Major environmental issues are not only of concern to the general public, but are challenging problems for the chemical industry and for chemical engineers. The goal is to provide an appreciation of the impacts that human activities can have on the environment. Also, the importance of healthy ecosystems are illustrated as they affect human welfare, the availability of natural resources, and economic sustainability.
When considering the potential impact of any human activity on the environment, it is useful to regard the environment as a system containing interrelated sub-processes. The environment functions as a sink for the wastes released as a result of human activities. The various subsystems of the environment act upon these wastes, generally rendering them less harmful by converting them into chemical forms that can be assimilated into natural systems. It is essential to understand these natural waste conversion processes so that the capacity of these natural systems is not exceeded by the rate of waste generation and release.
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The impact of waste releases on the environment can be global, regional, or local in scope. On a global scale, man-made (anthropogenic) greenhouse gases, such as methane and carbon dioxide, are implicated in global warming and climate change. Hydrocarbons released into the air, in combination with nitrogen oxides originating from combustion processes, can lead to air quality degradation over urban areas and extend for hundreds of kilometres. Chemicals disposed of in the soil can leach into underground water and reach groundwater sources, having their primary impact locally, near to the point of release.
The timing of pollution releases and rates of natural environmental degradation can affect the degree of impact that these substances have. For example, the build-up of greenhouse gases has occurred over several decades. Consequently, it will require several decades to reverse or stall the build-up that has already occurred. Other releases, such as those that impact urban air quality, can have their primary impact over a period of hours or days.
The environment is also a source of raw materials, energy, food, clean air, water, and soil for useful human purposes. Maintenance of healthy ecosystems is therefore essential if a sustainable flow of these materials is to continue. Depletion of natural resources due to population pressures and/or unwise resource management threatens the availability of these materials for future use.
A short review of environmental issues, including global energy consumption patterns, environmental impacts, ecosystem health, and natural resource utilization are given here:
1. Global Energy Issues:
The availability of adequate energy resources is necessary for most economic activity and makes possible the high standard of living that developed societies enjoy. Although energy resources are widely available, some such as oil and coal are non-renewable, and others, such as solar, although inexhaustible, are not currently cost effective for most applications. An understanding of global energy usage patterns, energy conservation, and the environmental impacts associated with the production and use of energy are therefore very important.
Often, primary energy sources such as fossil fuels must be converted into another form such as heat or electricity. As the second law of thermodynamics dictates, such conversions will be less than 100 per cent efficient. An inefficient user of primary energy is the typical automobile, which converts into motion about 10 per cent of the energy available in crude oil.
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The global use of energy has steadily risen since the dawn of the industrial revolution. Currently, fossil fuels make up roughly 85 per cent of the world’s energy consumption, while renewable sources such as hydroelectric, solar, and wind power account for only about 8 per cent of the power usage. Nuclear power provides roughly 6 per cent of the world energy demand, and its contribution varies from country to country. The United States meets about 20 per cent of its electricity demand, Japan 28 per cent, and Sweden almost 50 per cent from nuclear power.
The disparity in global energy use is illustrated by the fact that 65-70 per cent of the energy is used by about 25 per cent of the world’s population. Energy consumption per capita is greatest in industrialised regions such as North America, Europe, and Japan.
Another interesting aspect of energy consumption by industrialised countries and the developing world is the trend in energy efficiency, the energy consumed per unit of economic output. Future chemical engineers will need to recognise the importance of energy efficiency in process design.
Many environmental effects are associated with energy consumption. Fossil fuel combustion releases large quantities of carbon dioxide into the atmosphere. During its long residence time in the atmosphere, CO2 readily absorbs infrared radiation contributing to global warming. Further, combustion processes release oxides of nitrogen and sulphur oxide into the air where photochemical and/or chemical reactions can convert them into ground level ozone and acid rain. Hydropower energy generation requires widespread land inundation, habitat destruction, alteration in surface and groundwater flows, and decreases the acreage of land available for agricultural use.
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Nuclear power has environmental problems linked to uranium mining and spent nuclear rod disposal. ‘Renewable fuels’ are not benign either. Traditional energy usage (wood) has caused widespread deforestation in localised regions of developing countries. Solar power panels require energy-intensive use of heavy metals and creation of metal wastes. Satisfying future energy demands must occur with a full understanding of competing environmental and energy needs.
The atmosphere allows solar radiation from the sun to pass through without significant absorption of energy. Some of the solar radiation reaching the surface of the earth is absorbed, heating the land and water. Infrared radiation is emitted from the earth’s surface, but certain gases in the atmosphere absorb this infrared radiation, and re-direct a portion back to the surface, thus warming the planet and making life, as we know it, possible. This process is often referred to as the greenhouse effect.
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The surface temperature of the earth will rise until a radiative equilibrium is achieved between the rate of solar radiation absorption and the rate of infrared radiation emission. Human activities, such as fossil fuel combustion, deforestation, agriculture and large-scale chemical production, have measurably altered the composition of gases in the atmosphere. Some believe that these alterations will lead to a warming of the earth-atmosphere system by enhancement of the greenhouse effect. Figure 1.2 summarises the major links in the chain of environmental cause and effect for the emission of greenhouse gases.
Table 1.1 is a list of the most important greenhouse gases along with their anthropogenic (man-made) sources, emission rates, concentrations, residence times in the atmosphere, relative radiative forcing efficiencies, and estimated contribution to global warming. The primary greenhouse gases are water vapour, carbon dioxide, methane, nitrous oxide, chlorofluorocarbons, and tropospheric ozone. Water vapour is the most abundant greenhouse gas, but is omitted because it is generally not from anthropogenic sources. Carbon dioxide contributes significantly to global warming due to its high emission rate and concentration.
ii. Photochemical Smog:
Photochemical smog is initiated by the photochemical dissociation of NO2 and the consequent secondary reactions involving unsaturated hydrocarbons, other organic compounds and free radicals, leading to the formation of organic peroxides and ozone. This phenomenon takes place during sunny days with low winds and low level inversion. Photochemical smog and the consequent formation of aerosols reduce visibility, cause irritation to eyes and damage plants and rubber goods.
The oxidation of SO2 can also take place by interaction with the free radical HO’ present in photochemical smog –
Chemical oxidation of SO2 may also take place in water droplets, present in aerosols. This reaction is accelerated in the presence of NH3 and catalysts, e.g. oxides of Mn, Fe, Cu, Ni.
Solid particles, such as soot, bring about catalytic oxidation of SO2 by providing a heterogeneous phase for contact. Soot is formed during combustion of solid and liquid fuels in domestic and industrial operations, and automobile emissions.
Sulphur dioxide is a pollutant responsible for smog formation, acid rains and corrosion of metals and alloys.
iii. Oxidation of Organic Compounds:
Organic compounds such as hydrocarbons, aldehydes and ketones absorb solar radiation and undergo various photochemical and chemical reactions involving free radicals. Some of these reactions are catalysed by particulate matter such as soot and metal oxides. Some of the resultant intermediates and final products contribute to photochemical smog formation.
The earth is heated by sunlight and some of the heat that is absorbed by the earth is radiated back into space.
However, some of the gases in the lower atmosphere, acting like glass in a greenhouse, allow solar radiations (in the range 300 to 2,500 nm, i.e. near UV, visible and near infrared region, while filtering the dangerous UV radiations, i.e. < 300 nm) but do not allow the earth to re-radiate the heat into space. In other words, these gases in the atmosphere are transparent to the sunlight coming in, but they strongly absorb infrared radiation, which the earth sends back as heat. A part of the heat so trapped in these atmospheric gases is reemitted to the earth’s surface.
The net result is the heating of the earth’s surface by this phenomenon, called the ‘greenhouse effect’. The gases that are responsible for this greenhouse effect are CO2, water vapour, CH4 and man-made chlorofluorocarbons (CFCs). Water vapour strongly absorbs infrared radiations in the range 4,000 to 8,000 nm and CO2 in the range 12,000 to 16,300 nm. The radiations in the range 8,000 to 12,000 nm escape unabsorbed and this is known as the region of atmospheric window.
Carbon dioxide is released by volcanoes, oceans, decaying plants as well as human activities, such as deforestation and combustion of fossil fuels. Automobile exhausts account for 30 per cent of CO2 emissions in developed countries.
Methane is released from coal mines, decomposition of organic matter in swamps, rice paddy cultivation, guts of termites in forest debris and stomachs of ruminants.
Chlorofluorocarbons (CFCs) are used as coolants in refrigerators, propellants in aerosol sprays, plastic foam materials like ‘thermocoles’ or ‘styrofoam’ and in automobile air conditioners.
In fact, the ‘greenhouse gases’ (particularly CO2 and water vapour) are responsible for keeping our planet warm and thus sustaining life on the earth. If the greenhouse gases were very less or totally absent then the average temperature on the earth would have been at sub-zero levels.
But, however, if the concentration of greenhouse gases increases, they may trap too much of heat, which may threaten the very existence of life on earth. For instance, the CO2 present in the atmosphere of the planet Venus, is about 60,000 times more than that on earth. Hence, the average temperature of Venus is about 425°C, making the existence of life impossible there.
Oceans and bio-mass are the major sinks for atmospheric CO2. Oceans convert CO2 into soluble bicarbonates. The photosynthetic activity in the green plants increases with the increase in CO2 level in the atmosphere. Forests are the places where lot of photosynthetic activity occurs. They also act as vast reservoirs of fixed but readily oxidisable carbon in the form of vegetation, wood and humus. Hence, forests maintain a balance in the atmospheric CO2 level, and deforestation upsets this balance and increases the atmospheric CO2 level.
It is estimated that the atmospheric CO2 content has increased by 25 per cent during the last two centuries. This is mostly attributed to the industrial revolution and is one of the reasons for the slight increase in the global temperature (about 0.5°C). Since the concentrations of greenhouse gases have been continuously increasing because of deforestation, industrialisation, increased burning of fossil fuels, mining, exhausts from increasing number of automobiles and other anthropogenic activities, there is an increasing concern about the possible ‘global warming’.
Some scientists fear that if proper precautions are not taken, the concentration of greenhouse gases in the atmosphere may double within the next 50-100 years. If this happens, the average global temperature may increase by 4°– 5°C. This will increase the evaporation of surface waters, which may influence climatic changes depending upon the pattern of cloud formation. For instance, low-level dense clouds may exert cooling effect whereas high-level thin cloud formation may exert heating effect due to increased greenhouse effect.
The projections from computer modelling regarding the climatic changes that could be triggered off due to ‘global warming’ reveal alarming scenarios. Even a 1.5°C rise in surface temperature can adversely affect food production in the world. Thus, the wheat growing zones in the northern latitude may be shifted from the USSR and Canada to the Polar Regions, i.e. from fertile soils to poor soils near the North Pole. The biological productivity of the ocean would also decrease due to warming of the earth’s surface layer, which in turn, may reduce the transport of nutrients from deeper layers to the surface by vertical circulation.
Computer modelling also indicates the following effects due to ‘global warming’ – melting of the polar ice caps; dry areas becoming drier; humid areas like the Amazon suffering more intense tropical storms; drastic drop in food production, particularly in lands within 35 degrees north and south of the Equator; increased breeding of pests and diseases due to more humid conditions; shorter, wetter and warmer winters and longer, hotter and drier summers, particularly in mid-continental areas.
Global warming may also trigger increased thermal expansion of oceans and melting of glaciers, which may result in an increase in the sea-level by 20 cm to 1.5 metres by the latter part of the 21st century. Thus, cities like Mumbai, Miami, London, Venice, Bangkok and Leningrad may become extremely vulnerable. Defences against the rising sea-levels and expanding oceans are very difficult and expensive, which many nations cannot afford. Further, a global temperature rise, is likely to cause more floods, hurricanes, and tornadoes.
There are differences of opinion among experts regarding the dynamics and effects of ‘global warming’ due to the complexity of natural phenomena that might be operating simultaneously. More accurate future climatic projections will be possible with better super-computer models, based on greater understanding of the complex natural climatic forces involved. But until that time, the possible devastating effect due to ‘global warming’ by the ‘greenhouse effect’ cannot be underestimated.
Some of the steps suggested to minimise the ‘greenhouse effect’ include reduction in the use of fossil fuels, encouraging the use of alternative sources of energy (e.g. solar, geothermal, wind, bio-gas, etc.), conservation of forests, extensive afforestation, encouraging community forestry, reduction in the use of automobiles, research in the development of more efficient automobile engines, ban on CFCs and nuclear explosions, development of environmentally compatible technologies with the help of intensive inter-disciplinary research, effective check on the growth of population and imparting of non-formal and formal environmental education.
v. Ozone Depletion in the Stratosphere:
There is a distinction between ‘good’ and ‘bad’ ozone (O3) in the atmosphere. Tropospheric ozone, created by photochemical reactions involving nitrogen oxides and hydrocarbons at the earth’s surface, is an important component of smog. A potent oxidant, ozone irritates the breathing passages and can lead to serious lung damage. Ozone is also harmful to crops and trees.
Stratospheric ozone, found in the upper atmosphere, performs a vital and beneficial function for all life on earth by absorbing harmful ultraviolet radiation. The potential destruction of this stratospheric ozone layer is therefore of great concern.
The stratospheric ozone layer is a region in the atmosphere between 12 and 30 miles (20-50 km) above ground level in which the ozone concentration is elevated compared to all other regions of the atmosphere. In this low-pressure region, the concentration of O3, can be as high as 10 ppm (about 1 out of every 1,00,000 molecules).
Ozone is formed at altitudes between 25 and 35 km in the tropical regions near the equator where solar radiation is consistently strong throughout the year. Because of atmospheric motion, ozone migrates to the Polar Regions and its highest concentration is found there at about 15 km in altitude. Stratospheric ozone concentrations have steadily declined over the past 20 years.
Ozone equilibrates in the stratosphere as a result of a series of natural formation and destruction reactions that are initiated by solar energy. The natural cycle of stratospheric ozone creation and destruction has been altered by the introduction of man-made chemicals. CFCs are highly stable chemical structures composed of carbon, chlorine, and fluorine. One important example is trichlorofluoromethane, CCI3F, or CFC-11.
CFCs reach the stratosphere due to their chemical properties; high volatility, low water solubility, and persistence (non-reactivity) in the lower atmosphere. In the stratosphere, they are photo-dissociated to produce chlorine atoms, which then catalyse the destruction of ozone –
The chlorine atom is not destroyed in the reaction and can cause the destruction of up to 10000 molecules of ozone before forming HCl by reacting with hydrocarbons. The HCl eventually precipitates from the atmosphere. A similar mechanism as outlined above for chlorine also applies to bromine, except that bromine is an even more potent ozone destroying compound. Interestingly, fluorine does not appear to be reactive with ozone. Figure 1.3 summarises the major steps in the environmental cause and effect chain for ozone-depleting substances.
CFCs were first introduced in the 1930’s for use as refrigerants and solvents. By the 1950’s significant quantities were released into the atmosphere. Releases reached a peak in the mid-eighties (CFC-11 and CFC-12 combined were about 700 million kg). Releases have been decreasing since about 1990. The Montreal Protocol, which instituted a phase-out of ozone-depleting chemicals, is the primary reason for the declining trend. The growth in accumulation of CFCs in the environment has been halted as a result of the Montreal protocol.
2. Air Quality Issues:
Air pollution arises from a number of sources, including stationary, mobile, and area sources. Stationary sources include factories and other manufacturing processes. Mobile sources are automobiles, other transportation vehicles, and recreational vehicles such as snowmobiles and watercraft. Area sources are emissions associated with human activities that are not considered mobile or stationary.
Examples of area sources include emissions from lawn and garden equipment, and residential heating. Pollutants can be classified as primary, those emitted directly to the atmosphere, or secondary, those formed in the atmosphere after emission of precursor compounds. Photochemical smog (the term originated as a contraction of smoke and fog) is an example of secondary pollution that is formed from the emission of volatile organic compounds (VOCs) and nitrogen oxides (NOx), the primary pollutants.
Air quality problems are closely associated with combustion processes occurring in the industrial and transportation sectors of the economy. Smog formation and acid rain are also closely tied to these processes. In addition, hazardous air pollutants, including chlorinated organic compounds and heavy metals, are emitted in sufficient quantities to be of concern. Figure 1.4 shows the primary environmental cause and effect chain leading to the formation of smog.
i. Criteria Air Pollutants:
Clean Air Act which charged the Environmental Protection Agency (EPA) with identifying those air pollutants which are most deleterious to public health and welfare, and empowered EPA to set maximum allowable ambient air concentrations for these criteria air pollutant EPA identified six substances as criteria air pollutants. Table 1.2 and promulgated primary and secondary standards that make up to the National Ambient Air Quality Standards (NAAQS). Primary standards are intended to protect the public health with an adequate margin of safety. Secondary standards are meant to protect public welfare, such as damage to crops, vegetation, and ecosystems or reductions in visibility.
Criteria pollutants are a set of individual chemical species that are considered to have potential for serious adverse health impacts, especially in susceptible populations. These pollutants have established health-based standards and were among the first airborne pollutants to be regulated.
ii. NOx, Hydrocarbons and VOCs—Ground-Level Ozone:
Ground-level ozone is one of the most pervasive and intractable air pollution problems in the United States and other developing countries. We should again differentiate between this ‘bad’ ozone created at or near ground level (tropospheric) from the ‘good’ or stratospheric ozone that protects us from UV radiation.
Ground-level ozone, a component of photochemical smog, is actually a secondary pollutant in that certain precursor contaminants are required to create it. The precursor contaminants are nitrogen oxides (NOx, primarily NO and NO2) and hydrocarbons. The oxides of nitrogen along with sunlight cause ozone formation, but the role of hydrocarbons is to accelerate and enhance the accumulation of ozone.
Oxides of nitrogen (NOx) are formed in high-temperature industrial and transportation combustion processes. Health effects associated with short-term exposure to NO2 (less than three hours at high concentrations) are increases in respiratory illness in children and impaired respiratory function in individuals with pre-existing respiratory problems. Industry makes a significant contribution to the ‘fuel combustion’ category from the energy requirements of industrial processes. Major sources of hydrocarbon emissions are the chemical and oil refining industries, and motor vehicles.
Solvents comprise 66 per cent of the industrial emissions and 34 per cent of total VOC emissions. It should be noted that there are natural (biogenic) sources of HCs/VOCs, such as isoprene and monoterpenes that can contribute significantly to regional hydrocarbon emissions and low-level ozone levels. Ground-level ozone concentrations are exacerbated by certain physical and atmospheric factors. High-intensity solar radiation, low prevailing wind speed (dilution), atmospheric inversions, and proximity to mountain ranges or coastlines (stagnant air masses) all contribute to photochemical smog formation.
Human exposure to ozone can result in both acute (short-term) and chronic (long-term) health effects. The high reactivity of ozone makes it a strong lung irritant, even at low concentrations. Formaldehyde, peroxyacetylnitrate (PAN), and other smog-related oxygenated organics are eye irritants. Ground-level ozone also affects crops and vegetation adversely when it enters the stomata of leaves and destroys chlorophyll, thus disrupting photosynthesis. Finally, since ozone is an oxidant, it causes materials with which it reacts to deteriorate, such as rubber and latex painted surfaces.
CO is a colourless, odourless gas formed primarily as a by-product of incomplete combustion. The major health hazard posed by CO is its capacity to bind with haemoglobin in the blood stream and thereby reduce the oxygen-carrying ability of the blood. Transportation sources account for the bulk (76.6 per cent) of total national CO emissions. Areas with high traffic congestion generally will have high ambient CO concentrations. High localised and indoor CO levels can come from cigarettes (second-hand smoke), wood-burning fireplaces, and kerosene space heaters)
Lead in the atmosphere is primarily found in fine particulates, up to 10 microns in diametre, which can remain suspended in the atmosphere for significant periods of time. Tetraethyl lead (CH3CH2)4 – Pb) was used as an octane booster and antiknock compound for many years before its full toxicological effects were understood. The Clean Air Act of 1970 banned all lead additives and the dramatic decline in lead concentrations and emissions has been one of the most important yet unheralded environmental improvements of the past twenty-five years. In 1997, industrial processes accounted for 74.2 per cent of remaining lead emissions, with 13.3 per cent resulting from transportation, and 12.6 per cent from non-transportation fuel combustion.
Lead also enters waterways in urban runoff and industrial effluents, and adheres to sediment particles in the receiving water body. Uptake by aquatic species can result in malformations, death, and aquatic ecosystem instability. There is a further concern that increased levels of lead can occur locally due to acid precipitation that increases lead’s solubility in water and thus its bioavailability. Lead persists in the environment and is accumulated by aquatic organisms.
Lead enters the body by inhalation and ingestion of food (contaminated fish), water, soil, and airborne dust. It subsequently deposits in target organs and tissue, especially the brain. The primary human health effect of lead in the environment is its effect on brain development, especially in children. There is a direct correlation between elevated levels of lead in the blood, especially in the urban areas of developing countries that have yet to ban lead as a gasoline additive.
Particulate matter (PM) is the general term for microscopic solid or liquid phase (aerosol) particles suspended in air. PM exists in a variety of sizes ranging from a few Angstroms to several hundred micrometres. Particles are either emitted directly from primary sources or are formed in the atmosphere by gas-phase reactions (secondary aerosols).
Since particle size determines how deep into the lung a particle is inhaled, there are two NAAQS for PM, PM2.5, and PM10. Particles smaller than 2.5 μm are called ‘fine’; are composed largely of inorganic salts (primarily ammonium sulphate and nitrate), organic species, and trace metals. Fine PM can deposit deep in the lung where removal is difficult. Particles larger than 2.5 μm are called ‘coarse’ particles, and are composed largely of suspended dust. Coarse PM tends to deposit in the upper respiratory tract, where removal is more easily accomplished.
Coarse particle inhalation frequently causes or exacerbates upper respiratory difficulties, including asthma. Fine particle inhalation can decrease lung functions and cause chronic bronchitis. Inhalation of specific toxic substances such as asbestos, coal mine dust, or textile fibres are now known to cause specific associated cancers (asbestosis, black lung cancer, and brown lung cancer, respectively).
An environmental effect of PM is limited visibility in many parts of the United States including some National Parks. In addition, nitrogen and sulphur containing particles deposited on land increase soil acidity and alter nutrient balances. When deposited in water bodies, the acidic particles alter the pH of the water and lead to death of aquatic organisms. PM deposition also causes soiling and corrosion of cultural monuments and buildings, especially those that are made of limestone.
vi. SO2, NOx, and Acid Deposition:
Sulphur dioxide (SO2) is the most commonly encountered of the sulphur oxide (SOx) gases, and is formed upon combustion of sulphur-containing solid and liquid fuels (primarily coal and oil). SOx are generated by electric utilities, metal smelting, and other industrial processes. Nitrogen oxides (NOx) are also produced in combustion reactions; however, the origin of most NO, is the oxidation of nitrogen in the combustion air. After being emitted, SOx and NOx can be transported over long distances and are transformed in the atmosphere by gas phase and aqueous phase reactions to acid components (H2SO4 and HNO3).
The gas phase reactions produce microscopic aerosols of acid-containing components, while aqueous phase reactions occur inside existing particles. The acid is deposited to the earth’s surface as either dry deposition of aerosols during periods of no precipitation or wet deposition of acid-containing rain or other precipitation. There are also natural emission sources for both sulphur and nitrogen- containing compounds that contribute to acid deposition. Water in equilibrium with CO2 in the atmosphere at a concentration of 330 ppm has a pH of 5.6. When natural sources of sulphur and nitrogen acid rain precursors are considered, the ‘natural’ background pH of rain is expected to be about 5.0. As a result of these considerations, ‘acid rain’ is defined as having a pH less than 5.0. Figure 1.5 shows the major environmental cause and effect steps for acidification of surface water by acid rain.
Major sources of SO2 emissions are non-transportation fuel combustion (84.7 per cent), industrial processes (8.4 per cent), transportation (6.8 per cent), and miscellaneous (0.1 per cent). The goal of this programme is to decrease acid deposition significantly by controlling SO2 and other emissions from utilities, smelters, and sulphuric acid manufacturing plants, and by reducing the average sulphur content of fuels for industrial, commercial, and residential boilers.
There are a number of health and environmental effects of SO2, NOx, and acid deposition. SO2 is absorbed readily into the moist tissue lining the upper respiratory system, leading to irritation and swelling of this tissue and airway constriction. Long-term exposure to high concentrations can lead to lung disease and aggravate cardiovascular disease. Acid deposition causes acidification of surface water, especially in regions of high SO2 concentrations and low buffering and ion exchange capacity of soil and surface water.
Acidification of water can harm fish populations, by exposure to heavy metals, such as aluminum which is leached from soil. Excessive exposure of plants to SO2 decreases plant growth and yield and has been shown to decrease the number and variety of plant species in a region.
Hazardous air pollutants (HAPs), or air toxics, are airborne pollutants that are known to have adverse human health effects, such as cancer. Currently, there are over 180 chemicals identified on the Clean Air Act list of HAPs (US EPA 1998). Examples of air toxics include the heavy metals mercury and chromium, and organic chemicals such as benzene, hexane, perchloroethylene (perc), 1, 3-butadiene, dioxins, and polycyclic aromatic hydrocarbons (PAHs).
The Clean Air Act defined a major source of HAPs as a stationary source that has the potential to emit 10 tons per year of anyone HAP on the list or 25 tons per year of any combination of HAPs. Examples of major sources include chemical complexes and oil refineries. The Clean Air Act prescribes a very high level of pollution control technology for HAPs called maximum achievable control technology (MACT). Small area sources, such as dry cleaners, emit lower HAP tonnages but taken together are a significant source of HAPs. Emission reductions can be achieved by changes in work practices such as material substitution and other pollution prevention strategies.
HAPs affect human health via the typical inhalation or ingestion routes. HAPs can accumulate in the tissue of fish, and the concentration of the contaminant increases up the food chain to humans. Many of these persistent and bio-accumulative chemicals are known or suspected carcinogens.
3. Water Quality Issues:
The availability of freshwater in sufficient quantity and purity is vitally important in meeting human domestic and industrial needs. Though 70 per cent of the earth’s surface is covered with water, the vast majority exists in oceans and is too saline to meet the needs of domestic, agricultural, or other uses. Of the total 1.36 billion cubic kilometres of water on earth, 97 per cent is ocean water, 2 per cent is locked in glaciers, 0.31 per cent is stored in deep groundwater reserves, and 0.32 per cent is readily accessible freshwater (4.2 million cubic kilometres).
Freshwater is continually replenished by the action of the hydrologic cycle. Ocean water evaporates to form clouds, precipitation returns water to the earth’s surface, recharging the groundwater by infiltration through the soil, and rivers return water to the ocean to complete the cycle. In the United States, freshwater use is divided among several sectors; agricultural irrigation 42 per cent, electricity generation 38 per cent, public supply 11 per cent, industry 7 per cent, and rural uses 2 per cent. Groundwater resources meet about 20 per cent of US water requirements, with the remainder coming from surface water sources.
Contamination of surface and groundwater originates from two categories of pollution sources. Point sources are entities that release relatively large quantities of waste-water at a specific location, such as industrial discharges and sewer outfalls. Non-point sources include all remaining discharges, such as agricultural and urban runoff, septic tank leachate, and mine drainage. Another contributor to water pollution is leaking underground storage tanks. Leaks result in the release of pollution into the subsurface where dissolution in groundwater can lead to the extensive destruction of drinking water resources.
Besides the industrial and municipal sources we typically think of in regard to water pollution, other significant sources of surface and groundwater contamination include agriculture and forestry. Contaminants originating from agricultural activities include pesticides, inorganic nutrients such as ammonium, nitrate, and phosphate, and leachate from animal waste. Forestry practices involve widespread disruption of the soil surface from road building and the movement of heavy machinery on the forest floor. This activity increases erosion of topsoil, especially on steep forest slopes. The resulting additional suspended sediment in streams and rivers can lead to light blockage, reduced primary production in streams, destruction of spawning grounds, and habitat disruption of fisheries.
Transportation sources also contribute to water pollution, especially in coastal regions where shipping is most active.