The phrase heavy metals is used here as a general name for metals with densities in excess of 5 g/cm3. About 15 species are of practical concern. Heavy metals may be applied to soils deliberately to correct nutrient deficiencies or to kill pests. Very small amounts are needed to correct deficiencies, and these do not cause pollution. Repeated applications of inorganic pesticides containing heavy metals (for example, in sprays applied to fruit trees) may add amounts to soils large enough to be harmful. In contrast to organic pesticides, heavy metals do not disappear through decomposition but remain in soil indefinitely. Additional sources of soil contamination by heavy metals are industrial and traffic exhausts, flooding of land by contaminated waters, sewage sludge applied to land, and disposal of other refuse.
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Heavy metals participate in several kinds of reactions in soils, and these affect their concentrations and solubility. The metal ions tend to be bonded to soil constituents through cation exchange; this may amount to substantial quantities even though concentrations in the soil solution are usually low. Some soil constituents seem to have specific affinities for heavy metal ions, resulting in their preferential adsorption over more abundant cations. The concentrations of heavy metals in the soil solution are also affected by equilibria with hydroxyl, carbonate, and phosphate ions. Precipitation of heavy metals by these anions can limit concentrations even though fairly large amounts are added to soil. On the other hand, some heavy metal ions are strongly chelated by organic substances of low molecular weight, thereby altering their adsorption behavior and permitting rather high concentrations in the soil solution. The actual concentration in a soil is thus a function of reactions of heavy metals with a variety of soil constituents.
Cadmium is considered as one of the most hazardous of the heavy metals because of its presumed effect on the development of vascular disease. Amounts of cadmium in soils are normally below 1 ppm, but values as high as 1700 ppm have been reported for surface samples collected near zinc-ore smelters. Cadmium is usually associated with zinc in nature, and the geochemical relationship between the two leads to their common occurrence with Zn/Cd ratios near 900. Cadmium is easily taken up by most plants. Some are quite sensitive to excess cadmium, and others are not.
(ii) Chromium (Cr)
This metal is a major component of the wastes of the plating industry. Cr is toxic for plant growth only at high concentrations. Chromium mobility within plants is extremely low. Soil pollution by chromium is seldom a problem because it is taken up by plants as chromate, a form that hardly occurs at prevailing pH values and redox potentials.
(iii) Cobalt (Co)
This can be highly toxic to plants. Most plant species cannot tolerate concentrations of cobalt exceeding 0.1 ppm. Usually cobalt contents of soil do not exceed 10 ppm. Preferential cobalt adsorption on soil constituents and fixation in clay mineral lattices might add to the problem.
(iv) Copper (Cu)
Copper is toxic to most plants at concentrations exceeding 0.1 ppm. Its concentration in drinking water for human consumption is considered safe when not exceeding 1.0 ppm. Concentrations above 20 ppm in feed and forage are toxic to sheep. Normal copper contents of soils are around 20 ppm. Mobility and displacement of copper in soils are low because of its strong bonding with organic matter and clay minerals.
(v) Lead (Pb)
This may accumulate in soils along roads from traffic exhausts and in the vicinities of lead-zinc smelters. Roadside concentrations as high as 2400 ppm have been reported. While (excessive) intake of lead by humans and animals is considered a serious health hazard, the primary pathway of such intakes is probably via surface contamination of crops and grasses (eaten by grazing animals) rather than via plant uptake. The mobility of lead in soil and plants tends to be low though in some cases considerable uptake by plants has been observed. Normal lead levels in plants range from 0.5-3 ppm. With respect to plant growth, lead toxicity levels appear to differ considerably for different plant species.
(vi) Mercury (Hg)
Extensive mercury poisoning was first reported at Minamata, Japan, in 1953. As a result of the strong interactions between mercury compounds and soil constituents, displacement of mercury in forms other than vapor is usually very low. Methylation of mercury, possibly occurring in nature under restricted conditions, constitutes one of the most serious hazards related to this element, because in this form mercury will accumulate easily in food chains. Because of this hazard, the use of alkylmercury fungicides for seed dressings has been banned in many countries.
(vii) Molybdenum (Mo)
This element is best known for its deficiency in certain soils. Under normal conditions molybdenum predominates in anionic form (molybdate), subject to adsorption by iron oxides and hydroxides much like phosphate. While normal molybdenum content in plants is around 0.1 ppm, toxicity symptoms have been observed at levels above 200-300 ppm (dry matter).
(viii) Nickel (Ni)
This element tends to be highly toxic to plants. As it is easily taken up by plants when present in soils, care must be exercised in disposal of waste containing nickel. Total nickel contents in soils range from 5-500 ppm, with 100 ppm as a rough mean value. The concentration in the soil solution is usually around 0.005-0.05 ppm, and contents in healthy plants do not exceed 1 ppm (dry matter).
(ix) Zinc (Zn)
The use of this element in galvanized iron is widespread. Zinc commonly occurs in soils at levels of 10-300 ppm, with 30-50 ppm as a rough average range. Sewage sludges may have very high zinc contents, and the possible accumulation of zinc in soil after disposal of such wastes deserves attention. In plants, zinc will become toxic at levels exceeding about 400 ppm (dry matter), where it probably interferes with the uptake of other essential elements. In soil, zinc appears to be rather mobile.
Wastes and soil pollution
The large amount of waste produced every day in towns and cities and other human settlements end up in soil. The most common kinds of wastes can be classified into four types: agricultural, industrial, municipal, and nuclear (Table 5.13).
Table 5.13. Wastes and Soil Pollution
Sources
Effects
Agriculture
(i) accumulation of animal manures
(ii) excessive input of chemical fertilizers
(iii) illicit dumping of tainted crops on land
Mining and Quarrying
(i) using of explosives to blow up mines
(ii) using of machineries which emit toxic byproducts and leaks to the ground
Sewage sludge
Improper sanitation system causes sludge to leak at surrounding soil
Household
(i) improper waste disposal system causes waste accumulation
(ii) improper sanitation system
Dredged spoils
Method of dredging at fertile land causes soil infertility, leaving the soil more prone to external pollution
Demolition and construction
Nonbiodegradable rubbles or debris which undergo chemical reactions and increase soil toxicity
Industrial
Poisonous/toxic gases which are not filtered or neutralized
Control of Soil pollution
The following general methods of controlling soil pollution are in use.
Effluents should be properly treated before discharging them on to soil.
Solid wastes should be properly collected and disposed of by appropriate method.
From the wastes, recovery of useful products should be done.
Microbial degradation of biodegradable substances reduces soil pollution.
5.5 Marine Pollution: Causes, Effects and Control
The sea, which covers around 70 per cent of the earth’s surface, is home to millions of fish, crustaceans, mammals, microorganisms, and plants. It is a vital source of food for both animals and people. Thousands of birds rely on the sea for their daily food supplies. Fishermen throughout the world catch over 90 million tons of fish every year, and in many developing countries, fish is the principal source of protein.
People also depend on the sea for many of their medicines. Marine animals and plants contain many chemicals that can be used to cure human ailments: an estimated 500 sea species yield chemicals that could help treat cancer.
But the oceans now are in a very bad shape. People have treated the sea as a dumping ground for thousands of years, offloading rubbish, sewage, and more recently – industrial waste. Marine pollution frequently originates on land, entering the sea via rivers and pipelines. This means that coastal waters are dirtier than the open seas, with estuaries and harbours being especially badly affected. Additional pollution is actually created at sea by activities such as dredging, drilling for oil and minerals, and shipping.
Marine Pollution
For close to thirty years, most academics studying the phenomena of marine pollution have adhered to a definition developed by a UN body, the Group of Experts on the Scientific Aspects of Marine Pollution (GESAMP), who define it as
“Introduction by man, directly or indirectly, of substances or energy into the marine environment (including estuaries) resulting in such deleterious effects as harm to living resources, hazard to human health, hindrance to marine activities including fishing, impairment of quality for use of sea-water, and reduction of amenities.”
The definition has two important aspects:
First, it is action oriented. Marine pollution results from human activity. Thus, earthquakes or volcanic eruptions in the ocean floor and subsequent damage or change to the ocean ecosystems is not considered as pollution.
Second, the definition is amenable to measurement. Marine pollution is harmful, and its danger can be identified in a variety of ways. For example, it is easy to see the deleterious effects that oil spills have on the sea birds and mammals that happen to run into them. Scientists likewise can readily identify various toxic substances found in the marine environment, measure their quantities, and provide estimates of their potential danger for the health of both marine life and humans.
The important sources of marine pollution are shown in Fig. 5.4.
Toxics
Toxic waste is the most harmful form of pollution to marine creatures. Once a form of toxic waste affects an organism, it can be quickly passed along the food chain and might eventually end up in seafood, causing various problems. Toxic wastes arrive from the leakage of landfills, dumps, mines and farms. Sewage and industrial wastes introduce chemical pollutants like DDT. Farm chemicals (insecticides and herbicides) along with heavy metals (e.g. mercury and zinc) can have disastrous effect on marine life.
Mercury the most dangerous toxic element
Top priority is usually given to control the pollutant that poses a threat to human health, the most serious being mercury. Major sources of mercury include rivers, marine outfalls and direct dumping of chemical waste. Natural inputs like the weathering of mercury-bearing rocks, volcanic gases also contribute to mercury in the ocean. Dissolved mercury in the sea is adsorbed onto particulate matter and also forms stable complexes with organic compounds occurring in the sea. Inorganic mercury can be easily accumulated by living organisms.
Fig. 5.4. Sources of marine pollution.
A classic example of mercury poisoning happened in Minamata, a small Japanese coastal town dependent on fishing for a livelihood. In 1952, a nearby factory producing vinyl chloride and acetaldehyde using mercuric sulphate as a catalyst dumped its wastes in Minamata bay. Typically 300-1000 g of mercury is lost for each ton of acetaldehyde produced, 5% of which is in the form of methyl mercury. Mercuric chloride when used as a catalyst produces 1 g of methyl mercury per ton of product. Accumulated contamination was as high as 200 ppm mercury at the factory outfall.
The effects began with the death of a large number of fish in the early 1950s. This affected birds, cats, pigs, and humans. Birds lost coordination to fly. Cats were seen running in circles and foaming at the mouth. Local residents called these occurrences “the disease of the dancing cats”. Later, the disease was termed “Minamata Disease” when humans began to have symptoms of methyl mercury poisoning.
Other Toxic Materials
Toxic materials are substances derived from industrial, agricultural, household cleaning, gardening and automotive products. They do not always kill wildlife, but they can threaten inland and coastal waters. Examples of toxic materials include:
Dioxins come from bleaching paper, incineration of solid wastes containing PVC and other materials, and the process of making herbicides. Dioxins and related compounds degrade slowly and are toxic to marine life. They cause genetic chromosomal aberrations in marine life and are suspected of causing cancer in humans.
PCBs are used in the making electrical equipments and hydraulic fluids. Developmental problems in children and reproductive problems in some other animals have been linked to PCBs. Slowly degrading PCBs accumulate as they pass along the ocean food web.
PAHs come from oil spills, road runoff, and burning wood and coal.
Marine life and people suffer ill effects from PAHs. PAHs cause genetic and chromosomal problems in fish and most marine organisms.
Sewage and fertilizers
The discharge of sewage can cause public health problems either from contact with polluted waters or from consumption of contaminated fish or shellfish. The discharge of untreated sewage effluents also produces long-term adverse impacts on the ecology of critical coastal ecosystems in localized areas due to the contribution of nutrients and other pollutants. Pollution due to inadequate sewage disposal causes nutrient enrichment around population centers, and high nutrient levels and even eutrophication near treatment facilities and sewage outfalls.
Around the world, untreated sewage flows into coastal waters, carrying organic waste and nutrients that can lead to oxygen depletion, as well as disease-causing bacteria and parasites that require closing beaches and shellfish beds. The inadequate number of sewage treatment plants in operation, combined with poor operating conditions of available treatment plants, and the disposal practices of discharging mostly untreated wastewater are likely to have an adverse effect on the ocean.
Oil
The sites most vulnerable for accidents are areas where tankers and barges move through restricted channels and in the vicinity of ports. In spite of regulations established, tankers and barges do not always use port facilities for the disposal of bilge and tank washing and wastes, and a significant amount of oil, which exceeds that from accidental oil spills, is discharged into the coastal areas.
The impact of oil pollution on the ecology of coastal and marine ecosystems is particularly destructive following massive oil spills caused by maritime accidents. However, gas exchange between the water and the atmosphere is decreased by oil remaining on the surface of the water, with the possible result of oxygen depletion in enclosed bays where surface wave action is minimal. Coral death results from smothering when submerged oil directly adheres to coral surfaces and oil slicks affect sea birds and other marine animals. In addition, tar accumulation on beaches reduces tourism potential of coastal areas.
Mining and Dredging
Mining affects the marine ecosystem and the habitat. Mining can erode beaches, degrade water quality, and spoil coastal habitats. Mining coral to process for lime can remove the habitat of local marine species and weakens coastal storm defense. Mined or dredged areas take a very long time to recover. Because of this, strict regulations govern the dredging of the ocean floor
Synthetic Organic Chemicals
Many different synthetic organic chemicals enter the ocean and become incorporated into organisms. Ingestion of small amounts can cause illness or death. Halogenated hydrocarbons are a class of synthetic hydrocarbon compounds that contain chlorine, bromine, or iodine are used in pesticides, flame retardants, industrial solvents, and cleaning fluids. The level of synthetic organic chemicals in seawater is usually very low, but some organisms can concentrate these toxic substances in their flesh at higher levels in the food chain. That is an example of biological amplification.
Marine debris
More garbage such as plastic bags, rope, helium balloons, and stray fishing gear, build up in the oceans every year. Synthetic materials stay in the environment for years, killing or injuring ocean species, like whales and turtles, which mistake litter for food or get entangled in it. Ghost fishing by lost nets not only kills innocent ocean creatures but also reduces fishers’ catches.
Plastic is not biodegradable and therefore affects the oceans for long periods of time. Sea turtles mistake plastic bags for jellyfish and die from internal blockages. Seals and sea lions starve after being muzzled by six-pack rings or entangled by nets.
Effects of Marine Pollution on Living Marine Resources
Tens of thousands of chemicals are used to meet society’s technological and economic needs. Marine pollution is not only attributed to oil and chemical spills, but much of the debris and toxic substances affecting marine animals, in actual fact, originate on land. Pesticides, plastic bags, balloons, cigarette butts, motor oil, fishing line, find their way into local waterways either though direct dumping, through storm drains (whatever is left on streets, parking lots, can be washed into storm drains which lead directly to local waters), or through sanitary sewers, affecting living marine resources.
The time taken by a few common types of litter to biodegrade is given in Table 5.14.
Two basic ways by which chemical contaminants can affect living marine resources are:
By directly affecting the exposed organism’s own health and survival, and
By contaminating those resources that other species, including humans, may consume.
Researchers have been studying this dual impact of contaminants using a variety of marine organisms ranging from bottom-dwelling invertebrates and fish to species such as salmon and marine mammals. These biological effects include:
Diseases such as liver lesions in bottom fish,
Decreased reproductive success in bottom fish,
Impaired immune competence in anadromous fish, and
Growth impairment in invertebrates.
Marine pollution can have serious economic impact on coastal activities and on those who exploit the resources of the sea. In most cases such damage is caused primarily by the physical properties of these pollutants creating nuisance and hazardous conditions.
Table 5.14. Degradation time of materials
Materials
Time to degrade
Materials
Time to degrade
Tin cans
50 years
Wool
1 year
Painted wood
13 years
Plastic rings
400+ years
Newspaper
6 weeks
Plastic bottles
450 years
Paper towels
2 – 4 weeks
Aluminium cans
200 years
Disposable diapers
450 years
Monofilament line
600 years
Polystyrene foam
Indefinite
Cardboard
2 months
Impact on coastal activities
Contamination of coastal amenity areas is a common feature of many spills leading to public disquiet and interference with recreational activities such as bathing, boating, angling and diving. Hotel and restaurant owners and others who gain their livelihood from the tourist trade can also be affected.
Oil and chemical spills can adversely affect industries that rely on a clean supply of seawater for their normal operations. If substantial quantities of floating or sub-surface pollutants are drawn through intakes, contamination of the condenser tubes may result, requiring a reduction in output or total shutdown.
Simply, the effects of marine pollution are caused by either the physical nature of the pollutants themselves (physical contamination and smothering) or by their chemical components (toxic effects and accumulation leading to tainting). Marine life may also be affected by clean-up operations or indirectly through physical damage to the habitats in which plants and animals live.
The main threat posed to living resources by the persistent residues of spilled oils and water-in-oil emulsions (“mousse”) is one of physical smothering. The animals and plants most at risk are those that could come into contact with a contaminated sea surface:
Marine mammals and reptiles.
Birds that feed by diving or form flocks on the sea.
Marine life on shorelines and
Animals and plants in Mari culture facilities.
Subsequently the inability of individual marine organisms to reproduce, grow, feed or perform other functions can be caused by prolonged exposure to pollutants, if not eventual death. Sedentary animals in shallow waters such as oysters, mussels and clams that routinely filter large volumes of seawater to extract food are especially likely to accumulate oil components and harmful chemicals, poisoning consumers.
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In addition to that, birds, whales and other marine creatures often mistake cigarette butts (which find their way into the waters) for food. The butts contain small plastic pieces that can interfere with the digestion of food, casing marine life to starve. Monofilament fishing line can be lethal to seals, sea lions, fish and other animals. Many marine species, including seals, herring, gulls, sharks, and shellfish have died or suffered injuries from plastic bags, nets and monofilament fishing lines.
Impacts on specific marine habitats
The impact that marine pollution can have on selected marine habitats are given below. Within each habitat a wide range of environmental conditions prevail and often there is no clear division between one habitat and another.
In coastal areas some marine mammals and reptiles, such as turtles, may be particularly vulnerable to adverse effects from contamination because of their need to surface to breathe and to leave the water to breed. The impact of oil on shorelines may be particularly great where large areas of rocks, sand and mud are uncovered at low tide. The amenity value of beaches and rocky shores may require the use of rapid, effective clean-up techniques, which may not be compatible with the plants and animals.
In tropical regions, mangrove trees have complex breathing roots above the surface of the organically rich and oxygen-depleted mud in which they live. Oil may block the openings of the air breathing roots of mangroves or interfere with the trees’ salt balance, causing leaves to drop and the tress to die. Fresh oil entering nearby animal burrows can damage the root systems and the effect may persist for some time inhibiting decolonization by mangrove seedlings.
Living corals grow on the calcified remains of dead coral colonies that form overhangs, crevices and other irregularities inhabited by a rich variety of fish and other animals. If the living coral is destroyed the reef itself may be subject to wave erosion.
Birds which congregate in large numbers on the sea or shorelines to breed, feed or molt are particularly vulnerable to oil pollution. Although oil ingested by birds during preening may be lethal, the most common cause of death is from drowning, starvation and loss of body heat when their body surfaces are coated with oil.
Impact on fisheries and Mariculture
The pollutants in the waters, especially in the case of oil spills can also damage boats and gears used for catching or cultivating marine species. Floating equipment and fixed traps extending above the sea surface are more likely to become contaminated by floating oil whereas submerged nets, pots, lines and bottom trawls are usually well protected, provided they are not lifted through an oily sea surface.
An oil or chemical spill can also cause loss of market confidence since the public may be unwilling to purchase marine products from the region irrespective of whether the seafood is actually tainted. Bans on the fishing and harvesting of marine products may be imposed following a spill, both to maintain market confidence and to protect fishing gear and catches from contamination.
5.6 Noise Pollution
Noise usually means unwanted sound of appreciable intensity which goes on for a length of time (seconds to hours) that irritates people. The noise may emanate from factories, offices and market place, roads (traffic-related), running and shuttling of trains, landing and take-offs of aircrafts at airports, use of loudspeakers in meetings, rallies, celebrations, etc. When the quality and the intensity of the noise is practically constant (varying less than ±5 dBA) over an appreciable time (seconds or longer), it is often called “steady-state” noise. The first reaction to any form of unwanted sound is annoyance, followed by irritation, restlessness and extreme reaction. Since noise travels through air, all forms of noise are considered as polluting air and noise is considered as an air pollutant.
Sound is defined as a pressure variation that the human ear can detect. Just like dominoes, a wave motion is set off when an element sets the nearest particle of air into motion. This motion gradually spreads to adjacent air particles further away from the source. Depending on the medium, sound propagates at different speeds. In air, sound propagates at a speed of approximately 340 m/s. In liquids and solids, the propagation velocity is greater, 1500 m/s in water and 5000 m/s in steel.
Compared to the static air pressure (105 Pa), the audible sound pressure variations are very small ranging from about 20 µPa (20 Ã- 10-6 Pa) to 100 Pa. The sound pressure level of 20 µPa corresponds to the average person’s threshold of hearing. A sound pressure of approximately 100 Pa is so loud that it causes pain and is therefore called the threshold of pain. The ratio between these two extremes is more than a million to one.
Sound pressure level alone is not a reliable indicator of loudness. The frequency or pitch of a sound also has a substantial effect on how humans will respond. While the intensity (energy per unit area) of the sound is a purely physical quantity, the loudness or human response depends on the characteristics of the human ear.
A direct application of linear scales (in Pa) to the measurement of sound pressure leads to large and unwieldy numbers. Therefore, the acoustic parameters are conveniently expressed as a logarithmic ratio of the measured value to a reference value. This logarithmic ratio is called a decibel or dB. Using dB, the large numbers are converted into a manageable scale from 0 dB at the threshold of hearing (20 µPa) to 130 dB at the threshold of pain (~100 Pa). Some examples of common noise and their decibel levels are given in Table 5.16.
The decibel scale is open-ended. 0 dB or dBA should not be construed as the absence of sound. Instead, it is the generally accepted threshold of best human hearing. Sound pressure levels in negative decibel ranges are inaudible to humans. On the other extreme, the decibel scale can go much higher. For example, gun shots, explosions, and rocket engines can reach 140 dBA or higher at close range. Noise levels approaching 140 dBA are nearing the threshold of pain. Higher levels can inflict physical damage on such things as structural members of air and spacecraft and related parts.
Table 5.16. Equivalent sound levels in decibels normally occurring inside various places
Place
Leq (decibels)
Small Store (1-5 persons)
60
Large Store (more than 5 persons)
65
Small Office (1-2 desks)
58
Medium Office (3-10 desks)
63
Large Office (more than 10 desks)
67
Miscellaneous Business
63
Residence
Typical movement of people – no TV or radio
Speech at 10 feet, normal voice
TV listening at 10 feet, no other activity
Stereo music
40-45
55
55-60
50-70
How is noise measured?
Basically, there are two different instruments to measure noise exposures: the sound level meter and the dosimeter. A sound level meter is a device that measures the intensity of sound at a given moment. Since sound level meters provide a measure of sound intensity at only one point in time, it is generally necessary to take a number of measurements at different times during the day to estimate noise exposure over a workday. This measurement method is generally referred to as area noise monitoring.
A dosimeter is like a sound level meter except that it stores sound level measurements and integrates these measurements over time, providing an average noise exposure reading for a given period of time such as an 8-hour workday. The dosimeter measures noise levels in those locations in which a person works or spends long intervals of time. Such procedures are generally referred to as personal noise monitoring.
Human hearing is limited not only to the range of audible frequencies, but also in the way it perceives the sound pressure level in that range. In general, the healthy human ear is most sensitive to sounds between 1,000 Hz – 5000 Hz, and perceives both higher and lower frequency sounds of the same magnitude with less intensity. In order to approximate the frequency response of the human ear, a series of sound pressure level adjustments is usually applied to the sound measured by a sound level meter. The adjustments, or weighting network, are frequency dependent.
The A-scale approximates the frequency response of the average young ear when listening to most ordinary everyday sounds. When people make relative judgments of the loudness or annoyance of a sound, their judgments correlate well with the A-scale sound levels of those sounds. There are other weighting networks that have been devised to address high noise levels or other special problems (B-scale, C-scale, D-scale etc.) but these scales are rarely, if ever, used in conjunction with highway traffic noise. Noise levels are generated in the A-scale as dBA. In environmental noise studies, A-weighted sound pressure levels are commonly referred to as noise levels.
Sources of noise
Various sources of noise (Table 5.17) are industry, road traffic, rail traffic, air traffic, construction and public works, indoor sources (air conditioners, air coolers, radio, television and other home appliances), etc. In Indian conditions, indiscriminate use of public address system and diesel generator (DG) sets, has given a new dimension to the noise pollution problem.
Noise in Industrial Areas. Mechanized industry creates serious noise problems, su
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