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jump to: [Introduction] [Urban Runoff] [Soil Erosion] [Water Pollution] [Solid Waste] [Solutions][Self Test]
1. IntroductionOne of the striking features of the distribution of the world's population is the tendency for large human concentrations to occur near vast expanses of water. Since the beginning of the Industrial Revolution, urban development have influenced the flow and storage of water, as well as the quality of available fresh water.
Urbanization has had significant impacts on the hydrology of the environment by controlling:
2. Urban Runoff
In cities and suburbs, where much of the land is paved or covered -- streets, buildings, shopping centers, airport runways-- rainwater runs off as much as ten times faster than on unpaved land. Since this water cannot be absorbed into the soil, it flows rapidly down storm drains or through sewer systems, contributing to floods and often carrying debris and other pollutants to streams.
Figure 1 illustrates the components of water balance in the hydrological cycle. [Precipitation is either absorbed by the ground (A), or it becomes surface runoff (S). The absorbed water is divided between baseflow (B) (underground runoff to rivers) and evapotranspiration (E) , i.e., A=B+E ). The total runoff (T) is equal to the surface runoff plus the baseflow.] Urbanization has caused the surface runoff to increase, and the replenishment of underground water to decrease.
Impervious surfaces and sewers cause runoff after a rainstorm to occur more rapidly and with a greater peak flow than under nonurban conditions (see Figure 2). In turn, larger peak flows increase the frequency of floods. In general we can say that urbanization increases flood volume, frequency and peak value . The net result of this flushing effect may be to increase turbidity, pollutant loads, and bank erosion.
It has been observed that the size
of small floods is increased by urbanization, while during large, infrequent
floods there are not significant differences between the way rural and
urban areas behave (Martens, 1968; Hollis, 1975). An explanation for this
difference has been presented by Hollis, who explains that in a large flood,
rural catchments may saturate, acting as impervious surfaces.
3. Soil ErosionRunoff on bare land leads to erosion. Grass, trees, and other plants hold the soil in place and avoid erosion. The highest rates of erosion are produced in urban areas during construction, when there is a large amount of exposed ground and disturbances produced by excavation. Suspended sediment concentration in rivers and streams, where construction area water drains, has been found to be much higher (up to 100 times) than those in undisturbed areas. Once construction ceases, the rates of erosion fall dramatically. In general, erosion is the source of sediment that fills streams, pollutes water, kills aquatic life, and shortens the useful life of dams and reservoirs.
4. Water PollutionWater pollution includes chemicals, sediments, disease causing organisms, and heat. If the source of pollutants is discharge of wastewater from a fixed point such as a municipal or industrial plant effluent pipe, the pollution is referred to a point source pollution. If the pollutants do not enter the watercourse as a fixed point, such as surface runoff from precipitation, the pollution is referred to as a nonpoint source. Point source pollution is most evident and easy to control. As a matter of fact, industrial and municipal point sources in many cities today represent a small component of water pollution. It is the nonpoint source pollution derived from urban and agricultural runoff that are responsible for most pollution.
Sewers and drains, carry domestic
and industrial wastes that might have been widely deposited on the soil,
but have been channeled into rivers and other bodies of water. The storm
sewer system is designed to carry the excess water that flows over land
during and after a rainfall directly into a river, stream or lake. The
sanitary sewer system carries sewage from residential, commercial , and
industrial sources to the waste water treatment plant. At a waste water
treatment plant most harmful substances are removed from sewage before
it is piped to a nearby body of water . A combined sewer system allows
both sewage and stormwater to flow in one pipe, which goes into a water
treatment plant. (see Figure 3). Sometimes when it rains hard, the
water treatment plant cannot handle the large volume of sewage and stormwater.
In this instance, in order to avoid flooding the combined waste water treatment
plant, untreated or inadequately treated waste waters may be directly diverted
to a stream . This discharge into a stream is known as a combined sewer
After sewage is treated properly, it is clean enough to be pumped to a river, lake or ocean, but it is not safe to drink. To make water from rivers and lakes safe to drink, many communities depend on water purification plants. Here water is filtered and chlorinated to kill disease causing organisms. By EPA standards water is safe to drink if the coliform bacterial count is 0/100 ml; water is safe for swimming if the coliform bacterial count is less than 200/100 ml.
Table 2 presents a comparison of contaminant profiles for urban surface runoff and raw domestic sewage.
Suspended Solids include slit and clay particles from soil runoff, plankton, industrial waste, and sewage. High concentration of suspended solids reduces water clarity, contributes to a decrease in photosynthesis, can bind with toxic compounds and heavy metals, and can lead to an increase in water temperature through greater absorption of sunlight by surface waters. Warm water holds less oxygen than cold water, and so it can be detrimental to some aquatic life.
Eutrophication is the process by which a body of water (often shallow) becomes (either naturally or by pollution) rich in dissolved nutrients with a seasonal deficiency in dissolved oxygen. Cultural eutrophications refers to the enrichment of water, usually by phosphorus, from human activities.
Total Phosphorus includes organic phosphorus and inorganic phosphates. Organic phosphorus is a part of living plants and animals. Inorganic phosphates are ions bonded to soil particles, and phosphates present in laundry detergents. Anthropogenic sources of phosphorus are:
Total Nitrogen is a fundamental plant nutrient and it is required by all living plants and animals for building protein. In aquatic ecosystem, nitrogen is present in many different forms. In its molecular form, nitrogen is useless to most aquatic plants, except green-blue algae, which convert molecular nitrogen into nitrates and ammonia. Plants then take ammonia and nitrates through their roots. Aquatic organisms obtain their nitrogen by eating aquatic plants. Bacterial decomposition of aquatic plants and animals results in ammonia, which is in turn oxidized by specialized bacteria to form nitrates and nitrites. This process is oxygen demanding and, like in the case of phosphorus, can also cause eutrophication. Nitrogen, however, is not a limiting growing factor in plants (unlike phosphorus), so plants are not as sensitive to increases in ammonia and nitrates. Anthropogenic sources of nitrogen are:
Anthropogenic depletion in dissolved oxygen can be caused by the presence of organic materials, such as sewage, discharge from food processing plants, and decaying plants and animals. Decomposition of organic matter is carried out by bacteria in an aerobic process. Excessive bacterial consumption of oxygen can bring down dissolved oxygen to levels which can cause major shifts in the kinds and diversity of aquatic organisms.
The presence of excessive organic nutrients (phosphorus and nitrogen) can also be a major force in high biochemical oxygen demand. The presence of nutrients can increase plant growth, and thus increase the demand of oxygen for aerobic decomposition by bacteria.
Fecal Coliform Bacteria are derived from the feces of humans and other animals. These bacteria can contaminate rivers, lakes and oceans by direct discharge, from agricultural and storm runoff, and from sewage discharge into the water. In a combined sewer system, after heavy rains, untreated or inadequately treated waste may be diverted into a body of water.
Fecal coliform bacteria by themselves are not pathogenic, that is, they do not cause illness or disease. If coliform bacteria are present in amounts greater than 200 colonies/ml of water, it is likely that other pathogenic organisms are present. These include bacteria, viruses and parasites that cause disease and illness. Since pathogens are usually scarce in water, they are difficult to measure; instead coliform levels are monitored, because of their greater abundance, and their association with pathogenic organisms.
Chlorides: The use of salt for de-icing has increased the chloride content in some areas' runoff.
An excellent example of water pollution by municipal and industrial wastes and how this problems are being addressed is provided by the River Rouge Program in Southeastern Michigan.
5. Solid WasteOne of the major problems associated with urbanization is the disposal of solid waste. For example, Mexico City produces approximately 10,000 tons of domestic garbage per day. Until 1987 most of the residues were disposed of in open fields. Recently, a more modern system of sanitary filling into dredged pits has become operational. Waste disposal will remain a problem until a better disposal service is implemented.
Waste disposal generally falls into two categories: (a) throwaway methods, like incineration, landfilling, and ocean dumping, and (b) low-waste methods, like recycling and composting.
Incineration produces air pollution, while landfills give off gases that contribute to global warming, and pollute underground water supplies. Landfills provide a ready dumping ground for non-hazardous waste, but these spaces are running out. Some landfills create their own problems. For example, older ones have no lining and wastes can filter down through the soil and into the water supply. In most landfills little air is available to decompose the waste. Items buried 20 years ago have been dug up intact.
The ocean floors are also being affected because of the vast bulk of waste material that humans are creating. Rivers are an important conduit for the transport of waste to the oceans; direct dumping of urban and industrial waste is also an important source. Waste-solid disposal by coastal cities is sufficiently large to modify shorelines, and it covers adjacent ocean bottoms with characteristic deposits on a scale large enough to be geologically significant. Not only are the rates of sedimentation high, but the anthropogenic waste tend to contain high contents of such substances as carbon and heavy metals.
6. SolutionsRapid Urban Growth creates the following problems:
The rapid population growth also demands increased efficiencies in agricultural productivity, distribution and marketing. Present problems in agriculture include competition for water and soil degradation due to erosion, leaching and poor cultivation practices in many regions of the world. There is a solution available to the urban waste problem, which is also agreeable to the problems encountered in agriculture: reformat urban waste for agricultural application to increase soil fertility. This is possible because more than half of urban waste consists of soiled paper, degradable sludge, and yard and food wastes. By changing the composition of waste input, (sorting the input waste and processing of the resultant materials) rich compost can be made available for improving soil fertility and biological activity. Waste water can also be properly processed for irrigation and aquaculture.
The urban/agricultural interface
has, not only environmental, but also social and economic benefits. For
example, it provides economic savings to farmers and urban residents and
provides new opportunities for privatization in waste transformation and
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