Acid rain is often associated with images of damaged forests, eroded statues and polluted lakes. But its effects are not limited to landscapes or historic buildings. Changes in rainfall chemistry can alter freshwater ecosystems, mobilise toxic metals and make water treatment more challenging. In some catchments, acid deposition also interacts with other contaminants, including nutrients, industrial chemicals and persistent substances such as PFAS.
Understanding acid rain means looking beyond the rain itself. The real issue is the movement of pollutants through the atmosphere, soil and water cycle—and the long-term consequences for drinking water supplies and aquatic life.
What is acid rain?
Acid rain is precipitation that is more acidic than normal because it contains elevated levels of sulfuric and nitric acids. These acids form when sulfur dioxide (SO2) and nitrogen oxides (NOx) react with oxygen, water vapour and other chemicals in the atmosphere.
Rain is naturally slightly acidic. Carbon dioxide in the atmosphere dissolves in raindrops and forms weak carbonic acid. Clean rain typically has a pH of around 5.6, although the exact value varies according to local conditions. Acid rain generally refers to precipitation with a substantially lower pH, often below 5, caused mainly by human-generated air pollution.
The term “acid rain” is also used more broadly to describe acid deposition. This includes:
- Wet deposition: acidic rain, snow, sleet or fog.
- Dry deposition: acidic gases and particles that settle on buildings, soil, vegetation and water surfaces between rainfall events.
Dry deposition can later be washed into rivers and lakes by stormwater. In other words, a blue-sky day does not necessarily mean that an ecosystem is free from acidic pollution.
What causes acid rain?
The main chemical drivers are sulfur dioxide and nitrogen oxides. These pollutants are released when fossil fuels are burned, particularly coal and oil. Major sources include:
- Coal-fired power stations
- Industrial boilers and manufacturing facilities
- Metal smelting and refining
- Oil and gas combustion
- Road transport and shipping
- Large-scale agricultural and industrial combustion processes
Once released, sulfur dioxide can oxidise in the atmosphere and form sulfuric acid. Nitrogen oxides can undergo a series of reactions that produce nitric acid. Winds may carry these pollutants hundreds of kilometres, meaning emissions in one region can affect rainfall and water quality far away.
This long-range transport explains why acid rain has historically been a major transboundary issue in Europe and North America. A lake may be affected by emissions from distant power stations rather than from any pollution source located nearby.
Natural events can also contribute to atmospheric acidity. Volcanic eruptions release sulfur compounds, while wildfires emit nitrogen oxides and other gases. However, persistent regional acid deposition in industrialised areas has primarily been linked to human activity.
How acidic precipitation affects soil and forests
Soils do not all respond to acid deposition in the same way. Some contain alkaline minerals, such as calcium carbonate, that can neutralise acids. This buffering capacity protects nearby streams and groundwater. Other soils, especially those formed from granite or sandstone, have limited ability to neutralise acidity.
When acidic water passes through poorly buffered soils, it can leach essential nutrients such as calcium, magnesium and potassium. These nutrients are important for plant growth. Acidification can also release aluminium from soil minerals. Dissolved aluminium is toxic to many tree roots and aquatic organisms.
Forests exposed to acid deposition may become more vulnerable to drought, cold weather, pests and disease. Acid rain does not always kill trees directly. Instead, it can weaken them gradually by damaging roots, reducing nutrient availability and interfering with their ability to absorb water.
High-elevation forests can be particularly exposed because they receive acidic cloud and fog water as well as rainfall. Evergreen needles may capture pollutants efficiently, increasing the amount of acidic material reaching the forest floor.
Effects on lakes, rivers and wetlands
The clearest water-quality impact of acid rain is a fall in the pH of surface water. Some lakes and streams can absorb acidic inputs without a major change in pH. Others may acidify rapidly once their natural buffering capacity is exhausted.
As water becomes more acidic, several ecological effects can occur:
- Fish eggs may fail to hatch.
- Young fish and amphibians may experience impaired growth or reproductive problems.
- Sensitive species, including some aquatic insects and molluscs, may disappear.
- Food webs can become less diverse and less resilient.
- Aluminium can become more soluble and damage fish gills.
- Decomposing organic matter and nutrient cycling may be disrupted.
Fish are often affected not only by acidity itself but also by dissolved metals. Aluminium, for example, can accumulate on fish gills and interfere with oxygen uptake. During snowmelt or heavy rainfall, a sudden pulse of acidic water and dissolved metals can create particularly stressful conditions.
Some species are more tolerant than others. A lake may continue to look clear while losing sensitive organisms beneath the surface. Clear water is not necessarily healthy water—a useful reminder for anyone assessing environmental quality by appearance alone.
Acid rain and drinking water quality
Acid rain does not usually make drinking water “acidic” in the same way as a household product. Water treatment plants, groundwater minerals and natural buffering processes can substantially reduce changes in pH before water reaches consumers. Nevertheless, acid deposition can affect drinking-water sources through several pathways.
Metal mobilisation is one of the most important. As soils and sediments become more acidic, metals such as aluminium, iron, copper, lead and manganese may become more soluble. These substances can enter reservoirs, rivers and groundwater. The risk depends on local geology, land use, infrastructure and the effectiveness of treatment processes.
Acidic water can also be more corrosive to pipes and plumbing. Where water has low alkalinity, it may interact with metal distribution systems and increase the release of lead or copper. This is a water-quality concern because contamination can occur after treatment, inside the network or within household plumbing.
Water suppliers typically monitor pH, alkalinity, conductivity, turbidity, metals and other indicators. Treatment may include pH adjustment, alkalinity addition, filtration, coagulation or activated carbon, depending on the contaminants present and the characteristics of the source water.
Acidification can also change the behaviour of organic matter and other pollutants. When soil chemistry shifts, contaminants that were previously bound to particles may become more mobile. The result is not always predictable: some pollutants become easier to transport, while others may bind more strongly to certain minerals or organic materials.
Does acid rain affect PFAS in water?
PFAS are a separate class of contaminants from the acids associated with acid rain. They are highly persistent, widely used industrial chemicals found in products such as water-resistant textiles, firefighting foams, food packaging and some manufacturing materials.
Acid rain does not create PFAS, and lowering the pH of water does not destroy them. PFAS are chemically persistent and require specialised treatment approaches. However, changes in soil and water chemistry may influence how contaminants move through an environment. Acid deposition can alter mineral surfaces, organic matter and groundwater conditions, potentially affecting the transport of multiple pollutants within a catchment.
This is why water-quality assessment should not focus on a single contaminant in isolation. A source water affected by acidic drainage may also contain metals, nutrients, pesticides, industrial chemicals or PFAS. Each substance has different chemical properties and treatment requirements.
For PFAS removal, treatment options may include granular activated carbon, ion exchange and high-pressure membranes such as reverse osmosis. These technologies are not designed simply to neutralise acidity, so a treatment train may be needed when several water-quality problems occur together.
How scientists measure acidification
pH is an important starting point, but it does not tell the whole story. A water body with a pH of 6 may be more vulnerable than one with a pH of 5 if it has very little alkalinity and limited capacity to neutralise further acid inputs.
Researchers and water managers commonly examine:
- pH: a measure of acidity on a logarithmic scale.
- Alkalinity: the capacity of water to neutralise acids.
- Acid-neutralising capacity: an indicator of vulnerability to acidification.
- Sulfate and nitrate: useful indicators of atmospheric deposition and catchment processes.
- Calcium and magnesium: nutrients that may be depleted from soils.
- Aluminium and other metals: potential indicators of soil and sediment mobilisation.
- Biological indicators: fish populations, aquatic insects, amphibians and plant communities.
Long-term monitoring is essential. A single sample can capture conditions after a storm but miss seasonal trends. Measurements taken during snowmelt, drought or heavy rainfall can reveal how quickly a catchment responds to acidic inputs.
Has acid rain been reduced?
In many regions, air-quality regulations have significantly reduced sulfur dioxide emissions. The United States Clean Air Act and its Acid Rain Program helped drive major reductions, while European policies, including the Convention on Long-range Transboundary Air Pollution, contributed to lower sulfur emissions across Europe.
These improvements have produced measurable environmental benefits. Some lakes and streams have shown chemical recovery, and sulfur deposition has declined substantially in many monitored areas. However, recovery is often slow because soils may have lost essential base cations and ecosystems can retain an “acidification legacy” for decades.
Nitrogen pollution remains a concern in many regions. Nitrogen oxides from transport and combustion, together with ammonia from agriculture, can contribute to acidification and nutrient enrichment. These pollutants may also drive eutrophication, which occurs when excessive nutrients promote algal growth and reduce oxygen levels in water.
Climate change adds another layer of complexity. More intense rainfall can increase runoff and erosion, while drought can concentrate pollutants and reduce dilution. Wildfires may alter soil chemistry and generate pulses of metals and nutrients when burned landscapes receive heavy rain.
What can be done to protect water quality?
The most effective response is to reduce pollution at its source. This includes improving energy efficiency, replacing high-sulfur fuels, expanding renewable energy, controlling industrial emissions and reducing vehicle pollution. Strong emissions standards remain essential because atmospheric pollution does not respect administrative boundaries.
At the catchment level, practical measures can include:
- Monitoring vulnerable lakes, rivers and groundwater sources.
- Protecting wetlands and forest soils that help regulate water chemistry.
- Managing stormwater and erosion from disturbed land.
- Testing private wells in areas with acidic soils or mining activity.
- Controlling corrosion within drinking-water distribution systems.
- Using treatment systems suited to the contaminants actually present.
- Combining pH, metal and PFAS testing where multiple pollution sources are possible.
For households, unusual changes in taste, colour or staining may justify professional water testing, but appearance alone cannot identify acidification or PFAS. Accredited laboratory analysis is the reliable route. Private well owners should be especially attentive because wells are not always covered by the same monitoring requirements as public supplies.
Why acid rain still matters
Acid rain is a strong example of how air pollution becomes a water-quality problem. Emissions released from a chimney, vehicle or industrial site can travel through the atmosphere, enter soils and eventually affect rivers, lakes and drinking-water sources.
Regulation has reduced acid deposition in many countries, but the environmental recovery is incomplete. Sensitive ecosystems remain vulnerable, and new pressures—including climate change, wildfire, urban runoff and persistent chemical contamination—can complicate the picture.
Protecting water quality therefore requires more than checking whether rain feels acidic. It means tracking the chemistry of entire catchments, understanding how pollutants interact and investing in prevention as well as treatment. Clean air and clean water are not separate objectives; they are two parts of the same environmental system.
Sources and further reading
- UK Government, Department for Environment, Food & Rural Affairs: air pollution and acid deposition information.
- US Environmental Protection Agency: Acid Rain Program and effects of acid rain.
- European Environment Agency: air pollutant emissions, deposition and ecosystem impacts.
- United Nations Economic Commission for Europe: Convention on Long-range Transboundary Air Pollution.
- US Geological Survey: water chemistry, acidification and dissolved metals.