While the concentrations of pollutants in local areas can build up to very high levels (see introduction, page 386), those pollutants won’t necessarily stay in place. Wind and water transport pollutants around the biosphere, crossing geographical and political borders—a phenomenon called transboundary pollution. For instance, heavy metals and pesticides released in Europe and North America have been found in both terrestrial and aquatic ecosystems far to the north in the Arctic.

The atmosphere and the oceans are circulating constantly and redistributing material, including pollutants, throughout the biosphere (see Figure 8.2, page 232). The 1991 eruption of Mount Pinatubo in the Philippines, for example, provided an unprecedented opportunity to study the dispersal of materials by atmospheric circulation. There had been much larger volcanic eruptions within historic times, but never before had there been satellites in place equipped with sensors that could document the spread of materials injected into the atmosphere by the eruption. Mount Pinatubo injected approximately 20 million metric tons of SO₂ into the stratosphere, which were gradually converted chemically into 30 million metric tons of sulfuric acid and water aerosols.

An aerosol consists of tiny particles of solid material or tiny liquid droplets suspended in air or other gas. In the case of the stratospheric aerosols resulting from the Pinatubo eruption, the droplets were made up of a mixture of sulfuric acid and water. These aerosols were initially confined to a tropical band encircling Earth, but they spread through the stratosphere over a period of two years (Figure 13.9). Gradually, atmospheric circulation and gravity removed these acidic aerosols from the stratosphere into the troposphere (the lower atmosphere) and to the surface of Earth.

Pollutants emitted into the lower atmosphere are also widely dispersed across Earth. During his voyage on the H.M.S. Beagle, Charles Darwin noted the fallout of sand and dust on the ship far out in the North Atlantic Ocean off North Africa. Despite the ship’s great distance from Africa, he inferred that the material was being blown by prevailing winds from the Sahara Desert. We know now that Darwin was correct and that, in fact, dust is transported from the Sahara Desert all the way across the Atlantic to the Amazon River Basin (Figure 13.10). The atmosphere can also transport other pollutants long distances. For example, ozone and mercury produced by the burning of coal in China are deposited across North America.

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The 2011 nuclear accident at Fukushima, Japan, provided dramatic evidence of oceanic transport. The tsunami that disabled the Fukushima nuclear power plant caused immense destruction of property and much of the debris created by the tsunami was dragged out to sea with the receding water. The massive collection of debris, which contained whole ships and largely intact houses, was transported with oceanic currents across the Pacific Ocean to the coasts of Canada and the United States (Figure 13.11).

Pollutants can also be transported across the oceans with the help of organisms. For example, radioactive isotopes leaked during the nuclear accident at Fukushima were found in juvenile bluefin tuna caught off the California coast approximately 1 year later (Figure 13.12). These young tuna had taken up radioactive cesium-134 while swimming in the waters off Japan and then swam across the Pacific Ocean to their feeding grounds off the West Coast of North America. The levels of cesium-134 in the tuna did not present a health hazard, but it demonstrates how a contaminant can be quickly transported halfway around the planet.

Surface water and groundwater carry pollutants. A watershed, also known as a drainage basin or catchment, is the land area drained by a water system, such as the Chesapeake Bay watershed (Figure 13.13). Precipitation in a watershed has several potential fates. Some precipitation immediately evaporates and returns to the atmosphere as water vapor. A portion runs off the land as surface flow, eventually entering waterways, ranging from small streams to rivers. Some precipitation infiltrates into the soil, where it may be taken up by terrestrial plants and stored as tissue moisture or transpired as water vapor back into the atmosphere. Yet another fraction of the precipitation, which soaks into the ground, becomes groundwater (see Chapter 6, page 159). Flows of surface water and groundwater determine where the pollutants will move and, as shown by the map of the Chesapeake Bay watershed, these flows often cross political boundaries.

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The concept of an airshed is similar to a watershed but related to movements of air rather than water. We define an airshed as a part of the atmosphere that has a consistent airflow, primarily due to the location of mountains and prevailing wind patterns. Since the air mass within an airshed typically behaves in a united and orderly way, a focus on airsheds can be used to track and manage air pollution. Figure 13.14 shows the airshed for the deposition of nitrogen oxides in the Chesapeake Bay, along with its watershed. Because it is less confined, the airshed is generally much larger than the watershed.

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The large area of airsheds has significant consequences for managing pollution. For example, in December 2013, eight Mid-Atlantic and Northeast states filed a petition with the EPA to require nine Midwest and Appalachian states, which were upwind of them, to reduce air pollution within their borders. The states issuing the petition claimed that prevailing winds were transporting pollutants to them from outside their borders and that the upwind states are part of an airshed that crosses many state boundaries.