Issue 2: Pollution of existing supplies |
| Description |
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The quality of water from different sources varies widely. Precipitation absorbs gases from the atmosphere and removes particles from the air. When precipitation strikes the ground it becomes surface water runoff or enters the ground. Surface water flows into larger and larger channels, ponds, lakes and rivers until some of it reaches the sea. Along its course, surface water picks up both organic and mineral particles, bacteria and other organisms as well as salts and other soluble substances. Water in fresh water bodies sometimes acquires odours, tastes and colours from algae and other organisms and from decaying vegetation. For hundreds of years heavy metals from mining and pathogens from cities have caused serious, although localised, contamination. Since the industrial revolution, water pollution problems have become first regional, then continental and now global in nature. Much water is polluted when it is used in industry and agriculture or for domestic purposes. Mining is the major cause of metal contamination, whereas other industries contribute to acidification. The intensification of agricultural activities has led to the contamination of groundwater by fertilisers and other chemicals. Moreover, irrigation projects often cause a rapid rise in the level of groundwater, which leads to waterlogging and soil salinity. Recent Global Environmental Monitoring System (GEMS) assessments have found that the main water pollutants are sewage, nutrients, toxic metals and industrial as well as agricultural chemicals. Conclusions drawn from the GEMS assessment include: · The nature and level of freshwater pollution strongly depends on socio-economic development; · the most common water pollutant is organic material from domestic sewage, municipal waste and agro-industrial effluent; · the high water nitrate levels found in western Europe and the United States are a result of the nitrogen fertilisers and manure used for intensive agriculture; · the GEMS assessment also noted a dramatic increase in the use of fertilisers in developing countries, particularly where intensive irrigation allows for double or triple cropping. Other conditions highlighted in the GEMS report include deforestation, eutrophication, suspended particulate matter (SM) and salinity. Deforestation, i.e. the clearing of land for agriculture and urban development, often leads to water contamination. When the soil is stripped of its protective vegetative covering, it becomes prone to erosion. This in turn leads to higher water turbidity, because of the increased amounts of suspended matter, to nutrient leaching and to a decreased water-retention capacity of the soil. There is also concern about the destruction of wetlands, which destroys the habitat of many species and removes natural filter mechanisms, permitting many common pollutants to reach water supplies. Eutrophication is the enrichment of waters with nutrients, especially phosphorus and nitrogen. It can lead to enhanced plant growth and depleted oxygen levels as this plant material decays. It is not always a human-induced problem, but is often linked to organic waste and agricultural runoff. Today 30 to 40 percent of the world's lakes and reservoirs are eutrophic. Not all intervention has been successful, but eutrophication can be reversible if mid- and long- term strategies are enacted. Laws and measures introduced to reduce tripolyphosphates (used mostly in detergents) and to remove phosphorus from wastewater have had positive effects. Suspended consists of materials that float in suspension in water. There are three main sources of suspended matter: natural soil erosion, matter formed organically within a water body and material produced as a by- product of human activity. Suspended matter settles on the sediment bed and forms deposits in rivers, lakes, deltas and estuaries. River damming affects the amount of suspended matter flowing from rivers to the oceans because reservoirs act as effective sinks for suspended matter. An estimated 10 percent of the global suspended matter discharge to the sea is trapped in reservoirs. Approximately 25 percent of the water currently flowing to the oceans has been previously stored in a reservoir. Damming can also greatly modify water quality; waters flowing out of reservoirs not only have reduced suspended matter quantities, they are also depleted of nutrients and are often more saline, which consequently has detrimental effects on downstream agriculture and fisheries. Salinity is a significant and widespread form of freshwater pollution, particularly in arid, semi-arid and some coastal regions. The primary cause of salinisation is a combination of poor drainage and high evaporation, rates which concentrate salts on irrigated land. Salinity can adversely affect the productivity of irrigated crops and is also detrimental to industrial and household water users. The estimated global gross area of irrigated land is 270 million ha. About 20 to 30 million ha are severely affected by salinity while an additional 60 to 80 million ha are affected to some degree. Waterlogged soil, which aggravates the problem of salinity, is usually caused by overwatering and a lack of proper drainage systems. Runoff from agricultural areas fertilised with manure and chemicals pollutes watercourses and groundwater by increasing levels of nutrients. Source: UNEP. 1991. Freshwater Pollution. UNEP/GEMS Environmental Library. No. 6, Nairobi. |
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GIWA definition |
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This issue refers to pollution of surface fresh waters as a result of point or diffuse sources. WHO criteria for drinking water are employed as they are the basis of many national standards. Other criteria for monitoring surface waters vary widely but a measure of potability is a good surrogate for more general environmental health of fresh water systems. |
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Environmental impacts |
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1. Modification of riparian habitats 2. Depletion of fish stocks and species diversity 3. Changes in terrestrial and aquatic biodiversity and food webs 4. Potential for impacts on migratory species such as water birds |
Indicators | |
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The following are environmental impact indicators for pollution of existing supplies. Select the environmental impact indicators that are relevant from the list below and input data into the appropriate indicator sheets. | |
| POL |
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| PAR |
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| HYD |
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| EFF |
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| BIO |
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| FW |
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| PROX |
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