Chemistry Of Water Pollution

Type of physical science: Chemistry

Field of study: Environmental chemistry

Water pollution occurs when physical and chemical change has been made to water which limits its usefulness to humans, animals, and plants. A variety of physical and chemical substances are considered water pollutants. Anthropogenic activities are usually the cause of polluted water.

Overview

Water pollution describes the condition where water is made undesirable or unfit for human, animal, and plant consumption because it contains unwanted chemicals or biological substances called contaminants. Water is called the universal solvent because, to some degree, it readily dissolves almost all materials. Thus, water quality is easily degraded by liquid, gaseous, and solid materials, which are dissolved during the migration of water through the earth. In addition, even the most insoluble solids and hydrophobic liquids may be transported by surface or ground waters, either as solid particles carried along by moving water or as liquids transported on the surface of the water. Most water pollution is the result of human activities involving disposal of industrial products, chemicals, or wastes, but in certain instances, pollution may result from natural processes. Pollution resulting from direct or indirect human activity is termed "anthropogenic pollution." Most pollutants are inorganic or organic chemicals and compounds; however, they can also be physical substances such as litter and sediment. If the contaminant is from a single source or location of input, such as the outfall from a chemical plant, a waste treatment plant, or a storm sewer, it is termed "point-source pollution." Nonpoint source pollution results from more widely distributed sources. Runoff of pesticides and herbicides from agricultural fields or lawns or from atmospheric deposition are examples of nonpoint source pollution.

Regardless of the contamination source or the water reservoir that is affected, there are four important aspects to consider concerning chemical pollution of water: the type of chemical contaminant, the danger a contaminant presents because of its toxicity, the concentration or amount of the contaminant present, and the behavior of contaminants in the environment. A variety of chemicals may cause water pollution. These include inorganic elements such as the heavy metals lead, cadmium, and mercury, and inorganic nonmetals such as arsenic and selenium. A large number of organic compounds are also identified as pollutants, including insecticides, pesticides, industrial wastes, by-products of fuel and waste combustion, and hydrocarbon fuels. Nutrients and sediments can also cause water pollution, albeit indirectly, by causing overfertilization of plants and other organisms, thereby resulting in water "unfit" for consumption.

The danger a chemical pollutant presents is discussed in terms of contaminant toxicity.

For example, lead (Pb) is often referred to as a dangerous "toxic" metal. Toxicity refers to a chemical's ability to disrupt cellular and tissue functions in organisms. If the disruption is severe, cells may stop functioning and, if severe enough, the organism may eventually die. It is the exposure or dose that describes the degree an organism has interacted with a pollutant. The dose is a function of the amount or concentration and the length of time over which exposure occurs.

Generally, the larger the concentration an organism is exposed, the shorter the length of time before the organism is affected. The chemical form, or speciation, of a pollutant also affects its toxicity. For example, mercury (Hg) is highly toxic and dangerous when it is the form of methyl-mercury but is less of a problem when it is present as metallic mercury.

Pollutants, and, in fact, most chemicals, are toxic when some critical dose or exposure is surpassed. Particularly good examples are the trace metals, many of which, such as copper and zinc, are essential for proper cell function when present at low concentrations. At elevated doses, however, these same metals can become very toxic to organisms. The same concept applies to many organic compounds; however, as a general rule-of-thumb, organic compounds are usually toxic at lower doses. There will be some level at which all chemicals will be dangerous. These levels are different for each chemical pollutant and for each organism and are related to the dose or length of time of exposure. Based on doses from laboratory studies of animals and case studies of humans, government health agencies have prepared lists of recommended safe drinking water concentrations for many potential chemical pollutants. These concentrations are designed to minimize the dose, and therefore, the health risks of polluted water.

Because it is exposure or dose that makes chemicals dangerous, it is very important to consider both the chemical type and abundance when water pollution is evaluated for the danger it presents. The amount or concentration of a chemical in water is expressed in terms of weight per unit volume or mass of water. Units of concentration commonly used are milligrams per liter of water, parts per million of water, and parts per billion of water. As an example of how small a total contaminant amount these concentrations represent, 1 part per billion represented slightly greater than 4 centimeters of the total circumference of the earth ,077 kilometers). Very small concentrations of extremely toxic contaminants, at levels such as several parts per billion, may cause water to be unfit for use, while less toxic pollutants may be dangerous when they are present in the part per million range or higher. Even at these low concentrations, contaminants are often thousands or millions of times greater in polluted water than in natural, noncontaminated water. As a result, a number of problems are faced when remedial actions are taken to clean up contaminated water, mainly because of the amounts of toxic pollutants that can be dispersed in large volumes of lake, river, or groundwater. Many times, polluted water will contain elevated concentrations of compounds that are so toxic that their effects cannot be easily removed by simply diluting the polluted water with clean water. This case illustrates that, in some cases, a very small total amount, and concentration, of a toxic contaminant may cause water to be unsafe.

Any discussion of water pollution and its causes must focus on the different, interconnected reservoirs of water that make up the earth's hydrosphere. These reservoirs are the atmosphere, the oceans, surface waters (lakes, streams, rivers, swamps), glaciers, and the underground reservoir termed "groundwater." The hydrologic cycle refers to the pathways of water movement between these reservoirs, as well as the volumes of water in each reservoir and the rate of transfer between reservoirs. Because each of these reservoirs may be affected by different types or classes of chemical and physical pollutants, the study of water pollution generally follows the hydrologic cycle.

Applications

The major sources of pollution to atmospheric water are gases and dust particles that enter the atmosphere. These pollutants are generated from the exhaust and particulates emitted during the burning of coal, gasoline, and solid wastes. In some areas, atmospheric pollution results from wind-borne dust from mining activities. The gases react with water in the atmosphere, while the dust particles act as nuclei for rain drops, which carry both types of pollutants to the earth's surface.

The two groups of gases that are most important as pollution sources are nitrous oxide compounds and sulfur oxide compounds. Nitrous oxide compounds are formed and emitted during the burning of fuel in internal combustion engines. Sulfur oxide compounds are generated primarily during the burning of coal as the result of oxidation of sulfide minerals contained in the coal. Both gases dissolve into rain and snow during cloud condensation and can also be washed out of the air by falling precipitation. Once dissolved, they quickly convert to sulfuric acid (H₂SO₄) and nitric acid (HNO₃), respectively, which are the main agents of acid rain. These acids may migrate hundreds of kilometers in the atmosphere before being swept to the ground by precipitation. In many areas far from ground-based sources of pollution, atmospheric precipitation of acids, nonburnt hydrocarbons, and minute dust and ash particles are the primary sources of water pollution.

Because of the ease with which they are accessed by humans, surface waters such as lakes, streams, and the ocean are perhaps the most easily polluted reservoirs in the hydrosphere.

Traditionally, lakes and rivers were used as disposal sites for untreated anthropogenic waste products. As a result, the most abundant contaminants of surface water pollution are the chemicals found in industrial and municipal wastes. These wastes supply contaminants such as heavy metals and organic compounds. Waste contamination is often introduced from point sources such as municipal waste treatment plants, sewer outfalls, and industrial outfalls.

Surface waters are also affected by nonpoint sources of pollution. The most important of these are direct atmospheric deposition of particles, along with overland runoff of precipitation. Large inputs of dust, ash, and other particles may occur in lake basins in arid regions or near heavily populated regions where fuels or other materials are burned. Overland flow of precipitation draining off the land also delivers dissolved contaminants to streams and rivers. This source, along with groundwater, transports to surface water bodies many pollutants such as fertilizers, pesticides, and herbicides, which are applied to crops, fields, and lawns.

Perhaps the most recognized example of this type of pollutant is the once widely used pesticide DDT (dichloro-diphenyl-trichloromethane), which was banned after having been linked to declines in populations of fish-feeding birds. These types of chemicals are often termed "persistent" because they are very slow to break down, degrade, and dissipate from the environment. Thus, these pollutants may remain active in the environment many years after application, still moving through the hydrologic cycle far from the site of their use, and still having deleterious effects on plants and animals for many years. Another important group of chemical pollutants delivered to surface water by overland flow are the essential nutrients phosphorus (P) and nitrogen (N). These chemicals originate from animal and human waste and are also major components of fertilizer. Nutrient pollution occurs from nonpoint sources, including fields and animal feedlots, as well as from point sources, such as waste treatment outlets. The result of excess nutrient input to surface water is a condition termed "eutrophication": the rapid growth of plants and algae in lakes resulting from the oversupply of essential nutrients. Another type of surface water pollution is thermal pollution, which results from inputs of hot water from such sources as electrical and steam generation plants.

An important concept concerning the fate of pollutants in surface water is the ability of these reservoirs to cleanse themselves naturally of pollutants once the pollution source is removed. This ability results from two natural processes occurring in lakes and rivers: The first of these processes involves the continual replenishment and flushing of water through lakes and rivers. As a result, clean water replaces polluted water relatively rapidly, but only if the source of pollution is eliminated. This flushing is fastest in rivers and slowest in large lakes or lakes that do not have rivers flowing from them. The other mechanism occurs through the deposition and burial of sediment particles on river and lake beds and on the ocean floor. Many contaminants are hydrophobic; that is, they readily associate with sediment and solid organic debris rather than remaining dissolved in the water. As they settle to the bottom, solid particles will scavenge contaminants from water, thereby removing them from the water. While the overall effect is to cleanse dissolved contamination from surface water bodies, the sediment and organic debris may, in fact, act as a long-term source of pollution to the overlying water. This is caused by a number of processes, among which are changes in the oxidation state of the sediment after burial. These changes may cause the sediment to dissolve and release contaminants back to the overlying water column. Resuspension of contaminated sediment by floods, storm waves, or human activity can also reintroduce the contaminant to the overlying water. Additionally, some plants may extract contaminants directly from sediment and water. Finally, ingestion of the contaminants by worms and other sediment feeds can introduce the contaminant into the food chain, including fish eaten by humans.

Major surface water contamination also occurs by oil spills such as the 1989 accident of the Exxon Valdez in Alaska or the actions in the Persian Gulf in 1991. After a spill, the petroleum rapidly changes through evaporation of the most toxic, lower-molecular-weight components. The heavier components tend to spread and are altered through microbial activity and oxidation. The chemical evolution of a spill depends upon the type of petroleum involved, the temperature of the air and water, ocean turbulence, and any actions taken to control or contain the effects of the spill.

Groundwater is the portion of the hydrosphere that lies below the ground surface; it is the water that moves by gravity through the open void space of consolidated and nonconsolidated geologic materials. Geologic materials that are saturated and supply water are called aquifers.

Layers that cannot supply water and act as barriers to water flow are called aquicludes or aquitards. Aquifers that are surrounded by aquitards or aquicludes are called confined. Confined aquifers are usually deeper and are protected by aquitards from surface sources of pollution.

Pollution of confined aquifers can occur if contaminants are introduced through such activities as drilling or in areas of soil excavation. Aquifers having an upper water surface, termed the "water table," which is connected to the atmosphere through the soil, are called unconfined aquifers.

These aquifers are particularly sensitive to pollution from spilled liquids and polluted surface water that infiltrates into the ground. Polluted water may directly enter an unconfined aquifer, and clean water, infiltrating through contaminated soil or wastes, can pick up chemicals and transfer them into underlying aquifers as well.

A variety of soluble and insoluble or hydrophobic chemicals may pollute groundwater.

Inorganic chemicals that are soluble in water may be introduced from concentrated solutions, such as saline oil-field brines, or heavy metals concentrated in waste waters. Organic chemicals of particular importance are the persistent organic compounds, especially aromatic organic compounds, which contain chlorinated benzene derivatives. Examples of these compounds include benzene, toluene, ethylbenzene, and xylene, referred to as BTEX, a suite of compounds often used as indicators for the presence of other aromatic compounds in groundwater. Another group of these compounds include polychlorinated biphenyls (PCBs). Many organic compounds are naturally degraded with time by the actions of bacteria, sunlight, and oxygen penetrating into soils. Nevertheless, many classes of chlorobenzene compounds are toxic to bacteria and are not readily degraded; therefore, they often last for many years in slow-moving groundwater. The density of organic compounds may either be lighter than or heavier than water, depending on their atomic makeup. This group of chemicals may therefore either float on the groundwater table or sink to the bottom of aquifers if they are more dense than water. Sources for organic chemical pollution include leakage from chemical or gasoline storage tanks, broken pipelines, waste storage ponds, and landfills.

Solid materials and solid wastes are also cited as contamination sources. These include road salt, solid waste piles (both municipal and industrial wastes), ash generated from coal or refuse combustion, and any other solid material that can be dissolved by precipitation or washed off the surface of a solid material.

Unlike surface waters, which may cleanse themselves quickly if the source of contamination is eliminated, groundwater will remain polluted long after the contamination source is removed. This results from the fact that the slow flow rates typical of groundwater eliminate the rapid flushing of contaminants from aquifers, and because of the lack of rapid means for removing contaminants by processes such as sedimentation. Reactions between the dissolved chemicals and the clays and other minerals in the aquifer may be the only means by which contaminants are removed from the water. If the solid materials in an aquifer become contaminated, however, they may act as an almost permanent source of recontamination of fresh groundwater entering the aquifer.

Context

The study of water pollution grew out of the awareness of the importance of maintaining a healthy environment. Perhaps the first major book recognizing the problem was SILENT SPRING (1962) by Rachel Carson, who documented the threat of environmental pollution. The problem of widespread pollution first surfaced in the 1950's and 1960's after a series of incidents of human illness and obvious degradation of the environment were brought to light. Major examples of these incidents include mercury poisoning, which occurred in Minamata Bay, Japan; the disappearance of pelicans along the California coast; and the extreme eutrophication and metal pollution that has occurred in the Great Lakes, especially Lake Erie.

In Minamata Bay, Japan, an outburst of illness and disability, affecting more than a hundred people and leading to the deaths of many, was related to inorganic mercury used in an industry along the bay. Some of the mercury, lost during processing, was flushed into the bay and poisoned the food chain and ultimately the people who ate seafood along the bay. In California, the rapid decrease in the number of seabirds was linked to the movement of the persistent pesticide DDT off the land and into the ocean food chain. This chemical affected the ability of the birds to reproduce. The resulting ban on DDT gradually has allowed a return of pelicans in the area. Pollution of the Great Lakes, once recognized, has led to local, state, and federal actions to reduce the sources of contamination and overfertilization. This is gradually allowing the cleanup of these important waterways to occur.

Although the importance of maintaining a clean environment is now recognized and major steps have been taken to return the earth's water to an unpolluted state, many problems remain. Perhaps the most important is the existence of toxic sites, where both soil and water have been polluted through industrial activity or waste disposal. Even where the industrial and disposal activities were done legally and with consideration for environmental needs as they were then understood at the time, advances in environmental chemistry now show these disposal sites are "time bombs" that threaten the health of the population and the usefulness of the land for hundreds of years to come. Major strides have been made in understanding the nature of water pollution and the interaction of the chemicals with different parts of the hydrosphere. Much remains to be done to develop efficient means of detecting, confining, and neutralizing pollution of the environment.

The study of water pollution, its sources, behavior, migration, and effects on organisms is of extreme importance, since without clean, safe, pure drinking water, humans, plants, and animals cannot survive. Because water plays such a vital role in many different aspects of life and Earth processes, aspects of water pollution are studied by many sciences such as chemistry, geology, geochemistry, biology, forestry, zoology, and civil, mechanical, and chemical engineering. In addition, because of the vital economic role that water has in human endeavors, water pollution is also studied in fields such as resource management and utilization.

Principal terms

CONCENTRATION: the amount of a substance present in water or soil, usually expressed in terms of weight per volume or mass of water

CONTAMINANT: an unwanted chemical, biological, or physical substance that degrades the quality of water

DOSE: the concentration and amount of a substance to which an organism is exposed

EUTROPHICATION: the condition caused by overfertilization of a lake or river, which involves the addition of large amounts of nutrients to the water

HYDROPHOBIC: refers to a material that does not readily associate with water

HYDROSPHERE: the reservoirs of the earth that contain water, including the atmosphere, oceans, surface waters, and groundwater

NONPOINT SOURCE: a widespread source of pollution that may be hard to isolate and identify; examples include runoff from agricultural lands and automobile exhaust

PERSISTENT CHEMICAL: a chemical that is not readily decomposed or neutralized in the environment

POINT SOURCE: a single isolated source of pollution (such as a sewage outfall) that is often easily identified

WATER POLLUTION: the degradation of a water mass because of physical and chemical changes

Bibliography

Berner, E. K., and R. A. Berner. THE GLOBAL WATER CYCLE: GEOCHEMISTRY AND ENVIRONMENT. Englewood Cliffs, N.J.: Prentice-Hall, 1987. A review of the global water cycle that discusses the important natural and anthropogenic inputs of chemicals into the reservoirs of the hydrosphere.

Canter, Larry W., Robert C. Knox, and Deborah M. Fairchild. GROUND WATER QUALITY PROTECTION. Chelsea, Mich.: Lewis, 1987. A high-level text dealing with groundwater flow, water pollution, and water monitoring, analysis, and management.

D'Itri, F. M., and L. G. Wolfson. RURAL GROUND WATER CONTAMINATION. Chelsea, Mich.: Lewis, 1987. While many of the classic problems in water pollution are related to the urban environment, this excellent book, dealing with nonindustrial sources of pollution, treats a wide variety of point and nonpoint sources of pollution having an impact on the quality of ground water in the United States.

Fetter, C. W. APPLIED HYDROGEOLOGY. 2d ed. Columbus, Ohio: Merrill, 1988. An introductory textbook that covers the whole range of groundwater studies. Contains an excellent section giving examples of groundwater pollution and groundwater restoration.

Moore, James W., and S. Ramanmoorthy. HEAVY METALS IN NATURAL WATERS: APPLIED MONITORING AND IMPACT ASSESSMENT. Berlin: Springer-Verlag, 1984. Discusses the chemistry of eight inorganic metals in the environment, along with monitoring approaches and political considerations of heavy metal contamination.

Moore, James W., ORGANIC CHEMICALS IN NATURAL WATERS: APPLIED MONITORING AND IMPACT ASSESSMENT. Berlin: Springer-Verlag, 1984. This review book discusses the chemistry of the different groups of organic pesticides in the environment, emphasizing their behavior in water, sediment, and organisms, and the strategies for prioritization of contamination risks.

Neely, W. B. CHEMICALS IN THE ENVIRONMENT: DISTRIBUTION, TRANSPORT, FATE, ANALYSIS. New York: Marcel Dekker, 1980. A general-to-specialty-level text that details much about geochemical cycling and the behavior of different organic and inorganic chemicals in the hydrosphere. Includes good discussions of mathematical models to aid in environmental studies.

The Chemistry of Air Pollution

Concentration in Solutions

Solutes and Precipitates