Acid and Mercury Deposition

Emissions resulting from the combustion of fossil fuels are comprised of a wide range of chemical compounds. Some of these compounds, such as water vapor, are relatively harmless, but others may be more damaging to the environment. For example, mercury (Hg), sulfur dioxide (SO2), and oxides of nitrogen (NOX) are also emitted and can travel with weather patterns and be deposited downwind from the emissions source. SO2 and NOX react with other molecules in the atmosphere to form acids. Following the weather patterns, these compounds eventually fall out of the atmosphere as rain, snow, or even dust. The phenomenon is often called “acid rain” but is more accurately described as “acidic deposition.” Similarly, Hg being deposited from the atmosphere, in one form or another, is called “mercury deposition.” Each of these compounds has unique properties and contributes to its own type of environmental degradation.

Acid Deposition and its Effect on the Environment

While the northeastern United States enjoys a relative abundance of freshwater, the quality of our water has degraded over the past century as a consequence of human activities. The deposition of acids is an important cause of this decline. Acid deposition is related to the combustion of fossil fuels, such as gasoline, oil, and coal, which releases sulfur dioxide (SO2) and nitrogen oxides (NOX) into the atmosphere. These gases mix with water vapor in the air and react to form acidic compounds, which fall to earth as rain or snow, sometimes hundreds of miles from their source. Over time, acid deposition, which includes acid rain, snow, and other kinds of acidic inputs, affects lakes and rivers. From decades of acid deposition, the acidity in many bodies of water across the Northeast become too acidic to support aquatic life.  However, emissions of these pollutants began to decline in the 1990’s and the acidity has begun to decline in some water bodies.  This “recovery” is not uniform and may take decades or even centuries for some water bodies to recover and support any biological recovery.  Forests and forest soils have been much slower to respond to acid deposition and may respond differently.  Research continues to better understand the chemical and biological recovery of lakes, streams and forests from acid deposition and how climate change might alter this recovery.

Mercury, Human Health, and the Environment

Mercury is a toxic pollutant that accumulates in fish and affects humans and animals that eat fish. Two-thirds of the mercury in the world is released into the environment by human activities, such as electricity production and waste incineration. As this mercury is released into the air, it enters remote forested watersheds through atmospheric deposition.

Even small levels of mercury can pose serious health and environmental risks. In 2003, 44 states and American Samoa issued fish consumption advisories because of mercury contamination. From 2002 to 2003, the number of river miles with advisories increased by 60%. According to the U.S. Environmental Protection Agency (EPA), warnings not to eat fish have been issued for nearly one out of every four American rivers and one out of every three lakes. There are currently 104 fish consumption advisories due to mercury for specific waters in New York State.

While fish mercury concentrations have generally declined in recent decades consistent with regional declines in mercury emissions and deposition in the eastern U.S., changes have also been linked with other disturbances including land use, nutrient supply, climate change and changes in air pollution. Inconsistent patterns were found in mercury concentrations in yellow perch in Adirondack lakes, and in different fish species for additional lakes across New York.  Research into patterns and trends in mercury deposition, cycling and bioaccumulation are ongoing.