U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2007–3057

July 2007

Prepared in cooperation with the National Park Service

Acid Rain in Shenandoah National Park, Virginia

Printed on recycled paper

Visitors to Shenandoah National 

Park (SNP) enjoy the animal and plant 

life and the scenery but may not realize how vulnerable these features are to 

various threats, such as invasion of exotic 

plants and insects, improper use of park 

resources by humans, and air and water 

pollution. The National Park Service 

strives to protect natural resources from 

such threats to ensure that the resources 

will be available for enjoyment now and 

in the future. Because SNP has limited 

influence over the air pollution that 

envelops the region, acidic deposition—

commonly known as acid rain—is one of 

the more challenging threats facing park 

managers. With the help of U.S. Geological Survey (USGS) scientists, park 

managers can understand how acid rain 

interacts with ground- and surface-water 

resources, which enables them to explain 

why reductions in air pollution can help 

preserve park resources. Such understanding also provides essential insight 

into ecosystem processes, as managers 

strive to unravel and resolve other environmental problems that are interrelated 

to acid rain.

Even though reductions in emissions 

of acid-forming compounds mandated by 

the Clean Air Act and Amendments have 

occurred since 1990, acid rain continues to fall in the eastern United States, 

including SNP. The combination of acid 

rain (currently about 4.6 pH units) falling 

onto an environment that has little inherent ability to neutralize the acidic input 

and decades of exposure to acid rain have 

resulted in a fragile environment. When 

the effects of acid rain are combined 

with stressors, such as forest defoliation 

caused by the gypsy moth or coniferous tree demise caused by the hemlock 

woolly adelgid, significant environmental 

problems can develop, and park resources 

become threatened.

Understanding pH and 

Acid-Neutralizing 

Capacity

The severity of acid rain and acidic 

streamwater is measured by the pH 

scale—a quantitative measure of how 

acidic (low pH) or alkaline (high pH) a 

solution is (fig. 1). A neutral solution, 

which is neither acidic nor alkaline, has 

a pH of 7 units. The pH scale is logarithThe pH scale is a measure of how acidic (low pH) or alkaline 

(high pH) a solution is. Rainwater is considered normal at 5.6 pH 

units. Shenandoah National Park rain typically is 10 times more 

acidic than normal rain.

Figure 1.  Relations

of common solutions

and acid-neutralizing

capacity to the pH

scale.

mic, which means that each whole-number change indicates a 10-fold change in 

acidity or alkalinity. For example, a pH of 

4 is 10 times more acidic than a pH of 5. 

Rainwater is considered normal at 5.6 pH 

units; therefore, rain with a pH of 4.6, 

which typically occurs in SNP, is about 

10 times more acidic than normal rain. 

Acid-neutralizing capacity (ANC)—

the ability of a solution to neutralize 

acidic inputs—is a quantitative measure 

associated with pH (fig. 1). In general, 

water with a high ANC (for example, 

500 units1) also has a high pH and is 

good at neutralizing acidic inputs. In 

contrast, water with a low ANC (for 

example, 25 units) has a low pH and is 

a poor neutralizer. Stream animals and 

plants (the biota) can be sensitive to and 

negatively affected by changes in ANC, 

particularly when it decreases to near 

zero units, which can cause the biota to 

suffer “acid shock.” Research on fish 

indicates that many species are sensitive 

at ANC values less than 50 units.

1

ANC concentration typically is expressed in 

microequivalents per liter. “Unit” is used here for 

simplifying purposes.

0 1 2 3 4 5 6 7 9 10 11 12 13 14

Battery acid Vinegar Fish begin

to die

Normal

rain

Baking

soda Ammonia Lye

Acidic Alkaline

Neutral

Low acid-neutralizing capacity Intermediate acid-neutralizing capacity High acid-neutralizing capacity

8

Acid rain
Streamwater

pH = 4.6

ANC = 15

Streamwater

pH = 4.5

ANC = 5

Streamwater

pH = 6.1

ANC = 150

Streamwater

pH = 7.2

ANC = 250

Poor environment for a

diverse population of fish

and other aquatic biota

More desirable environment

for fish and other aquatic biota

Streams start as 

seeps on a hillface

Basaltic Bedrock

Siliciclastic Bedrock

Soil

 Acid Rain Facts

Acid rain is caused by combustion of fossil fuels (gas, oil, coal) and to a much 

lesser extent by natural processes (volcanic emissions, forest fires).

Combustion emits sulfur dioxide and nitrogen oxides (the two major contributors 

to acid rain) into the atmosphere; these oxides can travel hundreds of miles from 

the source of emission.

Sulfur dioxide and nitrogen oxides convert to sulfuric and nitric acids in the 

atmosphere; the acids are deposited to the earth by wet deposition (rain, snow, 

fog) and dry deposition (dry particles, gases), collectively referred to as “acidic 

deposition.”

Some of the largest emissions in the United States originate in the Ohio River 

Valley and are carried downwind through the atmosphere toward Shenandoah 

National Park.

The Clean Air Act, designed to reduce emissions, was passed in 1970, amended 

in 1990, and amended again in 1995.

In Shenandoah National Park, the annual average pH values of precipitation 

for the following periods were: 4.53 for 1981–1990; 4.59 for 1991–1995; 

4.62 for 1996–2005. These pH values suggest incremental improvements in 

response to the Clean Air Act amendments.

Rain tends to be most acidic during the summer; longer periods between storms 

allow acids to build up in the atmosphere before being “rained out.”

Individual pH values of rain well below 4.0 have been measured in Shenandoah 

National Park.

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Shenandoah National Park 

Streams 

Shenandoah National Park streams 

start near the top of a mountain, usually 

as a spring or seep on a hillface. As the 

water flows downhill, the stream increases 

in size as more tributaries and ground 

water contribute to the streamflow. Such 

streams are called “headwater streams.” 

Because these streams begin with little 

water, contributions of acid rain can make 

the streamwater more acidic. During 

storms, acid rain quickly reaches headwater streams and can decrease the ANC 

and pH of the stream temporarily. As the 

pulse of acidic water flows downstream, 

the streamwater recovers to the pre-storm 

ANC and pH values.

Although the acidity of rain is spatially uniform across SNP, streamwater 

ANC and pH values vary substantially 

across the park. These differences are 

related to the underlying geology (bedrock 

type) and topography (steepness of the 

slope) of the watersheds. The three main 

bedrock types in SNP are basaltic, granitic, 

and siliciclastic. Basaltic rocks are the 

best of the three bedrock types at neutralizing acidic inputs, siliciclastic rocks are 

poor, and granitic rocks are intermediate. 

Each of the three bedrock types underlies 

approximately one-third of SNP.

Shenandoah National Park has 231 

headwater streams in which ANC and pH 

decrease to various degrees, depending on 

the geology and topography. In general, 

the steeper the slope, the more quickly acid 

rain reaches the stream with less chance of 

being neutralized along 

the way. Therefore, a 

small stream near the 

crest of a mountain, 

on a steep slope, and 

underlain by siliciclastic bedrock, is likely 

to have some of the 

most acidic water in 

SNP (fig. 2). Such 

streamwater does not 

support a diverse population of aquatic biota. 

Conversely, a large stream 

with a gentle slope and 

underlain by basaltic bedrock will have near-neutral 

water and, consequently, a 

diverse population of aquatic 

biota (fig. 2).

Figure 2.  Two stream systems with different characteristics that result in differences in water quality.

Whether or not the aquatic biota will be diversified is strongly controlled by the geology and topography.
Skyland

Panorama

Byrd Visitor Center

Elkwallow

Big Meadows

Mathews Arm

Loft Mountain

Lewis Mountain

Front Royal Entrance Station

Rockfish Gap Entrance Station

Thornton Gap Entrance Station

Dickey Ridge Visitor Center

Swift Run Gap Entrance Station

Skyline Drive

Skyline Drive

Skyline Drive

Shenandoah

National Park

CHESAPEAKE

BAY

ATLANTIC OCEAN

Washington, D.C. Front Royal

Waynesboro

Richmond

VIRGINIA

Less than or equal to 50 units (poor ability to neutralize acidity)

Greater than 50 units (better ability to neutralize acidity)

EXPLANATION

MINIMUM ANC VALUES

Shenandoah National Park boundary

Watershed boundary

0

0 5 10 KILOMETERS

5 10 MILES

78o45'

78o15'

78o30'

38o45'

38o30'

38o15'

Shenandoah National Park Facts

Consists of 308 square miles (nearly 

200,000 acres)

Straddles the crest of the Blue Ridge Mountains

Stretches from Front Royal in the north to 

Rockfish Gap in the south, with Skyline Drive 

traversing the entire length

Touches eight counties (Albemarle, Augusta, 

Greene, Madison, Page, Rappahannock, 

Rockingham, and Warren)

Has 231 headwater streams

Contains three main types of bedrock 

(basaltic, granitic, and siliciclastic)

Has land-surface elevations that range 

from about 600 to 4,051 feet

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Figure 3.  Predictions of Shenandoah National Park watersheds that are likely to have a minimum average acid-neutralizing

capacity (ANC) less than or equal to 50 units (red) or greater than 50 units (blue) for 6 consecutive hours about once every

4 years. The watersheds shown in red have the greatest likelihood of losing native aquatic species because of extended periods

of low ANC.
Models and Predictions

Scientists studied streamwater data 

from five SNP streams and developed a 

specific mathematical model for each of 

the five watersheds based on mathematical relations among ANC decreases in 

each stream and the geologic and topographic characteristics of the watershed 

(Rice and others, 2005). The models for 

the five watersheds were transferred to 

the rest of the watersheds on the basis of 

known geology and topography in each 

watershed.

The models were designed to predict 

the magnitude, frequency, and duration

of decreases in ANC in each of the 231 

streams. The magnitude of a decrease 

in ANC is important because as water 

approaches zero units, fewer fish species are able to tolerate the increased 

acidity. The frequency of a decrease 

in ANC is important because the more 

often the ANC changes, the less time 

the aquatic biota has to recover between 

storm events. For example, during the 

spring when many fish spawn and other 

organisms reproduce, fish larvae and 

young fry are especially sensitive to acid 

shock. Two back-to-back events causing streamwater to have zero units ANC 

could destroy an entire population of 

aquatic organisms. The duration of a 

decrease in ANC is important because 

aquatic biota may be able to survive acid 

shock for a short time, but the longer the 

water remains acidic, fewer organisms 

will survive.

Predictions of annual minimum 

ANC values for specific durations that 

could occur on average once every 

4 years for each of the 231 watersheds 

are summarized on maps of SNP. These 

predictions were made for 6-hour, 1-, 3-, 

and 7-day durations. The 6-hour duration map (fig. 3) indicates the watersheds 

that are predicted to have a minimum 

average ANC value less than or equal 

to 50 units (red) or greater than 50 units 

(blue) for 6 continuous hours during any 

4-year period. During the 6 hours, ANC 

values may increase or decrease, but the 

average ANC value for the 6 hours will 

be lower than for any other 6-hour period. 

Stated another way, the map indicates the 

watersheds that are predicted to have a 

25-percent chance in any single year to 

have a minimum ANC value less than or 

equal to 50 units for 6 continuous hours.

One-half of the SNP watersheds are 

predicted to have ANC values less than or 

equal to 50 units for 6 continuous hours 

approximately once every 4 years. This 

does not mean that the other half of the 

SNP watersheds will never have these 

same conditions, but the probability of 

having them in any given year is predicted to be less than 25 percent.

Fish Health Threatened

Of the 231 SNP watersheds, 52 are 

predicted to occasionally have 3-day 

periods when the average ANC value is 

less than zero units, and 34 of these will 

have less than 2 years between occurrences of this condition. These predictions indicate that within the next 40 to 

100 years, these 34 watersheds will have 

4 consecutive years when this condition 

occurs at least once each year, assuming there is no change in the current 

(2007) acid rain conditions. The probability for these predictions to be accurate 

is greater than 90 percent. Several fish 

species, including native brook trout (the 

most acid-tolerant species in SNP), have 

maximum life spans less than 4 years. 

Less acid-tolerant species, such as rosysided dace, are even more vulnerable.

Karen C. Rice, Frank A. Deviney, Jr., 

and Gordon Olson

For additional details about the 

information presented here, refer to 

Rice, K.C., Deviney, F.A., Jr., Hornberger, G.M., and Webb, J.R., 2005, 

Predicting the vulnerability of streams 

to episodic acidification and potential 

effects on aquatic biota in Shenandoah 

National Park, Virginia: U.S. Geological Survey Scientific Investigations 

Report 2005–5259, 51 p. (online at 

http://pubs.water.usgs.gov/sir2005-

5259/).

Mission of USGS: The USGS serves the 

Nation by providing reliable scientific 

information to describe and understand 

the Earth; minimize loss of life and property from natural disasters; manage water, 

biological, energy, and mineral resources; 

and enhance and protect our quality of 

life.

Mission of NPS: The NPS preserves 

unimpaired the natural and cultural 

resources and values of the national

park system for the enjoyment, education, and inspiration of this and 

future generations. The Park Service 

cooperates with partners to extend the 

benefits of natural and cultural 

resource conservation and outdoor 

recreation throughout this country 

and the world.

For information about Shenandoah 

National Park, see http://www.nps.

gov/shen/index.htm

These results are of concern because 

they indicate a high probability for loss 

of native aquatic species in the park.

For information about water resources 

in Virginia, see http://va.water.usgs.gov

or contact Mark Bennett, Director, 

U.S. Geological Survey Virginia Water 

Science Center, 1730 East Parham Rd., 

Richmond, VA 23228; 804-261-2643 

(dc_va@usgs.gov).

Brook trout in Shenandoah National Park