Acid Rain Continued


 

In the lab

Once precipitation samples are brought back from the field, technicians then analyze them for many different things - including compounds such as nitrate, elements such as carbon and nitrogen, conductivity, and pH.

The HBEF Pierce laboratory is equipped to measure pH and conductivity, though the samples must be shipped to different labs for other types of analyses. At the HBEF, each pH value is recorded, becomes part of the long-term dataset, and is eventually analyzed by scientists.

Some scientists examine acid precipitation patterns and trends, while others study its effects on animals, plants and trees, soils, or streams. And yet another type of scientist might examine effects on entire ecosystems - terrestrial, aquatic, or both. Over the course of the Hubbard Brook Ecosystem Study, these scientists have learned much about the effects of acid precipitation.


HBEF acid precipitation trends

Scientists have measured the pH of precipitation falling on Watershed 6 (the reference watershed) and in other parts of the HBEF since 1964. While these data show that precipitation has become slightly less acidic over the past 40 years, the pH is still ~ 10 times lower than naturally occurring precipitation

The data used to produce these graphs were provided by Gene E. Likens with funding from the National Science Foundation and The Andrew W. Mellon Foundation.

Effects of acid precipitation


Acid precipitation has altered soils in areas of the Northeast

  • Acid deposition has increased the leaching of base cations from soils — elements such as calcium and magnesium that are important plant nutrients and help soils counteract negative effects of acid precipitation. For example, the amount of available calcium in the soil at the HBEF appears to have declined more than 50 percent over the past several decades. (Click here to read about a HBEF watershed calcium experiment.)
  • High levels of sulfuric and nitric acids in precipitation have resulted in accumulation of sulfur and nitrogen in forest soils across the region.
  • Acid deposition has increased the concentration of dissolved inorganic aluminum in soil water. Dissolved inorganic aluminum is an ecologically harmful form of aluminum. At high concentrations, aluminum can hinder the uptake of water and essential nutrients by tree roots.
  • Soils that are affected by acid deposition are less able to neutralize additional amounts of acid deposition, provide poorer growing conditions for plants, and delay ecosystem recovery.

Acid precipitation has stressed trees in areas of the Northeast

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 tree 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, and research has shown that:

  • Acid deposition leaches essential calcium from needles (i.e., foliage) of red spruce, making them more susceptible to freezing injury. Increased freezing injury has lead to the mortality of more than half of large canopy red spruce trees in some forests in the Northeast.
  • Mortality among sugar maples in Pennsylvania appears to result from deficiencies of base cations (in soil and trees) combined with other stresses such as insect defoliation or drought.

Acid deposition has impaired lakes and streams in the Northeast.

Acid deposition has impaired the water quality of lakes and streams in three important ways: lowering pH levels (i.e., increasing the acidity); decreasing acid-neutralizing capacity (ANC); and increasing aluminum concentrations. High concentrations of aluminum and increased acidity have reduced the species diversity and abundance of aquatic life in many lakes and streams in the Northeast. Fish have received the most attention to date, but entire food webs are often negatively affected. Recent water quality data show that:

  • 41 percent of lakes in the Adirondack Mountain region of New York and 15 percent of lakes in New England exhibit signs of chronic and/or episodic acidification.
  • Only modest improvements in ANC, an important measure of water quality, have occurred in New England. No significant improvement in ANC has been measured in the Adirondack or Catskill Mountains of New York.
  • Elevated concentrations of aluminum have been measured in surface waters throughout the Northeast.

The Clean Air Act and its Amendments

Research at the HBEF has directly affected the Clean Air Act and its Amendments (CAAA), federal legislation passed in 1970 and 1990 to address acid precipitation. Many people believe that the problem of acid rain was solved with the passage of the CAAA. However, further research at the HBEF and other study sites in the northeastern United States demonstrates that acid deposition is still a significant problem.

The Clean Air Act has had positive effects, but emissions and deposition remain high compared to background conditions. Acid deposition delivers acids to the Earth’s surface, which then move through soil, vegetation, and surface waters and, in turn, set off a cascade of negative ecological effects.

Recent research shows that the ability of some ecosystems to neutralize acid deposition has diminished over time, delaying the recovery of forests, lakes, and streams. Moreover, while the Clean Air Acts of 1970 and 1990 have improved air quality somewhat, the emissions reductions mandated in 1990 are not likely to bring about full ecosystem recovery in sensitive areas of the Northeast.


Hubbard Brook Research Foundation
Acid Rain Revisited

Much of the information and text for this webpage was taken from "Acid Rain Revisited," a report written by scientists working with the Hubbard Brook Research Foundation. For further information about the HBRF or to read the rest of the report, click here.


The National Atmospheric Deposition Program

Because of national concern about acid deposition, the National Atmospheric Deposition Program (NADP) was developed in the late 1970s. The NADP is a nationwide network of precipitation monitoring sites, and has grown from 22 stations in 1978 to over 200 sites spanning the continental United States, Alaska, Puerto Rico, and the Virgin Islands. The HBEF is one of these sites. The purpose of the network is to collect data on the chemistry of precipitation for monitoring of long-term trends such as acid deposition. For more information on the NADP program, click here.


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