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New Hampshire Department of Environmental Services

Acid rain, more properly termed acid deposition, is the input of acidifying compounds from the atmosphere to the earth's surface. It is comprised primarily of sulfuric and nitric acids that are formed when emissions of sulfur dioxide (SO2) and nitrogen oxides (NOx) react in the atmosphere with water, oxygen, and other compounds. Chlorine emissions can also lead to hydrochloric acid, but this usually only accounts for a small fraction of acid rain. The source of sulfur dioxide and nitrogen oxides is principally from the burning of fossil fuels by electric utilities, industries, and motor vehicles.

Once the acids form in the atmosphere, they can travel long distances before being deposited. Acid deposition occurs in three forms: wet deposition (rain, snow, sleet, and hail), dry deposition (particles, gases, and vapor), and cloud or fog deposition in high altitudes and coastal areas. Within New Hampshire, the high elevation mountain-tops receive the highest acid deposition on an annual basis. While these high elevations receive wet and dry deposition at rates similar to the lower elevations within the state, they also get a good dose of acid fog from the frequent low-level clouds that "sock-in" the mountain-tops.

Acidity is measured in units called pH (based on the number of free hydrogen ions available in solution) on a scale from 1 to 14. A pH of 7 is considered neutral, a pH lower than 7 is acidic, and a pH greater than 7 is alkaline. Because the pH scale is logarithmic, a small difference in pH really means a large difference in acidic, i.e., a pH of 4 is ten times more acidic than a pH of 5, and 100 times more acidic than a pH of 6. Natural rainwater is already somewhat acidic with a pH as low as 5.0.

The pH of precipitation in New Hampshire is typically between 4.0 and 4.8, but clouds have been measured as acidic as 2.1 (similar to lemon juice). In general, low pH levels in lakes and streams create conditions that harm fish and other aquatic organisms. Similarly, low pH conditions change forest soil chemistry, degrading growing conditions for some tree species.

The deposition of acidic compounds back to earth has a negative impact on aquatic and terrestrial ecosystems, public health, visibility and materials and structures. Further, the same emissions that cause acid rain contribute to other important environmental issues, such as smog, climate change, mercury contamination in fish, and over-fertilization of coastal waters.

Impacts on soil, vegetation, & forests: Acid deposition depletes nutrients such as calcium and magnesium from the soil, slowing growth of trees and other vegetation. Trees stripped of nutrients fall susceptible to insect infestation, drought, freezing, and ozone damage. For example, the decline of red spruce in New Hampshire, Vermont and New York has been directly linked to the impacts of acid deposition. Acid deposition also leaches aluminum from soils and rocks and carries it to soil water, vegetation, lakes, and streams where it can limit trees ability to absorb water and nutrients and be toxic to plants, fish, and other organisms.

Acid deposition has resulted in an accumulation of sulfur and nitrogen in forest soil. This accumulation will continue to leach into nearby water bodies and cause substantial slowing of the recovery of water ecosystems to the affects of acid deposition.

Impacts on lakes and streams: Acid deposition harms water quality in New Hampshire’s water bodies in three important ways: lowering pH levels (i.e., increasing the acidity), decreasing acid neutralizing capacity (ANC) (i.e., the ability of water to neutralize acid inputs), and increasing aluminum concentrations. Decreases in pH and increases in aluminum impact the survival of aquatic ecosystems by impairing the ability of certain fish, aquatic plants and other aquatic organisms to reproduce, grow, and survive. This has reduced the diversity and abundance of aquatic organisms in many of New Hampshire’s lakes and streams.

New Hampshire’s water bodies are especially vulnerable to the impacts of acid deposition because the state has a predominance of granite bedrock that is not rich in the minerals (e.g., limestone) that counteract the impacts of acid inputs. High elevation lakes are especially affected because of their small watersheds, which are characterized by shallow-to-no soils and excess precipitation and fog.

For many years, the state has monitored the effects of acid deposition on water bodies in New Hampshire by regularly taking water samples from lakes and ponds. As defined by the US Environmental Protection Agency, waters that have an ANC of zero or less, which corresponds to a pH of about 5.2, are considered to be acidified. A 2005 evaluation of lake data revealed that 3 percent of all lakes and 10 percent of remote, mostly high elevation ponds are acidic based on this definition. These values are unchanged from a similar assessment conducted five years previously. The acidified lakes in NH, as defined by an ANC value of zero or less and based on the most recent upper water layer sample, are listed in the table below.

Acidified Lakes in New Hampshire
(Based on Acid Neutralizing Capacity of Zero or Less)
Babbidge Reservoir Roxbury 5.4 -1.2
Baker Pond Chesterfield 5.2 0
Barrett Pond Washington 5.3 0
Bear Hill Pond Allenstown 4.5 -1.3
Black Mountain Pond Sandwich 5.7 0
Bowker Pond Fitzwilliam 4.8 -0.3
Brackett Pond Wentworth 4.7 -0.8
Cone Pond Thornton 4.5 -1.1
Constance Lake Piermont 4.8 -0.5
Corser Pond Errol 5.3 -0.6
Darrah Pond Litchfield 4.8 -0.9
Divol Pond Rindge 4.6 -1.2
Echo Lake Conway 5.4 0
Four Mile Pond Dixs Grant 5.1 -0.2
Gordon Pond Lincoln 4.6 -0.8
Juggernaut Pond Hancock 5 -0.4
Kiah Pond Sandwich 4.9 -0.3
Kilburn Pond Winchester 4.5 -1.3
Kinsman Pond Lincoln 4.5 -1.9
Lily Pond Alstead 5 -0.2
Lonesome Lake Lincoln 4.7 -0.7
Long Pond Lempster 5.2 -0.2
Loon Pond Lincoln 4.8 -1
Lovewell Pond Nashua 4.3 -3
Mountain Pond New Ipswich 4.9 -0.5
Nancy Pond Livermore 4.7 -0.8
North Pond Washington 5 -0.1
Nubanusit Lake Nelson 5.6 -0.4
Pisgah Reservoir Winchester 4.4 0
Robbins Pond Rindge 4.9 -0.2
Round Pond Nottingham 4.2 -3.7
Signal Pond Errol 4.5 -0.7
Solitude, Lake Newbury 4.9 -0.3
Sportsman Pond Fitzwilliam 4.8 -1
Spruce Pond Deerfield 4.8 -0.3
Three Ponds, Middle Warren 5.4 -0.6
Willey Pond, Big Strafford 4.8 -2
Willey Pond, Little Strafford 4.6 -1
Winkley Pond Barrington 5.1 -0.2

More significantly, negative biological impacts to aquatic organisms begin to occur at a pH of 6.0 or less. The 2005 evaluation of lake data indicates that over 15 percent of all lakes in the summer – but about 45 percent in the winter – and nearly 60 percent of remote ponds have a pH value of 6.0 or less. Winter pH values are lower than summer values because photosynthesis by algae can artificially inflate the pH (make it les acidic) during the summer growing season.

acidity classes bar graph

Programs to Control Acid Rain

State Programs:

Due to inaction by the federal government to control acid rain forming emissions, New Hampshire took the initiative to enact acid deposition legislation and passed the New Hampshire Acid Rain Control Act in 1985. The goal of the Acid Rain Control Act of 1985 was to reduce emissions of sulfur dioxide from stationary sources (power plants and industrial facilities) in the state by 25 percent and to set an annual SO2 emissions cap on major sources to ensure continued limits on SO2 emissions. The Act is implemented through the acid deposition control program established in the Rules to Control Air Pollution under Chapter Env-A 400.

New Hampshire’s Clean Power Act, passed in 2002 and amended in 2006, calls for annual reductions of multiple pollutants, including SO2, NOx, carbon dioxide and mercury. New Hampshire rules (Env-A 2900) were adopted to implement the Act which calls for an 87 percent reduction in SO2 emissions and a 70 percent reduction in NOx emissions from 1999 levels.

Federal Program:

The federal Acid Rain Program, passed as Title IV of the federal Clean Air Act Amendments of 1990, set a goal of reducing annual SO2 emissions by 10 million tons below 1980 levels. To achieve these reductions, the law required a two-phase tightening of the restrictions placed on fossil fuel-fired power plants. Phase I began in 1995 at the largest, higher emitting plants, mostly coal-burning electric utility plants. Phase II, which began in the year 2000, tightened the annual emissions limits imposed on these large, higher emitting plants and also set restrictions on smaller, cleaner plants fired by coal, oil, and gas. The federal Acid Rain Program affects existing utility units serving generators with an output capacity of greater than 25 megawatts and all new utility units. The only affected power plant in New Hampshire under Phase I was PSNH Merrimack Station in Bow. Additional facilities in New Hampshire affected under Phase II include Newington Station’s dual fuel oil or natural gas-fired unit in Newington and three coal units at Schiller Station in Portsmouth, as well as two new gas-fired plants - Granite Ridge Energy and Newington Energy - that began operation in 2002, although SO2 emissions from gas-fired plan are minimal.

Regional Program:

In June, 1997, the Conference of the New England Governors and Eastern Canadian Premiers (NEG/ECP) recognized that acid deposition continues to negatively impact the resources in the northeastern United States and Eastern Canada, in spite of the significant reductions of sulfur emissions that have taken place since 1990. In response to the need for further action, representatives of the New England states and Eastern Canadian provinces developed an Action Plan, finalized in June 1998, which identifies steps to address those aspects of the acid rain problem in the Northeast that are within the region’s control to influence. The Action Plan includes:

  • A plan for further reducing emissions of sulfur dioxide by 50 percent and nitrogen oxides by 20 to 30 percent greater than current commitments;
  • A research and monitoring agenda targeted at both improving the state-of-the-science for acid deposition, and increasing regional cooperative efforts in sharing research and data; and
  • A public education and outreach agenda to ensure the public continues to be educated and mobilized towards the overall goal of protecting the natural environment.

Implementation of state and federal acid rain laws and regulations has resulted in a decrease in sulfur dioxide emissions from in-state and out-of-state sources. This has resulted in a similar decline in sulfate deposition to the state and, to a lesser extent, a decline in sulfate concentrations in surface waters. Although sulfur deposition has declined, research from the Hubbard Brook Experimental Forest in Thornton, New Hampshire and from other study sites in the Northeast demonstrates that acid deposition is still a problem for several reasons.

First, while sulfur emissions have decreased, nitrogen emissions have not decreased substantially region-wide and wet deposition of nitrogen has remained largely unchanged since the 1980s, according to data from Hubbard Brook Experimental Forest.

Second, the loss of acid-neutralizing minerals from the soil and the long-term accumulation of sulfur and nitrogen in the soil have left many ecosystems more sensitive to the input of additional acids, further delaying recovery from acid deposition.

Deeper cuts in electric utility sulfur emissions (at least 80 percent beyond the Clean Air Act) will be needed for greater and faster recovery from acid deposition in the Northeast.

NH Department of Environmental Services | 29 Hazen Drive | PO Box 95 | Concord, NH 03302-0095
(603) 271-3503 | TDD Access: Relay NH 1-800-735-2964 | Hours: M-F, 8am-4pm

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