Water for people.pdf - WHO Thailand Digital Repository

T H E N A T U R A L W A T E R C Y C L E / 8 7a supply of drinking water with safe levels of arsenic and fluoridebut removal of the concentrations may be the only solution.Organic material from domestic sewage, municipal waste andagro-industrial effluent is the most widespread pollutant globally(UNEP, 1991). It is discharged untreated into rivers, lakes andaquifers, particularly in the more densely populated parts of Asia,Africa and South America and to a varying extent around certainurban centres over the remainder of the world. Its volume has risenover the last hundred years and this rise is likely to continue into thefuture as the pace of development accelerates. It contains faecalmaterials, some infected by pathogens, which lead to increased ratesof morbidity and mortality among populations using the water.This organic material also has high concentrations of nutrients,particularly nitrogen and phosphorus, which cause eutrophication oflakes and reservoirs, promoting abnormal plant growth and depletingoxygen. Nitrogen levels have also risen because of the increased useof nitrogenous fertilizers in agriculture, in both developed anddeveloping countries. There is concern because nitrate concentrationsin large numbers of sources of surface water and groundwater exceedthe WHO guideline of 10 milligrams per litre. In many parts of theworld trends in many heavy metal concentrations in river water haverisen due to leaching from waste dumps, mine drainage and melting,to the extent that they can reach five to ten times the naturalbackground level (Meybeck, 1998). Concentrations of organicmicropollutants from the use of pesticides, industrial solvents and likematerials have also increased. There is anxiety about the healtheffects of these and other pollutants, but the consequences ofexposure to these substances is often not clear.For the developed world, acidification of surface water was aserious problem in the 1960s and 1970s, particularly in Scandinavia,western and central Europe and in the north-east of North America,but since then sulphur emissions have decreased and the acid rainproblem has diminished. The main impact was on aquatic life whichgenerally cannot survive with pH levels below 5, but there are alsohealth problems because higher acidity raises concentrations ofmetals in drinking water. Acidification is likely to continue incountries and regions with increasing industrialization, such as Indiaand China.For the developing world, increasing salinity is a serious form ofwater pollution. Poor drainage, fine grain size and high evaporationrates tend to concentrate salts in the soils of irrigated areas in aridand semi-arid regions. Salinity affects large areas, some to a limitedextent, others more severely. In some cases natural salinity ismobilized from the landscape by the clearance of vegetation foragriculture and the increased infiltration this may cause.Shiklomanov (forthcoming) estimates that some 30 percent of theworld’s irrigated area suffers from salinity problems and remediationis seen to be very costly (Foster et al., 2000).Most rivers carry sediment in the form of suspended load andbed load, in some cases the latter is charged with metals and othertoxic materials (see map 4.5). This sediment load is adjusted to theflow regime of the river over time, and changes to this regimeaccompanied by increases or decreases in the load can causeproblems downstream. These include the progressive reduction ofreservoir volumes by siltation, the scouring of river channels andthe deposition of sediment in them, threatening flood protectionmeasures, fisheries and other forms of aquatic life. River diversions,including dams, can produce some of these effects on sediment, butin addition they may alter the chemical and biological characteristicsof rivers, to the detriment of native species. The world total ofsuspended sediment transported to the oceans is reported to be ashigh as 51.1 billion tons per year (Walling and Webb, 1996).Despite regional and global efforts to improve the situationsince the 1970s, knowledge of water quality is still incomplete,particularly for toxic substances and heavy metals (Meybeck, 1998).In addition, there appear to be no estimates of the world totalvolume of polluted surface water and groundwater, nor the severityof this pollution. Shiklomanov (forthcoming) provides estimates ofthe volume of wastewater produced by each continent, whichtogether gave a global total in excess of 1,500 km 3 for 1995. Thenthere is the contention that each litre of wastewater pollutes atleast 8 litres of freshwater, so that on this basis some 12,000 km 3of the globe’s water resources is not available for use. If this figurekeeps pace with population growth, then with an anticipatedpopulation of 9 billion by 2050, the world’s water resources wouldbe reduced by some 18,000 km 3 .Human impacts on water resourcesPreceding paragraphs have discussed various aspects of theinfluence of human activities on the hydrological cycle and on waterresources. In turn there are also those aspects of land use whichinfluence the hydrological cycle. Wetlands, for example, can haveprofound effects, many beneficial to humankind, including floodstorage, low flow maintenance, nutrient cycling and pollutanttrapping (Acreman, 2000). This view forms a key component of thepolicies stemming from the Convention on Wetlands (Davis, 1993)and those of many national initiatives, some concerned with theeconomic value of wetlands (Laurans et al., 1996).Studies of the hydrological impacts of changing land use have along and well documented history (Swanson et al., 1987; Blackie etal., 1980; Rodda, 1976; Sopper and Lull, 1967). The techniquescomprising the paired basin approach were developed in Switzerlandin the 1890s, with subsequent research following in Japan and theUnited States between 1910 and 1930. Similar basins, usually smalland contiguous with the same land use, were instrumented in orderto measure their water balances to quantify the effects of change,