"In the past 20 years, we have developed a better understanding of soil processes and how changes in soils are linked to changes in surface waters. Acid rain produces three primary changes in soil:

* depletion of calcium,

* mobilization of inorganic aluminum, and

* accumulation of sulfur and nitrogen.

Calcium is essential for wood formation in trees, and trees have a very high demand for calcium. Calcium in the soil also neutralizes acidity.

In the top layer of soil, the amount of calcium is very strongly related to the weathering potential of the local rocks. In the mineral horizon directly below the top layer, the correlation is much weaker. In the upper organic layer, very intense root activity recycles the calcium, preventing the calcium from leaching out. In the deeper soil, this process is not nearly as strong. As calcium is lowered, aluminum begins to be mobilized. The aluminum starts to control the chemistry and buffers any further decreases in calcium.

Aluminum can be mobilized by organic acids, forming organic aluminum, which is generally not toxic to aquatic life and does not impair tree growth. As the soil pH decreases, however, inorganic aluminum enters soil solution. At an aluminum-to-calcium ratio of greater than one, the growth of the tree is potentially impaired.

Nitrogen also is accumulating in forest soils to the point where some forests are not very effective at retaining nitrogen.... As the nitric acid leaches into the surface waters, it contributes to the calcium-depletion problem.

Factors other than acid rain can also change soils very significantly over decadal time scales. The changes in soils have reduced the stress tolerance of red spruce trees and contributed to the die-off of current-year needles during extremely cold periods as a result of the amount of calcium incorporated into the wood as the tree grows.

At the early states of acid rain, calcium actually was more available. The hydrogen ion was freeing up the calcium, making it easier for the tree roots to take it up. The higher calcium availability was reflected in the calcium in the tree tissue. With continued acidification of the soils, the calcium has leached out at a higher rate than the rate at which it was being added through the breakdown of the rocks and minerals. This situation has produced lower levels of available calcium, which is also reflected in the chemistry of the tree tissue.

Magnesium responds to acid rain leaching very similarly to calcium. If trees do not get enough calcium and magnesium, they are much more susceptible to stresses, such as defoliation from insects. If there is sufficient calcium and magnesium available to the foliage, the trees can recover from the defoliation. If there is not, they die. Acid Rain does not kill the trees directly; it reduces their ability to deal with stresses."

"Impacts arise because some areas have soil characteristics that make them sensitive to acid rain. Sensitive or not, soils are experiencing elevated accumulations of nitrogen and sulfur. When deposition is reduced, sulfur bleeds out of the soil. The sulfur that accumulated during the past 150 years represents a legacy that will endure. With the uptake of sulfur and nitrogen, soils exhibit a significant loss of nutrient cations, principally calcium and magnesium. The soil at Hubbard Brook, has lost more than half of its exchangeable, plant-available calcium. As the soil becomes more acidic, significant quantities of aluminum are leached out of the soil. That aluminum is toxic to plants and aquatic organisms.

In terms of the forest effects of acid precipitation, the Adirondacks and the Green Mountains have seen about a 50% loss of red spruce since 1980; and the White Mountains, about 25%. Acid rain causes the loss of membrane-bound calcium, which makes red spruce more susceptible to freezing damage. In the sugar maple, a loss of calcium and magnesium from the soil leads to less being taken up by the tree, making the tree more susceptible to insect infestation and drought stress.

Lakes and streams in high-elevation areas that have very shallow soils (and, subsequently, limited neutralization) are sensitive to acidification. Even lakes whose catchments have soils that can buffer the acid precipitation may become acidic during short periods, such as storm events or snowmelt.

Some lakes, though are chronically acidic. For the Adirondacks, about 40% of the chronically acidic lakes have been impacted by acid rain and, for the New England area, about 15%. With acidification of the lake water, the condition of the fish deteriorates, their numbers diminish, and species richness decreases.

A recovery of the calcium and magnesium base lines in soil will take a long time. Model calculations suggest that some sites in the Adirondacks are very sensitive and are still losing base resources."

"Of extreme concern are the chemical alterations of forest soils.... It has been estimated that, to eliminate this problem reductions in sulfur dioxide emissions from Canada as well as from the eastern U.S. on the order of about 75% would be needed.

"A calcium budget for lakes indicates that they are losing approximately 5 kg/hectare/yr. (Five pounds per acre per year.)

"Dendrogeo-chemistry has indicated that from 1920 to the mid 1950's iron and aluminum concentrations in wood did not change, but since 1950 iron and aluminum have increased."

"High-elevation spruce-fir forests are the most sensitive and we are seeing quite adverse consequences directly related to acidic deposition."

"Depletion of calcium from the soil is something we weren’t aware of in 1990. Calcium is essential for wood formation in trees. If they don’t have enough calcium, they don’t grow as fast. Calcium in the soil is the primary element that neutralizes acidity.

"Acid rain doesn't kill the sugar maple trees directly, but it reduces their ability to deal with stress."

"At the current levels for sensitive sites, we are talking many, many decades before we would see significant recoveries in chemical indicators that we would expect to lead to improvement in biological resources."

"The negative effects of acidification on fish are clear and unambiguous. Hydrogen ion, measured by low pH, is directly toxic to fish. Moreover, lower-pH waters leach aluminum from soils. Aluminum is quite lethal to fish and other aquatic organisms under acidic conditions, eliminating the most sensitive species first and lowering biodiversity. Fish can tolerate lower pH water if aluminum is absent.

There are about 30,000 miles of potential trout streams in Appalachia, about 30% of the probable historic trout streams in Virginia have become too acidic to support any fish.

For the Southern Appalachians, reductions in sulfur greater than 70% will be needed just to stop further losses, and very substantial reductions will be needed for recovery. For the Northeast the 1990 Clean Air Act amendments will not achieve chemical recovery of acidic streams."

"As sulfate deposition was declining significantly throughout the east, the estimated costs of clean-up were reduced from $7 or 9 billion down to $1 to 1.5 billion. When the Clean-Air Act is fully implemented health benefits will be about $50 billion per year."

"The compliance costs have been less than expected."

"Sulfur dioxide emissions in the Eastern U.S. are 92 to 95% of the nation’s total.

The mount of sulfur deposited in Ohio by wet deposition is about 10% of the amount that is emitted in Ohio." (It is wind blown into other states.)

"The clearly identifiable human health effects of ambient-air acidity follow their inhalation into the respiratory tract. In eastern North America, most of the acidity is associated with sulfuric acid aerosol, which is formed in the atmosphere as ultra fine particles (less than 0.1 microns in diameter) through the oxidation of sulfur dioxide emitted during fossil-fuel combustion. Those aerosols are gradually agglomerated into fine particles (between 0.2 and 1.0 microns in diameter) and also are neutralized to ammonium sulfate by reaction with ammonia gas, a product of anaerobic decay at ground level. Short-term peak exposures to acidic aerosols can cause asthma exacerbation and can decrease the lung’s ability to clear itself of inhaled particles. Long-term chronic exposures are closely associated with premature mortality, increased rates of emergency-room and hospital admissions, respiratory symptoms, and lost time from work and school. The people most affected lose perhaps 15 years of life span. Averaged over the entire US population, this is a loss of about 1.5 years in life span because of irritation to the cardiopulmonary system."