Definition: Acid deposition is the result of negatively charged particles, particularly sulphur dioxide (So2), nitrogen oxide (NOx) and ammonium (NH4) being discharged into the atmosphere where they react with various compounds and are transferred back to earth on water droplets in rain, sleet, snow or fog as "wet" deposition or attached to small airborne particles as "dry" deposition. Acid deposition can increase the acidity of soil and change the chemical balance of water bodies.
Acid deposition causes the low pH values generally less than 5.5 that are typical of acid-impacted waterbodies. These acidic waters act as leaching agents and dissolve naturally-occurring metals such as aluminum and manganese, which are commonly found in clays and soils. These metals in turn can stress or poison aquatic life and terrestrial plants.
The susceptibility of a watershed to acidification is related to the alkalinity of the watershed's underlying soil and rock. Alkalinity, or acid neutralizing capacity, refers to the ability to handle acid inputs. Limestone and other calcium and magnesium rich or "soft" rocks have relatively high alkalinity values, while granite, gneiss and schist and other "hard" rocks" have low alkalinity values. Much of the Appalachian highlands and much of the Adirondacks are composed of such "hard" rocks, and therefore they suffer greater impacts from acid deposition relative to other parts of the East.
Actual Ecosystem Stress: Acid deposition impacts forests and aquatic life directly. Acid rain flows to streams, lakes, and marshes after falling on forests, fields, buildings, and roads. Acid rain also falls directly on aquatic habitats. Most lakes and streams have a pH between 6 and 8, although some lakes are naturally acidic even without the effects of acid rain. Acid rain primarily affects sensitive bodies of water, which are located in watersheds whose soils and local geology have limited alkalinity, or the ability to neutralize acidic compounds, also called "buffering capacity". Lakes and streams become acidic (pH value goes down) when the water itself and its surrounding soil cannot buffer the acid rain enough to neutralize it. In areas where buffering capacity is low, acid rain also releases aluminum from soils into lakes and streams; aluminum is highly toxic to many species of aquatic organisms.
Acid deposition causing death of red spruce. Acid concentrations in precipitation tend to increase at higher elevations. Credit: NY State Dept. of Environmental Conservation
For forests, acidification of soils causes the eventual leaching of calcium and magnesium salts, which are replaced by aluminum ions. The loss of essential calcium ions and the toxicity of aluminum stunts plant growth and eventually can cause death.
Acidification of waters can be chronic or episodic. Chronically acidified waters have pH levels below 5.0 most of the time. Episodically acidified water suffer from pulses of acidic water during high runoff events. A classic example occurs when acidic snow accumulates over the winter, and during a spring melt or rain-on-snow effect. High volumes of surface runoff that do not have the opportunity to flow through the surrounding soil and be buffered flow into the stream system and essentially shock the aquatic ecosystem with a pulse of acidic water. Streams may register tolerable pH levels for much of the year, but episodic inputs of acidified water stress or eliminate aquatic life.
Low pH also adversely affects the growth rates and reproduction of fish. Although brook trout are tolerant of pH as low as 5.0, their tolerance is rapidly decreased when a low pH is present in combination with a heavy metal such as aluminum. Of the three main heavy metals typically found in AMD - aluminum, iron, and manganese - aluminum is the most toxic to aquatic life.
Acidic deposition also relates to nutrient pollution and eutrophication of water bodies (link to Energy in Streams) by serving as a source for nitrogen-based compounds that enter a water body from precipitation. The reactive nitrogen inputs then are taken up by algae and plant life, increasing biological activity, and often leading to algal blooms and associated decreases in dissolved oxygen.
Sources: Scientists have verified that the primary sources of acid deposition are sulfur dioxide (SO2) and nitrogen oxides (NOx). These pollutants enter the atmosphere from a mix of primarily manmade activities that involve the burning of fossil fuels. Cars, power plants and building heating are the main sources of these compounds. Ammonium is transferred to the atmosphere from the decomposition and volatization of animal wastes, including cattle, hogs and poultry. Natural sources include volcanic emissions, lightning and microbial processes. Once in the atmosphere, these compounds combine with water, oxygen and other chemicals to form mildly acidic compounds. Generation of sulphuric acid and nitric acids is increased by sunlight. These acidic compounds make their way back to land via "wet" or "dry" deposition.
Measures to Mitigate Acid Deposition: Acid deposition can ultimately only be stopped at its source - by greatly reducing or eliminating the discharge of acid deposition-causing pollutants in the atmosphere. Currently 14% of the Adirondack's 4,000 lakes are severely impacted by acid deposition, while 12% of mid-Atlantic Appalachian streams suffer significant impacts. But there is hope - the EPA's regulation of sulfur dioxide emissions from power plants under the Clean Air Act has shown some encouraging trends, and some lakes in the Adirondacks appear to be slowing benefiting from this effort with an increase in pH levels. Adirondacks Lake Survey; article on Adirondack Lakes
The main method of mitigating acid deposition in lakes and streams is through a process called liming, which periodically adds a basic mineral, often limestone sand, to an acidified stream or lake. Dissolution of the basic mineral raises the pH and allows life to return.
Liming of acidified waters can take a variety of methods. In wildnerness areas, limestone sand can be dumped by helicopter in intermittent tributaries that contribute runoff to streams during high rain or snowmelt events. On other streams or lakes, lime dosers dispense limestone sand periodically into waters.
|Examples of Restoration:|
|Left: Lime doser used to treat an acidified stream in Maryland. Liming is an effective method to mitigate streams suffering from both Acid rain and abandoned mine drainage. Credit: Alan Heft MD DNR.|
|Right: Limestone sand applied to a remote headwater area by helicopter. This method is used in federally designated wilderness areas where motorized vehicle use on the terrain is prohibited|