Scottish-American naturalist and Sierra Club founder, John Muir (1838–1914), wrote, "When we try to pick out anything by itself, we find it hitched to everything else in the Universe." Our rapidly growing, ever more industrialized human population exists within a carefully balanced global system of physical processes that circulates chemical elements through the solid earth, hydrosphere, atmosphere, and biosphere. From agricultural land and water management, to extraction and combustion of fossil fuels, to industrial and municipal disposal of waste products, modern human activity has overprinted natural Earth cycles with synthetic ones. In many cases, these man-made alterations to the natural environment negatively impact the very Earth systems that sustain human life. Contamination of the hydrosphere and atmosphere, depletion of radiation-shielding stratospheric ozone, and anthropogenic global climate change are examples of changes induced by human environmental pollution.
Accessible, uncontaminated water is essential to all human activities, and water pollution is a persistent environmental issue. Contamination of surface, ground, ocean, and atmospheric water occurs when chemical, radioactive, and organic waste is washed, spilled, or dumped into water reservoirs at point and non-point sources. Point sources of water pollution introduce concentrated waste products into rivers, aquifers, and oceans at focused entry points. Point sources such as oil spills, chemical leaks, and sewage discharges can often be easily corrected; the inflow of hazardous waste can be stopped, and the contaminated water reservoir can sometimes be cleansed. However, the immediate damage to ecosystems and water quality by highly concentrated chemicals at the spill site or pipe outlet may be irreversible, and cleanup is usually costly and difficult. The 1989 Exxon Valdez oil spill in Prince William Sound, Alaska was a dramatic example of a point source of marine water pollution.
Damaging materials also flow into streams and aquifers from diffuse, non-point sources like agricultural lands, logging tracts, mines, residential leach fields, and urban pavement. While non-point pollution is usually less concentrated, it is also more difficult to control, contain, and regulate. Furthermore, the environmental effects of non-point pollutants like fertilizers, pesticides, animal manure, and mining leachates often manifest themselves as systemic changes to aquatic environments that, in turn, reduce water quality. For example, addition of organic materials, fertilizers, and detergents to streams and lakes enhances the natural process of eutrophication, in which aquatic vegetation chokes a stream or lake, and eventually kills the reservoir's aquatic fauna. Even very low concentrations of toxic heavy metals like those found in leachates from mine tailings, or lead plumbing, can result in toxic contamination of fish and mammals in an aquatic ecosystem. Untreated sewage and agricultural runoff may introduce viral and bacterial pathogens that cause an array of human illnesses from typhoid to dysentery.
Groundwater pollution occurs when contaminants enter an aquifer from a point or non-point source in a recharge zone, contaminated surface water infiltrates, or buried tanks and landfills leak into the groundwater. Groundwater flow paths are complex, and the ultimate site of contamination is often difficult to predict. In karst aquifers, groundwater flows fairly rapidly through interconnected limestone dissolution cavities with little filtration of dissolved materials. Pollutants may thus be flushed from the groundwater in months or weeks, but contaminants often take unexpected paths through limestone aquifers, and eventually discharge, undiluted, at unexpected locations. In homogenous, porous aquifers, like the sandstone Ogalla aquifer in the south central United States, pollutants flow slowly from their points of entry, and are naturally filtered over time. However, it is difficult to flush contamination from a sandstone aquifer, and recharge with fresh water is extremely slow. Groundwater contamination is of particular concern for sitting buried landfills, petroleum tanks, and particularly nuclear waste repositories. Groundwater contamination by harmful radioactive waste buried at nuclear weapons laboratories in Hanford, Washington, and Oak Ridge, Tennessee has cast doubt on nuclear waste disposal schemes.
Though contamination is often introduced into the atmosphere at point sources like smokestacks and exhaust pipes, air pollution is usually diffuse because atmospheric circulation is unconfined. Sulfur dioxide emitted by coal-burning electrical generators disperses widely into the atmosphere before chemically combining with water vapor to form sulfuric acid. The resulting corrosive acid rain falls on widespread areas far downwind of the original point source. Nitrogen oxides released from automobile engines are a main component of the brown smog that blankets many cities. Nitrogen oxide and sulfur dioxide combine with other atmospheric chemicals in strong sunlight to form ozone, the component of smog that affects respiration and irritates humans' eyes. Ironically, ozone is harmful to humans in the lower atmosphere, but ozone in the outer atmosphere shields us from harmful, carcinogenic ultra-violet radiation. Another class of man-made chemicals, called chlorofluorocarbons (CFCs), has chemically destroyed the shielding ozone in the stratosphere over Antarctica, creating the "Ozone Hole." CFCs are common industrial chemicals used in air conditioners, aerosol spray cans, refrigerators, and foam packaging.
The dramatic decrease in air and water quality during the twentieth century has spurred the scientific community to better understand the types of environmental pollution described above, and to devise solutions that reduce contamination. Many governments have enacted legislation that encourages these solutions. In the United States, the Environmental Protection Agency's Clean Water Act of 1972, and Clean Air Act of 1990 strictly regulate industrial, agricultural and municipal sources of air and water pollution. Improved understanding of such complex processes as groundwater and atmospheric flow has led to safer methods of waste disposal, from properly-sited, lined landfills, to air filters on smoke stacks, to carburetors on automobiles. Countries that have enacted these relatively inexpensive measures now enjoy much cleaner air and water than existed in the 1970s. In 1987, the international community signed the Montreal Protocol that eventually bans production of CFCs. However, a handful of the thorniest environmental problems facing the Earth's human population have consequences we have yet to understand, let alone reverse. Solutions to the most threatening and highly-politicized environmental issues, including global climate change, overpopulation, and loss of biological diversity may require significant international socioeconomic changes.
