it may increase in temperature before being released in a controlled manner to nearby water sources. Increased water temperature may adversely affect ecosystems, including the growth of algae, which may disrupt the aquatic food chain.

  • Roads, highways, and bridges: As water washes over roads, highways, and bridges, sediment and pollutants may be carried to nearby bodies of water. The construction and maintenance of these structures may impair riparian habitats. Vegetation is often cleared to build roads; this may lead to increased soil erosion and the destruction of shading plants near bodies of water, both of which cause thermal pollution. Practices to reduce the negative impact of these structures include slope protection, sediment traps, and limiting land disturbances.

  • Forestry: Forestry, also called silviculture, may greatly impact water quality. According to the EPA, forestry accounts for 3-9% of NPS pollution nationally. Forestry's contribution to thermal pollution is perhaps the best documented of all nonpoint source polluters. Silviculture is primarily responsible for increasing sedimentation and, in turn, the turbidity of lakes, rivers, and other nearby bodies of water by accelerating soil erosion. As trees are harvested and roads are built to transport them to mills for processing, the surrounding soil is loosened. Soil and other organic material produced during harvest are carried to nearby bodies of water by water, ice, wind, or other processes. The smaller the body of water, the greater the effect. Cloudy or turbid water typically has less dissolved oxygen due to its higher water temperature. This temperature change may kill aquatic species, including plants, fish, and bottom-dwelling invertebrates (also called benthic invertebrates).

  • Silviculture may also affect water temperature by the removal of vegetation from the riparian zone, the area around a stream or river. The riparian zone provides shade and protection for aquatic life, maintaining cooler water temperatures and minimizing soil erosion along riverbanks. Forestry practices often remove this vegetation by harvesting, herbicides, or burning; these processes may greatly impact the riparian zone habitat. Decreased shade and increased soil erosion may increase water temperature, which may decrease the amount of oxygen in the body of water. This may speed up chemical processes in affected organisms by increasing oxygen demands. Silviculture's impact on the environment varies according to each project and its location. Road construction may greatly contribute to soil erosion. The EPA's guidelines for minimizing silviculture's negative impact on the environment include avoiding steep slopes or high-erosion areas when constructing roads, avoiding stream channels, and building proper drainage systems along roads. Reducing NPS involves careful planning and management. The EPA offers guidelines for reducing NPS, depending on the time of year or location. For example, the EPA suggests harvesting trees during times that minimize soil disturbance, such as during dry periods or when there is snow cover.

  • Hydromodification: Hydromodification is a change in the structure of bodies of water by building channels and dams or eroding the soil of shorelines or riverbanks. Hydromodification may increase sedimentation in bodies of water or remove vegetation from riparian zones, both of which may cause thermal pollution. Examples of hydromodification vary and include dredging or disturbing the bottom of bodies of water, stream relocation or straightening, construction of dams, construction along riverbanks or shorelines, and the removal of snags (dead trees). The EPA groups causes of hydromodification into three categories: channelization, dams, and stream bank and shoreline erosion. Hydromodification is the second leading source of the impairment of rivers, streams, lakes, ponds, and reservoirs. Any practice that affects the equilibrium of an aquatic environment can alter the temperature of that environment and subsequently cause thermal pollution. Hydromodification may alter the flow, sedimentation, turbidity, and depth of bodies of water, all of which affect water temperature. For example, changing the course of a stream may improve drainage, but it may also increase sedimentation and turbidity. The darker sediment particles may absorb heat and increase water temperature.

  • Marinas/boating: Marinas may contribute to poor water quality and thermal pollution by increasing soil erosion, disturbing sediment, and affecting riparian zone habitats. Construction, maintenance, or landscaping of marinas may reduce vegetation in riparian zones, leading to decreased shading of aquatic organisms and increased water temperature. These practices can also cause soil erosion. Storm water runoff and other processes may carry this sediment to nearby bodies of water, increasing the turbidity and water temperature. Boating may disturb the bottoms of lakes, ponds, etc., and stir up sediment. The effects of dredging, as it may be called, are usually temporary.

  • Landscaping: Residential and commercial landscaping may contribute to thermal pollution by creating polluted runoff and soil erosion. According to the EPA, lawn care represents 32% of residential outdoor water use, though it can vary by season and location. Using water dependent plants, watering plants at suboptimal times and levels, and not applying a comprehensive approach to water conservation may contribute to polluted runoff and soil erosion. To conserve water and mitigate pollution, the EPA suggests landscaping with drought tolerant plants, grouping plants with similar water requirements, or letting grass grow taller to promote soil water retention. Improvements in lawn irrigation may also be made. Cycle irrigation improves water penetration and reduces runoff by giving plants the right amount of water at the best time. Low-precipitation sprinklers deliver water at a lower pressure, allowing for deeper penetration of the soil and more uniform distribution. Bubbler/soaker systems and drip irrigation systems reduce runoff by delivering water at a slower rate, improving penetration. The EPA also supports the use of Xeriscape™ landscaping, which utilizes a comprehensive approach to water conservation. Xeriscape™ landscaping incorporates several principles of water conservation including "planning ahead, soil testing, selecting suitable plants for the area, choosing practical landscaping areas, efficient irrigation, use of mulches, and adequate maintenance. Xeriscape™ landscaping decreases storm water and irrigation runoff and water use. More than 40 states have Xeriscape™ projects.

  • Other residential outdoor water uses: Other residential outdoor water uses include washing automobiles and cleaning sidewalks and driveways. They may create thermal pollution by dumping cold or warm water into storm drains or over eroding soil. When car washing, the EPA suggests turning off the hose when not in use or washing the car on the lawn to reduce runoff. This could save as much as 150 gallons of water. Sidewalks or driveways can be swept instead of washing, which uses 50 gallons of water every five minutes.


  • General: Many industries and practices contribute to thermal pollution. Since the 1970s, scientists and regulatory agencies have studied the effects of thermal pollution. Several factors may influence the impact of thermal pollution on a body of water. They include the type of pollution, type of water body, time of year, watershed management, location of the water body, and water depth.

  • Studying thermal pollution: Scientists use different technologies to better understand thermal pollution's effects on the environment. Statistical analysis developed by the U.S. Environmental Protection Agency (EPA) may predict temperature dispersions in a body of water from a point source. Satellite imaging of bodies of water may illustrate temperature dispersion patterns and help identify the sources of thermal pollution. Videography is an important tool for understanding the impact of thermal pollution in deep waters, canals, or underwater pipes.

  • A recent study presented thermal infrared data from satellites of the Younggwang nuclear power station on the west coast of Korea. Researchers were able to characterize sea surface temperatures and found that water temperature changes were detected 50- 100km from the discharge. They called for urgent action to protect the coastal environment.

  • Scientists in British Columbia radio-tagged smallmouth bass and recorded their movements in and around a thermal discharge canal on Lake Erie. They found that many of the fish spent the majority of the winter in the canal and concluded that such infrastructures should be considered fish habitats and regulated accordingly.

  • Effects: Thermal pollution causes an increase or decrease in water temperature. Temperature may influence metabolic, growth, and reproductive rates of organisms and change the chemical composition of the water. Aquatic environments are particularly sensitive to temperature changes. Aquatic organisms often live within a narrow range of temperatures. If the temperature of the environment is altered by only a few degrees, it may impact the health of plants and animals. Therefore, thermal pollution may change the makeup of an entire ecosystem.

  • Chemical composition: Thermal pollution is often equated with thermal discharge, or the release of warmer water into a nearby river, lake, or ocean. Warmer water typically has less dissolved oxygen. This may speed up chemical processes in affected organisms by increasing oxygen demands. Metabolism, growth rates, and reproduction may be dramatically altered by changes in temperature. In warmer water, metabolic rates may speed up, causing organisms to eat more, and may subsequently lead to food shortages. Temperature changes may change the permeability of cell structures. In extreme cases, very high temperatures may destroy enzymes and break down cellular processes.

  • Ecosystem composition: Thermal pollution may have a dramatic effect on the composition of aquatic organisms living in a body of water. Because many aquatic organisms live in a narrow range of water temperatures, a shift in temperature may cause increased mortality in native populations and encourage the migration of populations better suited to that temperature. According to the EPA, less dissolved oxygen may disrupt the aquatic food chain, including killing plants, fish, and bottom-dwelling invertebrates. In extreme cases, an ecosystem may shift from one temperature range to another. For example, when a dam is built on a river creating a reservoir behind it, the warmer river environment will shift to a colder lake environment, changing the makeup of plants, insects, and fish in the ecosystem.

  • A study in India found that higher water temperatures created by discharge from a nuclear power plant had lethal effects on crabs. Researchers observed 100% mortality of the crabs at cooling pumps where the temperature had increased to 40 degrees Centigrade.

  • The effects of thermal pollution on an ecosystem may be positive or may have few consequences. Manatees, for example, prefer warmer waters and flock to the thermal effluents of manufacturing facilities. This may be called thermal enrichment, when increased temperature has a positive effect on the environment. It has been suggested that manatee populations will again decline if the effluents are removed or the water temperature are returned to normal.

  • A recent study in Canada examined the effects of thermal discharge on parasites that prey on species of flounder. Researchers found the diversity and abundance of algae, invertebrates, and fish were greater in the discharge waters. The flounder population was found to be unaffected by the warmer water, but parasites commonly found on and in the fish were affected.

  • A Russian study found that thermal pollution can cause an increase in microorganisms that prefer warmer water temperatures and negatively affect water quality.

  • Reducing effects: Many industries and practices contribute to thermal pollution. It may be produced by a single, easily identifiable source or by various, sometimes fundamentally different processes. Because of the diverse nature of the problem, many different approaches are being employed to mitigate the effects. Both voluntary and mandatory water and energy conservation programs instituted at many levels are helping to curb thermal pollution. Federal, state, and local agencies have spearheaded campaigns educating the public about water quality issues. Using less water and electricity are recurring themes. These campaigns, coupled with mandatory programs and new technologies, have a significant impact on water pollution. For example, 150 gallons of water may be saved during car washing alone when the hose is turned off when not in use. Power plants may drastically decrease thermal discharge by adopting closed loop systems (cooling systems that recycle warm water instead of discharging it), using cooling towers to get rid of waste heat, and discharging warmer waters into man-made retention ponds. Several laws are now in effect to curb pollution and improve water quality.

  • Regulation: Local, state, and federal regulations govern water quality and use and may address concerns regarding thermal pollution. The Clean Water Act is a federal statute protecting the chemical, physical, and biological integrity of surface (not ground) water throughout the United States. The main goal of the Clean Water Act is to protect and proliferate fish, shellfish, aquatic wildlife, and water recreation. After its passage in 1972, local and federal agencies focused on point source polluters such as power plants and analyzed ways to reduce their impact on the environment. Section 316 (b) of the Clean Water Act states that cooling water intake systems should use the best available technology to minimize negative environmental impact. Section 316 (c) addresses thermal discharge and states that limiting discharge will protect aquatic species.

  • According to the U.S. Department of Energy's Energy Information Administration, coolant water discharges from nuclear power plants dumped into nearby lakes, rivers, and oceans alter the ecology of these bodies of water. Whether the effect is negative and what the value of this effect is may determine how coolant water disposal is controlled.

  • The Clean Water Act created the National Pollutant Discharge Elimination System (NPDES) permit program. The program regulates point source pollution in the United States. It ensures water quality and limits polluting practices. The Water Quality Act of 1987 expanded the regulatory power of the Clean Water Act by making NPDES requirements apply to storm water discharge.

Health Impact/Safety

  • General: Thermal pollution is not thought to directly affect human health, but it may have a dramatic impact on aquatic organisms, including plants, insects, microorganisms, and fish. Many aquatic organisms are temperature-sensitive, meaning they live in only a narrow range of temperatures. The most important impact of warm water thermal pollution is a decrease in dissolved oxygen. Less oxygen in the environment may affect aquatic organisms and the overall health of the ecosystem.

  • Plant growth: Warm water may speed up the growth of plants by encouraging photosynthesis. Increases in algae and other plant populations may affect the amount of plant material decomposing in the environment. Bacteria require oxygen for decomposition, which means there is less available for other organisms.

  • Harmful algal blooms (HABs) occur when certain types of algae grow quickly in water, forming patches that may block sunlight and decrease the amount of dissolved oxygen in the water. HABs are increasing in coastal and surface waters across the United States, according to the U.S. Environmental Protection Agency (EPA). HABs effects on human health are not completely understood. The EPA has added certain microorganisms responsible for creating HABs to its Drinking Water Contaminant Candidate List, which identifies potentially harmful substances that should be investigated. The algae that may negatively affect human health include cyanobacteria, which may contaminate drinking water and have been associated with gastroenteritis, skin irritation, and liver damage.

  • Harmful marine algae produce toxins that can build up in shellfish. When ingested, these toxins may cause neurological or gastrointestinal problems or even death. Breathing the toxins may contribute to asthma attacks in susceptible individuals. Pfiesteria piscicida is found near dead fish, and water containing this organism may cause irritation and headache. In 2007, the Nassau County Health Department in northeastern Florida found that harmful algal blooms along the coastline were likely causes of respiratory illness in people working on a beach restoration.

  • Animal reproduction: The reproduction of aquatic species is often temperature-dependent. A change in water temperature may change when organisms spawn or if they do at all. For example, some marine bivalves (such as clams and oysters) spawn in the summer. Warmer water temperatures will encourage the organisms to spawn at an inappropriate time and for longer. This may affect the health of the population if the appropriate food is unavailable. On the other hand, the lugworm spawns in cold water and may not spawn at all if the water is too warm.

  • Adaptation to thermal pollution: Thermal pollution may increase water temperatures by as much as 30 degrees. Such drastic changes may be lethal to cold water organisms, such as trout and salmon. Thermal shock, or a sudden and rapid rise in temperature, can denature proteins, disrupt metabolic processes, and result in death. There is some evidence, though, that over time, some organisms may adapt to the warmer environment by producing heat shock proteins. Heat shock proteins protect cells from sudden changes in temperature.

  • Thermal discharge has been found to affect coral reefs. Coral living in thermal discharges may become bleached by the warmer water. There is some evidence that the organisms are able to produce heat shock proteins over time to mitigate the effects of the temperature changes.

  • Thermal enrichment: Thermal enrichment is also a potential side effect of thermal pollution. Thermal enrichment is considered to be beneficial to aquatic organisms. Warmer water temperatures may speed up the growth of commercial fishing stocks, extend fishing seasons, or encourage the migration of more desired organisms to an area. The benefits of thermal pollution are generally thought to not outweigh the negative consequences.

Future Research or Applications

  • General: Thermal pollution is a widespread and often disregarded problem that can dramatically impact the environment. Thermal pollution often combines with other types of water pollution, such as toxic chemicals, heavy metals, pathogens, and salts, or air pollution. It is difficult to address each issue independently. Protecting the health of waterways, lakes, and oceans requires understanding the different factors contributing to all types of pollution affecting those bodies of water.

  • Comprehensive programs: General watershed management may improve the integrity of the environment and is of particular importance in controlling nonpoint source (NPS) pollution. Cooperation among federal, state, and local agencies and individuals is necessary. An example of such an effort is the restoration of the Quinn River in Nevada. According to the U.S. Environmental Protection Agency (EPA), agriculture, recreation, and drought have contributed to the deterioration of the river habitat. Thermal pollution was cited as having a detrimental effect on aquatic life. Many entities were involved in restoring the river including the Forest Service, ranchers, the Boy Scouts, and federal and state agencies. Goals of the project addressed many issues. Riverbanks were stabilized, water temperatures reduced, grazing controlled, and successes publicized. The riparian environment was protected by a fence to discourage grazing along the river banks, water velocity was slowed by planting trees, and water temperature and erosion were also reduced by planting trees along the banks. Aquatic microinvertebrates were monitored during the project. From June to October in 1991, the concentration of organisms rose from poor to fair. The program also tackled NPS pollution in the form of runoff produced by lawn watering. A campaign was initiated to educate individuals about water conservation and the effects of NPS. Water use decreased by three percent the following year, in spite of higher ambient temperatures and 40,000 new settlers moving to the area.

  • Innovative technologies: The U.S. Department of Energy is researching different technologies to mitigate the negative environmental impact of power plants. Technologies that reduce power plant water consumption may, in turn, reduce thermal pollution. Hybrid cooling systems, which use water and air, use 50% less water than conventional closed- loop systems. Some technologies may use waste heat or remove it in innovative ways. Researchers at the University of Florida used waste heat to remove the salt from saline water and produce freshwater. Ceramic Composites, Inc. and their partner SPX Corporation are developing high-thermal conductivity foam that may eliminate thermal discharge. There is also interest in the use of impaired water such as mine pool water or wastewater to cool power plants. At the San Juan Generating Station in New Mexico, water from natural gas and oil extraction is used to cool the plant. Coupled with technologies that mitigate thermal discharge, these advances may reduce power plants' overall impact on the environment.

Author Information

  • This information has been edited and peer-reviewed by contributors to the Natural Standard Research Collaboration (www.naturalstandard.com).


Natural Standard developed the above evidence-based information based on a thorough systematic review of the available scientific articles. For comprehensive information about alternative and complementary therapies on the professional level, go to www.naturalstandard.com. Selected references are listed below.

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  7. Khan RA, Hooper RG. Influence of a thermal discharge on parasites of a cold-water flatfish, Pleuronectes americanus, as a bioindicator of subtle environmental change. J Parasitol. 2007 Oct;93(5):1227-30. View Abstract

  8. Natural Standard: The Authority on Integrative Medicine. www.naturalstandard.com.

  9. River Basin Center. www.rivercenter.uga.edu.

  10. Solovykh GN, Fabarisova LG, Nefedova EM, et al. Effect of thermal power stations on the sanitary and biological conditions of water reservoirs. Gig Sanit. 1998 Nov-Dec;(6):24-7. View Abstract

  11. State of Maine Bureau of Land and Water Quality. www.maine.gov.

  12. Union of Concerned Scientists. www.ucsusa.org.

  13. U.S. Department of Energy National Energy Technology Laboratory. www.netl.doe.gov.

  14. U.S. Energy Information Administration. www.eia.doe.gov.

  15. U.S. Environmental Protection Agency (EPA). www.epa.gov.

Copyright © 2013 Natural Standard (www.naturalstandard.com)

The information in this monograph is intended for informational purposes only, and is meant to help users better understand health concerns. Information is based on review of scientific research data, historical practice patterns, and clinical experience. This information should not be interpreted as specific medical advice. Users should consult with a qualified healthcare provider for specific questions regarding therapies, diagnosis and/or health conditions, prior to making therapeutic decisions.


March 22, 2017