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examBoard: Cambridge
examType: IGCSE
lessonTitle: Acid Rain Effects on Water Bodies
Acid rain is one of the most significant forms of water pollution affecting our planet. It occurs when air pollution, mainly from burning fossil fuels, mixes with water in the atmosphere and falls as acidic precipitation. This session explores how acid rain forms and its damaging effects on lakes, rivers and the creatures that live in them.
Key Definitions:
Normal, unpolluted rain has a pH of about 5.6, making it slightly acidic due to naturally occurring carbon dioxide forming carbonic acid. Acid rain, however, can have a pH as low as 4.3 or even lower in severe cases.
Sulphur dioxide (SO2) - Released when coal and oil are burned in power stations and factories. When mixed with water, oxygen and other chemicals, it forms sulphuric acid.
Nitrogen oxides (NOx) - Released from vehicle exhausts and power plants. These combine with water to form nitric acid.
1. Pollutants are released into the atmosphere
2. They react with water, oxygen and other chemicals to form acidic compounds
3. These compounds fall to Earth as wet deposition (rain, snow, fog) or dry deposition (gases and particles)
4. The acidic substances then enter water bodies directly or through runoff
When acid rain falls into lakes, rivers and streams, it can drastically alter their chemistry and harm the organisms living there. The severity of impact depends on the water body's buffer capacity, which varies based on surrounding soil and bedrock composition.
As acid rain enters water bodies, several chemical changes occur:
Acid rain lowers the pH of water bodies, sometimes to levels below 5.0. Most aquatic organisms thrive in water with pH levels between 6.5 and 8.5, so even small changes can be harmful.
Acidic water causes aluminium to leach from soils into water bodies. Aluminium is toxic to many aquatic organisms, particularly fish, even at low concentrations.
Acid rain disrupts nutrient cycles in water bodies, affecting the availability of essential elements like calcium. This impacts both plant growth and animal development.
The effects of acid rain cascade through aquatic ecosystems, harming organisms at all levels of the food web:
Direct effects: Damage to gills, making it difficult to extract oxygen from water
Reproductive failure: Eggs and young fry are particularly sensitive to acidity
Aluminium toxicity: Causes mucus to form on gills, suffocating fish
Population decline: Some species disappear entirely from affected lakes
Invertebrates: Many sensitive species like mayflies and freshwater shrimp disappear
Amphibians: Frogs and salamanders suffer developmental abnormalities
Algae and plants: Changes in species composition, with acid-tolerant species dominating
Microorganisms: Reduced decomposition rates affecting nutrient cycling
As lakes become more acidic, they undergo a process sometimes called "biological death" - a gradual loss of biodiversity:
Sweden has been particularly affected by acid rain, with thousands of lakes damaged. In the 1970s, scientists discovered that about 18,000 Swedish lakes had become acidified. Fish populations, especially brown trout and roach, declined dramatically or disappeared entirely from many lakes.
The acidification was traced to industrial emissions from the UK, Germany and other parts of Europe, carried by prevailing winds. This demonstrated how acid rain is a transboundary pollution issue requiring international cooperation.
Sweden began an extensive liming programme, adding limestone to affected lakes to neutralise acidity. While this helped restore some lakes, it requires continuous application and doesn't address the root cause of the problem.
Not all water bodies are equally vulnerable to acid rain. The impact depends largely on the surrounding geology:
Granite bedrock regions: Parts of Scotland, Scandinavia and northeastern North America
Thin soils: Mountainous regions with little soil to buffer acidity
Snowmelt areas: Where accumulated winter acids are released in spring thaws
Limestone regions: Areas with calcium carbonate-rich bedrock naturally buffer acidity
Deep soils: Thick soil layers can neutralise some acid before it reaches water bodies
Areas with alkaline dust: Some regions receive natural alkaline dust that counteracts acidity
Scientists use several methods to track acid rain and its impacts on water bodies:
Addressing acid rain requires action at multiple levels:
Reducing emissions of SO2 and NOx through cleaner energy sources, emission controls on power plants and catalytic converters on vehicles.
International agreements like the Convention on Long-Range Transboundary Air Pollution (1979) and national legislation such as the UK Clean Air Acts and US Clean Air Act Amendments.
Adding limestone (calcium carbonate) to acidified lakes to neutralise acidity a process called liming. This is a temporary solution that must be repeated regularly.
The US Acid Rain Program, established under the 1990 Clean Air Act Amendments, created a cap-and-trade system for SO2 emissions. Power plants were given allowances for emissions, which they could trade if they reduced emissions below their allowance.
Results have been impressive: Between 1990 and 2019, SO2 emissions from power plants fell by more than 90%. Many lakes and streams in the northeastern US and eastern Canada have shown signs of recovery, with pH levels slowly rising and some fish populations returning.
This demonstrates that with proper policies and technologies, we can reverse some of the damage caused by acid rain.
When acid rain inputs are reduced, water bodies can gradually recover, but the process is slow and complex:
Some lakes may never fully recover to their pre-acidification state, especially if they've experienced severe or prolonged acidification. This highlights the importance of preventing pollution rather than trying to fix the damage afterwards.
Acid rain's effects on water bodies represent one of the clearest examples of how air pollution can damage distant ecosystems. The good news is that through international cooperation and effective policies, we've made significant progress in reducing acid rain in many regions. However, continued vigilance is needed, especially as developing countries increase their industrial activities.
Understanding the science behind acid rain and its impacts helps us make better decisions about energy use, industrial practices and environmental protection. By reducing pollutant emissions and protecting vulnerable water bodies, we can help ensure that aquatic ecosystems remain healthy for future generations.
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