What Is Thermal Pollution?

Thermal pollution is a rapid change in temperature in a natural body of water. This pollution is most often caused by heated discharge from an industrial facility or another human activity. Thermal pollution can result in disruptions in natural systems and stress, disease, or even death for affected organisms.

Causes of Thermal Pollution

Natural phenomena such as wildfires, volcanoes, and underwater thermal vents can cause thermal pollution. However, it is more often the result of an industrial process or facility using large amounts of water from a natural source and releasing heated wastewater.

Power Plants and Industrial Facilities

Thermoelectric power plants fueled by coal, natural gas, nuclear, or biomass and other waste products constitute significant causes of thermal pollution. Power plants are typically built next to a river, lake, or ocean, which provide a steady supply of water. This gets converted to steam that drives turbines to generate electricity. Water is also used to cool machinery, which becomes very hot. The water absorbs heat, and what doesn't evaporate is typically discharged back to its source. 

In addition to power plants, other industrial facilities—such as petroleum refineries, pulp and paper mills, chemical plants, and steel mills—contribute to thermal pollution. They also use water to cool machinery and discharge water at elevated temperatures. 

This process of sucking in water from a lake, ocean, or river for industrial purposes and then releasing heated water back to its source is called once-through cooling. It has long been known to adversely affect aquatic and marine environments. Because of once-through cooling, fish and larvae that get trapped against intake screens are killed, and habitats are altered due to the discharge of warmer, often polluted water.  

Desalination Plants

Desalination plants also utilize once-through cooling. More than half of the seawater used in desalination gets flushed back into the ocean as wastewater, often at an elevated temperature.

In some parts of the world, desalination plants are clustered together, pouring massive amounts of heated, briny wastewater into shallow coastal regions. This can raise seawater temperature and salinity significantly.

Wastewater, Erosion, and Deforestation

Not all wastewater gets treated before it is released into water bodies. Untreated sewage, urban stormwater, and agricultural runoff can create thermal pollution in nearby water sources as the runoff is often warmer than the streams, lakes, or oceans they flow into.

Human land-use changes cause thermal pollution as well. Deforestation for timber harvesting or clearing land for crops and livestock grazing provokes erosion alongside rivers and streams, which leads to wider, shallower stream beds more prone to warming. In addition, clearing trees and vegetation from lakeshores and riverbanks creates more sun exposure, provoking water warming.

Impacts of Thermal Pollution

When heat moves rapidly into a water source, it generates direct and indirect environmental effects. Aquatic organisms can be highly sensitive to even small changes in water temperature. Some are unable to cope, suffering stress, disease, and even death. When populations of fish and other organisms decline, it may provoke ripple effects through the ecosystem. 

Thermal pollution also alters oxygen levels. The introduction of warmer water causes oxygen levels to drop, affecting aquatic life. Warmer water encourages the growth of algae, which absorb sunlight and cause further warming. These effects are often intensified if the discharge water contains a lot of nutrients, as is the case with agricultural runoff and untreated sewage. Warmer temperatures can increase aquatic organisms' vulnerability to chemicals present in these wastewaters, such as ammonia, heavy metals, and pesticides. Together, thermal pollution and nutrient loading can cause hypoxic “dead zones,” with very low oxygen levels. 

Examples of Thermal Pollution

In a 2016 global analysis of thermal pollution in rivers, the Mississippi River topped the list. Sixty-two percent of the river's heat emissions came from coal-fired power plants, and 28% from nuclear power generation. Other thermal pollution sources included agricultural runoff and wastewater. Europe's Rhine River also suffered significant impacts from power plant emissions, most notably from nuclear plants.

Water-scarce countries, particularly in the Middle East, have turned to desalination as a means of bolstering water security in the face of drought and climate change. A 2020 study of desalination plants in Ashkelon and Hadera along Israel's Mediterranean coast found that the mixing of cooling water to dilute briny wastewater created a heated plume 25% warmer than the natural seawater temperature, placing stress on local benthic organisms near the seafloor.

At California's last operating nuclear power plant, Diablo Canyon near San Luis Obispo, opponents have long raised concerns about the ecosystem impacts of the plant discharging millions of gallons of heated seawater each day back to the ocean. In 2021, PG&E, the utility that owns Diablo Canyon, reached a $5.9 million settlement agreement with the state over violation of permits meant to limit thermal pollution.

Mitigation of Thermal Pollution

Thermal pollution is a growing concern, especially with climate change exacerbating increases in water temperature from power plants, industry, agriculture, and other human sources. As of 2013, about a third of total power generation in the U.S. came from power plants that used once-through cooling. This is typical of older energy generation facilities.

A 2016 study found that half of the global freshwater heat emissions come from nuclear and coal plants from the 1970s and 1980s. Some older power plants that utilize once-through cooling are shutting down as aging equipment and increasing restrictions on air and water pollution, water consumption, and thermal discharge decrease profitability and increase liability. 

In the U.S., thermal pollution is regulated by the federal Clean Water Act, which requires states to set limits for thermal discharges from power plants to protect aquatic organisms and wildlife. Power plants must meet temperature discharge standards to qualify for the permits, or alternatively, prove that the discharge temperature has no adverse environmental effects. However, this doesn’t always happen in practice.

There is now a shift away from once-through cooling—not only because of thermal pollution but because of the overall strain that water-intensive processes place on aquatic and marine life, as well as increasingly scarce water resources. In 2010, California enacted a regulation to phase out once-through cooling at coastal power plants that use ocean water. 

Existing and emerging technologies provide other means of mitigating thermal pollution from power plants and other industrial sources. These include reducing the amount of water released by such facilities and capturing heated wastewater for other purposes, like desalination, to reduce discharges.