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It rains about once every three or four days in our region, on average, but most of us don't think about it very much unless we are in a severe drought. In many ways, however, rain is quite a remarkable phenomenon. In this issue of the EnvirocastTM Newsletter, we look at the role raindrops play in the Chesapeake Bay watershed. We will explore the potentially strong acidity of raindrops and learn about their amazing ability to cause erosion. We will discover how such tiny droplets can exert such a strong impact on the health of the Bay's ecosystem.

In order for a raindrop to form, a grain of dust or similarly-sized solid particle, known collectively as condensation nuclei, must first be present in the air. As water vapor condenses, the tiny water droplets will then adhere to each particle. About a million of these tiny droplets must join together to form a single raindrop that is heavy enough to fall out of the sky.

We often think of rain as being pure water, but it actually contains many diverse compounds. When rainwater is analyzed, it usually contains minute amounts of ammonium, cadmium, calcium, chloride, chromium, copper, fluoride, lead, magnesium, manganese, mercury, nickel, nitrate, phosphorus, sodium, selenium, sulfur, zinc, as well as some hydrocarbons and herbicides. These get into the raindrops because they serve as the condensation nuclei. As we learned in the last issue of the EnvirocastTM Newsletter, the deposition of many of these airborne pollutants can have a strong influence on the health of the streams, rivers, and estuaries of the Chesapeake Bay (see the March 2003 issue of the EnvirocastTM Newsletter for more information on atmospheric deposition).

Many viewers may be unaware that the rain that falls across the Chesapeake Bay ranks among the most acidic of any region in the country. Figure 1 depicts the national pattern in the acidity of rain, as measured in 2001 by the U.S. Geological Survey.

Figure 1. A pH distribution map showing how the acidity level of precipitation varies across the United States. - data from 2001 (source USGS).
© 2003 STORMCENTER COMMUNICATONS, INC. - ALL RIGHTS RESERVED

By its nature, rain tends to be slightly acidic. Unpolluted rain has a pH of about 5.6, due to the mixture of carbon dioxide and water vapor in the atmosphere which forms carbonic acid (for a primer on pH and acid rain, check out this month's Toolkit section). Rain can become even more acidic when raindrops react with gases and other particles in the atmosphere, such as sulfur dioxide and nitrogen oxide. These airborne pollutants are primarily emitted during the combustion of fossil fuels, such as coal, gas and oil. These reactions increase the acidity of rainfall by as much as two orders of magnitude.

Because of the reactions that can often take place with pollutants in the air, it is common to measure pH levels as low as 3.8 during some storms in our region, particularly if the winds are from the west or northwest. Winds from these directions are known for carrying with them acid rain precursors from the coal plants of the Ohio Valley. Indeed, according to air quality researchers, the annual average pH value in rain that falls throughout the Chesapeake Bay is consistently in the range of 4.2 to 4.5.

These pH levels are considered only mildly acidic, about the strength that you taste in beer, cider, wine, sauerkraut, or a dill pickle. Over time, however, the cumulative effect of our acid rainfall is strong enough to corrode and degrade stone and masonry, as has been recently documented at the monuments and buildings of our nation's Capital (as if the pigeons with their guano weren't bad enough).

Elsewhere in urban landscapes, scientists have documented that acid rain helps leach out zinc, copper and aluminum from our roof surfaces, metal pipes and copper flashing. Although acid rain is generally considered harmful to our environment, there are a few positive aspects. Some plants, for example, such as rhododendrons and azaleas, thrive in an acidic environment.

The underlying geology of the Chesapeake Bay watershed helps to partly neutralize the impacts of acid rain on our crops, forests, and streams, particularly when compared to nearby regions, such as the Northern Appalachian Mountains. Still, warning signs abound in the Bay's watershed, particularly with respect to the impact of acid rain on mountain streams. For example, 18% of the total stream miles in Maryland are currently impacted by acid rain (See Figure 2). In the mountains of Virginia, experts believe that half of all native trout streams are impacted by acid rain. Despite strong efforts to reduce emissions of acid rain precursors by 25% nationally over the last two decades, scientists have not observed significant improvement in either acid rain levels in our region or impacted mountain trout streams.

The size and terminal velocity of raindrops depend on the intensity of each rainstorm. The big raindrops of a summer thunderstorm are about 5 millimeters in diameter and fall at a speed of more than 20 miles per hour. By contrast during a light drizzle, raindrops are less than one millimeter in diameter, and fall out of the sky at a leisurely rate of about 1.5 miles per hour. Still, even tiny raindrops are quite powerful when they strike bare soil. Their force on impact can detach soil particles from the land and erode them easily.

Figure 3. Soil erosion by water begins when a raindrop splash dislodges soil particles. The impact of just a single raindrop can cause soil particles to be thrown as high as 1.5 meters in the air.
This is especially true if a thin film of
water is covering the ground.
Collectively, raindrops, especially heavy raindrops, can erode as much as 4
million tons of sediment in the
Chesapeake Bay watershed each year.

Image courtesy of USDA Natural
Resources Conservation Service.
© 2003 STORMCENTER COMMUNICATONS, INC.
- ALL RIGHTS RESERVED

As can be seen from Figure 3, a large raindrop can impact with such force that soil particles are splattered three or more feet into the air. Sheet flow, which occurs when water accumulates in a thin sheet on a slope; moves soil particles across the surface of the land, often filling the landscape with tiny rills and rivulets and occasionally even gullies. During larger or more intense storms, sediments are eroded from the land and delivered to small streams, which ultimately carry them to the Chesapeake Bay. The exact amount of sediment erosion depends on the soils, slope and vegetative cover on the land. Generally, forested lands with deep soils experience little soil erosion - about 200 to 400 pounds of sediment per acre per year. By contrast, when forestland is cleared and soils are exposed, annual sediment erosion rates are frequently measured in tons per acre.

Soils are most frequently exposed in two areas of the Bay's watershed: cropland and construction sites. There are about ten million acres of cropland in the Chesapeake Bay, mostly row crops such as corn and soybeans. Together, row crops comprise about a quarter of the total watershed area of the Chesapeake Bay. Croplands are very susceptible to soil erosion: erosion rates as high as 5 to 20 tons of sediment per acre per year are measured frequently at farms that do not use good soil conservation practices.

Rough estimates suggest that at least 100,000 acres of land are cleared each year for construction in the Chesapeake Bay watershed. These sites often experience the highest rates of sediment erosion in the Chesapeake Bay watershed. If effective erosion and sediment control practices are not used at construction sites, these sites can produce erosion rates of 20 to 100 tons of sediment per acre per year (see Figure 5). Together, cropland and construction sites generate at least half of the eight million tons of sediment delivered to the Chesapeake Bay in an average year of rainfall (this is equivalent to 220,000 dump truck loads).

Farms and construction sites need not experience severe soil erosion rates: careful land stewardship can keep soil in its place and sharply reduce downstream sediment loads. Basically, the erosion control prescription is simple - prevent raindrops from detaching soil particles by minimizing exposed soils and quickly establishing a vigorous vegetative cover.

At a farm, this means using good soil conservation practices, such as minimum tillage, cover crops, terracing and grassed waterways. At a construction site, this means minimal clearing, using grass or mulch to cover exposed soils, and installing silt fences and sediment basins to trap sediments. For more information on these and other erosion control practices, watch a slide show produced by the Center for Watershed Protection (http://www.stormwatercenter.net/slideshows/ESC.htm).

Picture of construction site and soil erosion. Image courtesy of the Center for Watershed Protection.

Governments throughout the Bay watershed have been diligently working with both farmers and builders to implement practices that can reduce sediment pollution on their lands. Several million acres of farmland in the watershed now use good soil conservation practices, and the recent U.S. Farm Bill (http://www.usda.gov/farmbill/) should provide millions of dollars to implement even more practices on cropland in coming years. Erosion control practices are also required by law at all construction sites larger than one acre in size in the Bay watershed (and, in many cases, at construction sites as small as a tenth of an acre). Builders must use effective practices during the entire construction process or face stiff penalties.

Homeowners can also play a key role in preventing soil erosion, particularly at this time of the year. After such a tough winter, many yards have dirt patches, mud puddles and others areas of bare ground. Check out this month's Tips for Viewers section to learn about some simple steps you can do to practice erosion control in your yard.

The Envirocast Newsletter is a joint project of the National Environmental Education & Training Foundation (NEETF) and the Center for Watershed Protection(CWP) and is produced in partnership with StormCenter Communications. It was developed under Cooperative Agreement No.830502010 awarded by the U.S. Environmental Protection Agency. The views expressed in this document are solely those of NEETF, CWP and StormCenter Communications. EPA does not endorse any products, commercial services or links mentioned in this newsletter.
email: envirocast@stormcenter.com web: www.stormcenter.com/envirocast