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Acid Rain: A Serious Regional Environmental Problem

Gene E. Likens¹,

F. Herbert Bormann

At present, acid rain or snow is falling on most of the northeastern United States. The annual acidity value averages about pH 4, but values between pH 2.1 and 5 have been recorded for individual storms. The acidity of precipitation in this region apparently increased about 20 years ago, and the increase may have been associated with the augmented use of natural gas and with the installation of particle-removal devices in tall smokestacks. Only some of the ecological and economic effects of this widespread introduction of strong acids into natural systems are known at present, but clearly they must be considered in proposals for new energy sources and in the development of air quality emission standards.

Acid rain is a popular term for the atmospheric deposition of acidified rain, snow, sleet, hail, acidifying gases and particles, as well as acidified fog and cloud water. The increased acidity of these depositions, primarily from the strong acids, sulfuric and nitric, is generated as a by-product of the combustion of fossil fuels containing sulfur or nitrogen, especially electrical utilities (power plants.) The heating of homes, electricity production, and driving vehicles all rely primarily on fossil fuel energy. When fossil fuels are combusted, acid-forming nitrogen and sulfur oxides are released to the atmosphere. These compounds are transformed chemically in the atmosphere, often traveling thousands of kilometers from their original source, and then fall out on land and water surfaces as acid rain. As a result, pollutants from power plants in New Jersey, Ohio or Michigan can impact forests, rivers or lakes in less developed parts of New Hampshire or Maine.

Acid rain was discovered in 1963 in North America at the Hubbard Brook Experimental Forest, in the White Mountains of New Hampshire at the initiation of the Hubbard Brook Ecosystem Study. The first sample of rain collected there had a pH of 3.7, some 80 times more acidic than unpolluted rain. Innovations for reducing fossil fuel emissions, such as scrubbers on the tall smoke stacks on power plants and factories, catalytic converters on automobiles, and use of low-sulfur coal, have been employed to reduce emissions of sulfur dioxide (SO₂) and nitrogen oxides (NO_x). As a result of increasing global economies, fossil fuel combustion is increasing around the world, with concomitant spread of acid rain, especially in areas such as China.

Precipitation chemistry

The source of the acids in the atmosphere is largely the result of the combustion of fossil fuels that produce waste by-products including gases such as oxides of sulfur andnitrogen. Oxidized sulfur and nitrogen gases are acid precursors in the atmosphere. For example, SO₂ reacts with water in the atmosphere to yield sulfuric acid:

SO₂ + H₂O + ЅO₂ = H₂SO₄

An analogous reaction of water with nitrogen oxides, symbolized as NO_x, yields nitric acid (HNO₃).

Ammonia (NH₃) is a by-product of some natural processes, as well as agriculturalsources (e.g., application of nitrogen-containing fertilizers; emissions from intensive animal feedlots, such as decomposition of organic matter). In its dissolved form, ammonium (NH₄⁺) contributes acidity to surface waters through the process of nitrification.

In addition to wet deposition (rain, snow, and fog), acidic deposition includes the deposition of dry, particulate, and gaseous acid precursors that become acidic in contact with water. This dry deposition is difficult to quantify and expensive to measure. Inferential methods indicate that dry deposition represents 20% to 80% of the total deposition of acids to the landscape, depending on factors such as location, season, and total rainfall.

Natural sources can also contribute additional acidity to precipitation. Natural emissions can come from wetlands and geologic sources. Major natural sources of NO_x include lightning and soil microbes. Organic acidity may arise from freshwater wetlands and coastalmarshes.

It is those natural sources that lead to the inference that pre-industrial precipitation in forested regions had a pH around 5.0. If true, then modern precipitation in the North and East is two to three times more acidic than pre-industrial.

The acidity of precipitation is still subject to misunderstanding. Even in pristine environments, precipitation pH is rarely controlled by the carbon dioxide (CO₂) reaction that has an equilibrium pH of 5.6:

H₂O + CO₂ = H₂CO₃

Because of the many sources of acidity in precipitation, pH 5.6 is not the benchmark `normal' pH against which the acidity of modern precipitation should be compared. Precipitation is a variable and complex mixture of particulates and solutes derived from local sources and long-range transport. For example, in arid or partly forested regions, dust from soil and bedrock typically neutralizes both the natural and human sources of acidity in precipitation, yielding a solution that may be quite basic (pH greater than 7). In the northeastern U.S. and eastern Canada, annual precipitation pH ranges from 4.3 in Pennsylvania, New York, and Ohio, to 4.8 in Maine and maritime Canada.

Effects on surface water quality

Lake acidification begins with the deposition of the byproducts acid precipitation (SO₄ and H ions) in terrestrial areas located adjacent to the water body. Hydrologic processes then move these chemicals through soil and bedrock where they can react with limestone and aluminum-containing silicate minerals. After these chemical reactions, the leachate continues to travel until it reaches the lake. The acidity of the leachate entering lake is controlled by the chemical composition of the effected lake's surrounding soil and bedrock. If the soil and bedrock is rich in limestone the acidity of the infiltrate can be reduced by the buffering action of calcium and magnesium compounds. Toxic aluminum (and some other toxic heavy metals) can leach into the lake if the soil and bedrock is rich in aluminum-rich silicate minerals. (Source:PhysicalGeography.net)

Surface water chemistry is a direct indicator of the potential deleterious effects of acidification on biotic integrity. Because surface water chemistry integrates the sum of processes upstream in awatershed, it is also an indicator of the indirect effects of watershed-scale impacts, such as nitrogen saturation,forest decline, or soil acidification.

Acid deposition degrades water quality by lowering pH levels (i.e., increasing acidity); decreasing acid-neutralizing capacity (ANC); and increasing aluminum concentrations. A recent survey in the Northeast concluded that 41 percent of lakes in the Adirondack region are still acidic or subject to short-term pulses in acidity associated with snowmelt or rain storms. In the Catskill region and New England as a whole, 15 percent of lakes exhibit these characteristics. Eighty-three percent of the impacted lakes are acidic due to acid deposition. The remaining 17 percent are probably acidic under natural conditions, but have been made more acidic by acid deposition. This survey presents a conservative estimate of lakes impaired by acid deposition. Data were collected from lakes that are one hectare or larger and included only samples that were collected during the summer, when conditions are relatively less acidic.

Stream data from the Hubbard Brook Experimental Forest, New Hampshire (HBEF) reveal a number of long-term trends that are consistent with trends in lakes and streams across the Northeast. Specifically, the concentration of sulfate in streams at the HBEF declined 20 percent between 1963 and 1994. The pH of streams subsequently increased from 4.8 to 5.0. Although this represents an important improvement in water quality, streams at the HBEF remain acidic compared to background conditions, estimated to be above 6.0. Moreover, a lake or stream's susceptibility to acid inputs - has not improved significantly at the HBEF over the past thirty years.

Chronic acidification

Surface waters become acidic when the supply of acids from atmospheric deposition and watershed processes exceeds the capacity of watershed soils and drainage waters to neutralize them. Surface waters are defined as `acidic' if their acid neutralizing capacity (ANC, analogous to alkalinity) is less than 0, corresponding to pH values less than about 5.2.

The chemical conditions that define acidity are that acid anion concentrations (sulfate, nitrate, organic acids) are present in excess of concentrations of base cations (typically calcium or magnesium), the products of mineral weathering reactions that neutralize acidity in soil or rock.

The National Surface Water Survey (NSWS) in the United States documented the status and extent of chronic acidification during probability surveys conducted from 1984 through 1988 in acid-sensitiveregions throughout the U.S. The NSWS estimated the chemical conditions of 28,300 lakes and 56,000stream reaches in all of the major acid-sensitive regions of the U.S.

The NSWS concluded that 4.2% of lakes larger than 4 hectares and 2.7% of stream segments in the acid-sensitive regions were acidic. The regions represented in that report are estimated to contain 95% of the lakes and 84% of the streams that have been anthropogenically acidified in the U.S. The Adirondacks had the largest proportion of acidic surface waters (14%) in the NSWS. The proportions of lakes estimated by NSWS to be acidic were smaller in New England and the Upper Midwest (5% and 3%, respectively), but the large numbers of lakes in these regions translate to several hundred acidic waters in each region.

The Valley and Ridge province and Northern Appalachian Plateau had 5% and 6% acidic sites, respectively. The only acid-sensitive region not assessed in the current report is Florida, where the high proportion of naturally acidic lakes, and a lack of long-term monitoring data, make assessment problematic.

A recent survey in the Northeastern U.S. concluded that 41 percent of lakes in the Adirondack Mt. region are still acidic or subject to short-term pulses in acidity associated with snowmelt or rain storms. In the Catskill region of New York and in New England, 15 percent of lakes exhibit these characteristics. Eighty-three percent of the impacted lakes are acidic due to acid deposition. The remaining 17 percent are probably acidic under natural conditions, but have been made more acidic by acid deposition. This survey presents a conservative estimate of lakes impaired by acid deposition. Data were collected from lakes that are one hectare or larger and included only samples that were collected during the summer, when conditions are relatively less acidic.

Stream data from the Hubbard Brook Experimental Forest, New Hampshire (HBEF) reveal a number of long-term trends that are consistent with trends in lakes and streams across the Northeastern U.S. Specifically, the concentration of sulfate in streams at the HBEF declined by about 45% between 1963-2008. The pH of streams subsequently increased from 4.85 to 5.15. Although this represents an important improvement in water quality, streams at the HBEF remain acidic compared to background conditions, estimated to be above 6.0. Moreover, ANC - an important measure of a lake or stream's susceptibility to acid inputs - has only improved slightly at the HBEF over the past forty-five years.

Biologically-relevant surface water chemistry

The main cause for concern over the effects of surface water acidification in the U.S. and elsewhere is the potential for detrimental biological affects. Typically, there is concern for biological impact if the pH is less than 6. At low pH values, aluminum may be present at concentrations that are toxic to biota, including sensitive life stages of fish and sensitive invertebrates. Aluminum is an abundant and normally harmless component of rocks and soils. However, it leaches from silicate minerals when they come in contact with low-pH waters. While much of the aluminum present in surface waters is organically-bound and relatively non-toxic, certain inorganic species are highly toxic. The best indicator of recovery in biologically-relevant chemistry would be a decrease in concentrations of inorganic monomeric aluminum, the most toxic form. Decreases in total aluminum would also suggest recovery, although the actual magnitude of the improvement in chemical conditions for biota would be unknown because we would not know how much of the decrease is due to inorganic vs. organic forms of aluminum.

Biological effects of acid rain

Effects on forest ecosystems

The 1990 National Acid Precipitation Assessment Program report to Congress concluded there was insubstantial evidence that acid deposition had caused the decline of trees other than red spruce growing at high elevations. More recent research shows that acid deposition has contributed to the decline of red spruce trees throughout the eastern U.S. and sugar maple trees in central and western Pennsylvania. Symptoms of tree decline include poor crown condition, reduced tree growth, and unusually high levels of tree mortality. Red spruce and sugar maple are the species that have been the most intensively studied.

Red Spruce

Since the 1960s, more than half of large canopy red spruce in the Adirondack Mountains of New York and the Green Mountains of Vermont and approximately one quarter of large canopy red spruce in the White Mountains of New Hampshire have died. Significant growth declines and winter injury to red spruce have been observed throughout its range. Acid deposition is the major cause of red spruce decline at high elevations in the Northeast. Red spruce decline occurs by both direct and indirect effects of acid deposition. Direct effects include the leaching of calcium from a tree's leaves and needles (i.e., foliage), whereas indirect effects refer to changes in the underlying soil chemistry.

Recent research suggests that the decline of red spruce is linked to the leaching of calcium from cell membranes in spruce needles by acid rain, mist or fog. The loss of calcium renders the needles more susceptible to freezing damage, thereby reducing a tree's tolerance to low temperatures and increasing the occurrence of winter injury and subsequent tree damage or death. In addition, elevated aluminumconcentrations in the soil may limit the ability of red spruce to take up water and nutrients through its roots. Water and nutrient deficiencies can lower a tree's tolerance to other environmental stresses and cause decline.

Sugar Maple

The decline of sugar maple has been studied in the eastern United States since the 1950s. Extensive mortality among sugar maples in Pennsylvania appears to have resulted from deficiencies of base cations, coupled with other stresses such as insect defoliation or drought. According to research studies, the probability of the loss of sugar maple crown vigor or the incidence of tree death increased on sites where supplies of calcium and magnesium in the soil and foliage were the lowest and stress from insect defoliation and/or drought was high. In northwestern and north central Pennsylvania, soils on the upper slopes of unglaciated sites contain low calcium and magnesium supplies as a result of more than half a million years of weathering combined with the leaching of these elements by acid deposition. Low levels of these base cations can cause a nutrient imbalance and reduce a tree's ability to respond to stresses such as insect infestation and drought.

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