Water pollution

Water pollution is the degradation of natural water as the result of human activities. The pollution of water supplies, which can be caused by activities such as mining, agriculture, and improper waste disposal, can have negative health impacts on plant and animal life.

Prior to industrialization, water pollution was usually caused by contamination from animal and human waste, leading to outbreaks of cholera and other waterborne diseases. Before the Industrial Revolution in the nineteenth century, humans produced relatively small amounts of refined metals and organic materials. The production of various alloys of copper, tin, lead, and zinc by heating the mineral ores or by using natural copper metal was also minor. During the Industrial Revolution, however, people began to produce cast iron on large scales, burning charcoal to heat iron ores at high temperatures. They also used other methods to refine other metals, such as nickel, aluminum, titanium, cobalt, platinum, chromium, niobium, and molybdenum. The extensive use of such metals resulted in massive increases in exploration, mining, and production, as well as increased use of energy, resulting in waste disposal problems and contamination of water supplies. In addition, growing urban populations produced concentrations of untreated human and animal wastes and associated disease-producing organisms in water bodies.

Inorganic Constituents

There are both natural and human sources of water contamination. Humans may increase natural contamination by, for example, mining natural resources and disposing of the waste, which may leach out dangerous constituents. Animal and plant health may be affected by enrichment or deficiency of certain dissolved constituents. Rocks and soils may have low concentrations of substances such as selenium, potassium, phosphorus, copper, cobalt, molybdenum, zinc, or iodine, which cause health problems in animals. Although humans may need to supplement their diets with certain minerals for optimal health, substances such as selenium, radioactive elements, copper, and zinc can be concentrated enough in some drinking waters to be harmful. Lead is particularly harmful when it contaminates drinking water, leading to lead poisoning.

Humans mine many elements and use them for manufacturing and other purposes faster than these elements weather out of natural rocks, producing a variety of atmospheric and water pollutants. For example, fertilizers have high concentrations of soluble nitrogen and phosphorus compounds, and animal wastes contain high levels of nitrate. The use of fertilizers thus can produce high nitrate (a nitrogen-oxygen compound) and phosphate (a phosphorus-oxygen compound) in natural waters. Nitrate can combine with hemoglobin so that oxygen transport in the body is inhibited. This is a potentially serious threat to infants. Water high in phosphorus can stimulate the growth of organisms such as algae. As the abundant algae die and drop to the bottom of a body of water, they may use up the dissolved oxygen in the water, which may, in turn, cause fish to die.

The use of salt (sodium chloride) to melt the ice on roads in winter can result in high dissolved sodium and chloride concentrations in natural waters. This adds unexpected sodium in drinking water, which may negatively impact individuals with health conditions such as high blood pressure, and it may cause surface waters to have toxic levels of chloride, impacting fish and amphibians. As climate change causes twenty-first century weather changes, such as unprecedented extreme cold, snow and ice, the use of salt in this way will increasingly pollute groundwater. In the past, deep groundwater with high salt concentrations brought up during petroleum extraction was placed in “evaporation pits” on the surface until the water evaporated. This would allow the salt to leak slowly into the water supply in the ground. Petroleum companies are required to inject these salty waters back into the ground at the level from which they came.

Another major pollutant in water results from the acidity produced by acid mine drainage and acid rain. Acid mine drainage results from the chemical reaction of sulfide minerals such as pyrite (iron sulfide) with water and oxygen from the atmosphere to produce sulfuric acid and dissolved metals in water. Most acid mine drainage comes from small amounts of pyrite in coal mines or waste piles from coal miners. Some acid mine drainage results from metal mines and wastes such as those found in lead and zinc mines. These acid waters can readily dissolve other metals, so acid mine waters may contain high concentrations of many poisonous metals.

Acid rain results from high concentrations of sulfur dioxide, carbon dioxide, and nitrogen oxide gases released into the atmosphere by industry. These gases dissolve in the water in the atmosphere to produce acidity. The worst acid rain thus occurs in industrial areas. Acid rain produces acid lakes and streams in areas that have little natural capacity to neutralize the acid. This results in the destruction of organisms that cannot live in such acidic waters. In some areas that have abundant limestone (a rock composed of calcium carbonate), acid rain reacts with the limestone to neutralize the acidity, so there is little problem with acidity of the natural waters. Areas without rocks that can neutralize the acid continue to have problems with acidity.

Organic Compounds

Organic compounds consist of carbon in a chemical combination with hydrogen, oxygen, sulfur, chlorine, or nitrogen. Many thousands of organic compounds are currently manufactured, and their classification is complex, but they may be grouped simplistically as alkanes, benzene derivatives, chlorinated hydrocarbons, and pesticides. Alkanes are straight chains of carbon atoms combined with hydrogen. Benzene derivatives consist of six-membered rings of carbon combined with other constituents chemically attached to the carbon atoms. Alkanes and benzene derivatives with six to ten carbon atoms are the organic compounds found in gasoline, diesel fuel, and other fuels. Alkanes in gasoline are not very soluble in water, but benzene derivatives are. Alkanes are also more easily degraded by bacteria than are benzene derivatives. The leakage of gasoline into the groundwater system can be a major pollution problem in areas around gasoline stations.

The term “groundwater” refers to any water found under the earth’s surface. The upper portions of groundwater contain both air and water in the pore space between mineral grains; the lower portions of groundwater contain only water in the pore space. The surface between the upper and lower zone is called the water table. Wells are usually drilled into the water table so some of the more soluble hydrocarbons of gasoline that have leaked into the groundwater system can dissolve in water and move in the saturated water zone. The insoluble alkanes and benzene derivatives of gasoline can also move as a separate fluid plume above the water table since they are lighter than water. The maximum allowable concentrations of these benzene derivatives in water are much lower than the allowable concentrations of the individual elements discussed previously.

Benzene derivatives are also fairly volatile. If they move in a liquid plume under buildings, they can move as vapors into basements or sewers, where they then may produce explosions or may cause illness in occupants of the affected buildings.

Chlorinated hydrocarbons contain the element chlorine in one or more parts of the compound. Many of these—such as dichloroethane, tetrachloroethane, and chloroform—are common organic pollutants found in waste disposal sites in the United States. Many are carcinogens (cancer-producing substances), and they become increasingly toxic at higher concentrations.

Pesticides

Pesticides are complex organic compounds that kill unwanted organisms such as insects. Examples of pesticides include Dichloro-diphenyl-trichloroethane (DDT), malathion, alachlor, atrazine, and chlordane. Pesticides may be carcinogenic, and some may not decompose readily in the food chain. DDT, for example, has been banned in the United States since 1972 because of its harmful effects and its slow decomposition in nature.

Pesticides vary greatly in the time it takes them to decompose naturally and move to the water table. At one extreme, pesticides such as prometon can last a long time and quickly move to the water table, thus rapidly contaminating the groundwater. At the other extreme, methyl parathion decomposes more readily and moves more slowly to the water table and is thus less likely to contaminate the groundwater. Among the pesticides often found in groundwater are dibromochloropropane (DBCP), aldicarb, carbofuran, chlordane, alachlor, and atrazine. In addition, a pesticide applied during times of high rainfall can rapidly move to streams, where the stream water can soak into the ground and contaminate the groundwater supply.

Another problem is that even if the original pesticide has been naturally destroyed by bacteria, the degradation products from the decomposition of the pesticide can be even more harmful to humans than the original pesticide. Few studies of these kinds of problems have been undertaken, and decay products from pesticides are often only poorly understood. The degradation products of a few pesticides, such as aldicarb, however, have been studied for a number of years. Such products may have entirely different movement and stability than the original pesticides.

Radioactivity and Heat Pollution

Humankind’s use of radioactive materials has created special problems in the areas of waste disposal and water pollution, in particular because radiation cannot be detected by the senses and can be very damaging if ingested. At one extreme, the radioactive element plutonium has a half-life (the time it takes for one-half of the radioactivity to decay) of about twenty-four thousand years, and it concentrates in the bones of vertebrates. This means that plutonium that has leaked into groundwater must be removed for hundreds of thousands of years. At the other extreme, radioactive materials with short half-lives that do not concentrate in organisms may not be of much concern, since the radioactivity will decay before it can be ingested.

Radioactive wastes are divided into low-level, intermediate-level, and high-level wastes. High-level wastes may be more than one million times more radioactive than what is considered acceptable for human exposure. Low-level wastes may contain radioactivity up to one thousand times more radioactive than what is considered acceptable. Intermediate-level wastes have radioactivity between these ranges. A wide variety of low-level radioactive wastes are produced by hospitals, the nuclear industry, and research laboratories. These wastes are often sealed in drums and buried under a thin layer of soil or diluted in water to acceptable levels of radioactivity and flushed into sewer systems.

High-level radioactive wastes are produced by nuclear fuel generation. Many high-level wastes have been stored for decades in double-walled stainless-steel tanks that are air-conditioned because of the intense heat given off by the radioactivity. Some of these tanks have leaked, and radioactive fluids have moved into nearby groundwater systems.

Thermal pollution is the heating of natural waters caused by the activities of industry, the burning of fossil fuels, or nuclear power production. Dumping heated waters directly into a body of water can kill many temperature-sensitive organisms living there. One method of reducing thermal pollution is to allow heated water to cool in ponds before it is returned to the source body of water.

Prevention and Remediation

In the long run, it is easier and less expensive to prevent the pollution of natural waters than to try to clean a polluted water supply or remediate sources of pollution. Preventive approaches include keeping contaminants contained so they cannot escape into water systems and banning the production and use of dangerous substances such as certain pesticides. One way of disposing of potential polluting substances would be to transport them to an area with a dry climate and low population density and place them in geologic materials that are impermeable to water flow. This would minimize the chances that moving water would carry hazardous constituents to groundwater. The costs of transporting waste materials are high, however, and this means that municipal wastes are usually disposed of locally. In areas that have high rainfall, waste management facilities must be especially careful to keep the waste contained in geologic materials, such as unfractured mudstone, that are impermeable to movement of water.

The expense of both prevention and remediation of water pollution is a source of ongoing conflict between environmentalists and the industries that produce polluting materials. In general, industries are interested in avoiding the costs associated with proper waste disposal, whereas environmentalists’ priority is to keep pollution to surface water and groundwater to a minimum.

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