Air pollution: effects on soil and water

Air pollution can significantly hurt the quality of soil and water resources. When we pollute our air, we are also polluting the precipitation that falls into water bodies and soils. This is an overarching concern given that soil and water are cornerstones to all life on earth, as they provide the homes for most organisms and many of the nutrients, minerals and elements that are essential for growth and biological functions. The flow of water from the atmosphere, through soils and into water bodies unifies these two precious resources.

Soils

Acid precipitation can alter the chemistry of the soil, which, in turn, can affect plant growth and water quality in a number of ways.

As soils become more acidic, their ability to retain many essential nutrients, minerals and elements, such as calcium (Ca), magnesium (Mg) and potassium (K), decreases. As a result, these nutrients, minerals and elements are transported, or leached, by water that flows through the soil, making them less available for land organisms to use.

Similarly, increased soil acidity can increase the mobilization of heavy metals found within the soil, such as aluminum (Al). These metals are then more able to flow into lakes, rivers and streams. Like many heavy metals, aluminum is poisonous to fish and other wildlife and remains suspended in the open water at higher acidities.

Soils are not equal in their vulnerability or resistance to acid precipitation. For instance, soils with higher concentrations of calcium carbonate (including limestone and dolomite) are more resistant to acid rain because the calcium carbonate to chemically neutralize acids. This is why “liming” is used as an ecological restoration method to adjust the pH of lakes affected by acid rain.

Soils that contain lower levels of calcium carbonate and are particularly vulnerable to acid rain include those found on quartzite, gneiss, granite and other metamorphic rock deposits, such as found throughout the Canadian Shield. Thin soils, such as those in high alpine areas, are also vulnerable. Unfortunately, any soil's ability to buffer against acid rain decreases with ongoing exposure.

Soils are also involved in the movement of persistent organic pollutants (POPs) through a process called the grasshopper effect. This is of particular concern in the northern communities of the Arctic where these pollutants tend to concentrate, and the focus of the Northern Contaminants Program.

Water

The effect of air pollution on water isn't always visible. Some rivers, lakes or coastal areas may seem clean, but still be polluted because of acid precipitation from rain, snow and particulate matter. Or they may be indirectly harmed when nutrients, elements and heavy metals leach from soils and suspended within the water column. Many Canadians rely on ground water for their water supply, and the effects of pollution aren't necessarily immediate or easily identified.

Water bodies can experience short-term but dramatic acidification, such as when acid precipitation in the form of snow is suddenly released into the water system during the spring melt. This "acid shock" can be lethal for many aquatic organisms such as fish. Alternatively, water bodies can become more acidic over the long term from ongoing exposure to acid precipitation.

Lakes affected by acid precipitation are often clear because vegetation and aquatic wildlife cannot withstand a lower pH. This is the case for many lakes around Sudbury, Ontario.

But not all acidified water bodies are clear. For example, aquatic ecosystems that are deficient in nitrogen, such as coastal waterways and estuaries, can experience rapid and significant increases in plant and algal growth because of acid precipitation formed from the release of nitrogen oxides (NO_x). This excessive growth within a water body is called eutrophication. Significant increases in plant growth are not necessarily a good thing, as the eventual decomposition of plants can deplete the oxygen resources needed by other organisms in a lake or pond.