8.6 - Thermal Pollution

Learning Objectives

I. Temperature and Dissolved Oxygen (DO)

• Solubility: Solubility is the ability of a solid, liquid, or gas (like oxygen) to dissolve into a liquid (water). Aquatic organisms like fish need to pull oxygen out of the water through their gills.

• Inverse relationship: There’s an inverse relationship between the temperature of a water source and the oxygen in that water source. As water temperature increases, the dissolved oxygen level decreases.

• Mechanism: Gases dissolve better in cold water than in warm water. When water warms up, its molecules (and the dissolved gas molecules) move faster, so more oxygen molecules have enough energy to escape from the water into the air. You can see dissolved gas leaving when you heat a pot of water: tiny bubbles form on the bottom and sides well before it boils. Those early bubbles are dissolved air, including oxygen, coming out of the water. The big bubbles at a rolling boil are different: they’re water vapor (steam), not oxygen.

• By the numbers: Fresh water at sea level can hold about 14.6 mg/L of oxygen at 0°C, 11.3 mg/L at 10°C, 9.1 mg/L at 20°C, and only 7.6 mg/L at 30°C. So if a power plant warms a river from 15°C to 25°C, the most oxygen the water can hold drops from about 10.1 to 8.3 mg/L, a loss of about 18%.

II. Sources and Effects of Thermal Pollution

• Definition: Thermal pollution occurs when hot water is released into a cooler body of water, which brings up the temperature and consequently brings down the dissolved oxygen level.

• Physiological effects:
• When less oxygen is available, organisms experience an increase in their rate of respiration. They try to pull in breaths more frequently, or pass water over their gills more frequently if they’re fish.
• That increased respiration rate causes physiological stress.
• If the dissolved oxygen level gets low enough, organisms can suffocate and die.
• A rapid temperature increase can cause thermal shock in the organisms living in the water body.
• Warm water also speeds up the metabolism of fish and other ectotherms (cold-blooded animals), so they need more oxygen at the same time the water holds less. That double hit is why thermal pollution is so harmful.

• Major sources:
• Power plants (biggest source): Power plants use cool water from nearby surface waters to turn into steam, which powers a generator, or to cool down hot machinery. The heated water is then released back into the surface body of water.
• Nuclear power plants: Especially big sources of thermal pollution, thanks to the intense heat released by nuclear fission. They use a ton of water to cool down the reactor core and prevent overheating.
• Example: France limits how much a power plant’s discharge water can warm a river. During heat waves, like the ones in 2003 and 2022, river water got so warm that EDF, France’s main power company, had to cut output at nuclear plants on the Rhône and Garonne rivers to protect fish.
• Industrial facilities: Large manufacturing facilities like steel mills or paper mills also use water to cool machinery and release the warmer water back into surface water.
• Urban runoff: Runoff water that passes over large areas of blacktop in parking lots can get warm in the sun. That warmer water then drains into streams and rivers, causing thermal pollution.

III. Mitigation via Cooling Towers

• Function: A cooling tower is a device used to cool water coming from industrial processes or power plants, especially nuclear.

• The hot water is sprinkled across an exchange surface, and airflow takes some of the heat off the water. Cool water in the base also helps absorb heat.

• Goal: Lower the temperature of the water back down as close as possible to the temperature of the surface body of water before releasing it.

• Caveat: Cooling towers are pretty standard procedure, particularly in nuclear power plants. So a better solution to propose for environmental problems may be increasing the efficiency of cooling towers, or holding the water for a longer period before discharging it.

Big picture: Thermal pollution occurs when industrial processes, especially power plants (including nuclear) and manufacturing facilities, release heated water into surface ecosystems. It’s dangerous because the solubility of oxygen decreases as water temperature increases. That reduction in dissolved oxygen causes aquatic organisms to experience physiological stress, leading to high respiration rates and potential suffocation, which is why mitigation through cooling towers is needed to safely lower discharge temperatures.