如题所述
Klaus-Dieter Balke
Institute for Geosciences,University of Tuebingen,Sigwartstr.10,D-72076 Tuebingen,Germany
A wise physician once said:it is the concentration that makes the difference between a substance acting as medicine or posion.
Most people believe that compounds which are highly concentrated endanger human be-ings.But because organisms need a certain cocktail of ingredients to grow up and to live,also the lack or deficiency of a substance can cause diseases.
Inorganic compounds dissolved in water are released from rocks by weathering processes and distributed by water.Therefore,the quality of surface-and groundwater is-besides the influence of temperature and the composition of precipitation-particularly governed by the kind of rocks outcropping in an area.
In limestone areas e.g.,water has high concentrations of calcium and hydrogencar-bonate,in sandstone areas it is often more or less acid,in rocks which are ore bearing high concentrations of heavy metals can be found,etc.
Drinking water contains inorganic and organic compounds.The knowledge of their concentrations and possible effects on human beings is decisive.Generally,the concentration of a sub-stance contained in drinking water must be kept within certain limits,too low as well as too high concentrations often cause health problems.To protect human beings against too high concentrations,limiting values for compounds dissolved in drinking water are fixed by several national and international organisations.Necessary substances which are not present or presentin too small concentrations in drinking water must be added artificially.
1 Biologically important substances
Surface water and groundwater contain-besides main ions such as Calcium（Ca）,Magnesium（Mg）,Sodium（Na）,Potassium（K）,Hydrogencarbonate（HCO3）,Sulphate（SO4）and Chloride（Cl）-ions in minor concentration and trace substances.
It has to be taken into consideration that even trace minerals which occur in concentrations of less than 10-1mg/L can be toxic and even lethal to humans,they also may have the tendency to accumulate in the body.Bioaccumulation in the food chain can also cause severe health problems.
1.1 Iron（Fe）
Iron,a heavy metal,may be a good example for illustrating the problem of concentration.I t is one of the most frequent elements on earth and can be found in nearly every rock,soil and water,enriched to very high concentrations in iron ores.
As a key element in the metabolism of all living organisms iron is of great biochemical importance.It is involved in a wide spectrum of physiological functions,it plays a major role in haemoglobin and myoglobin for oxygen uptake and transport as well as in peroxidases for the protection of cells from oxidative damage,in cytochromes for electron transport and in aconi-tase for energy production.Moreover,it stimulates the growth of cells.
Deficiency as well as overload of iron cause symptoms of disease.Iron deficiency causes anaemia,and is especially dangerous for children and pregnant women.
Iron overload causes liver cirrhosis,diabetes,cardiomyopathy,arthropathy,increased succeptibility to infections,and probably carcinoma.In Germany the concentration limit for iron in drinking water amounts to 0.2mg/L Fe.
1.2 Selenium（Se）
An element which is scarcely known to people is selenium Se.But there exist seleniferous areas,e.g.in China,where selenium dissolved in water with too high concentrations causes diseases.
Concerning the relation between concentrations of selenium and the health of human beings it was found that
（1）＜10 μg/d selenium cause deficiency symptoms，
（2）50 μg/d selenium can be resorbed,
（3）600 μg/d selenium work toxic.
It is recommended that a maximum individual selenium intake of 400μg/d should not be exceeded.
It is expected that at too high intake rates Keshan's disease will occur with symptoms such as acute or chronic heart function insufficiency,heart enlargement,gallop rhythm,arhytmia,ECG changes.
The limiting value for drinking water is 0.01mg/L Se in Germany.
1.3 Calcium（Ca）
But even a“common”element such as calcium Ca that is normally the main cation of surface-and shallow groundwater can be harmful.As an element of biologic importance it is an essential compound e.g.in bones and teeth.Drinking water should not have concentrations below 25mg/L in order to avoid decomposition of Ca-containing organs,and it shoud not have higher concentrations than 250mg/L.In areas where quartzitic sand or sandstone or acid igneous rocks such as granites or rhyolites are outcropping surface-and groundwater have very low calcium contents.Areas with high calcium concentrations in water are those with limestone and dolomite rocks.
1.4 Nitrate（NO3）
Nitrogen N is contained in all organisms and involved in many biologic processes.Nitrate NO3is an ion,formed by oxidation of nitrogen and it is part of salts such as sodium nitrate NaNO3（Chile saltpetre）and potassium nitrate KNO3.As nitrogen is needed for plant growth nitrate is spread out on farmland in order to fertilize soils.Because in many cases over-fertilizing is practiced by farmers,a certain portion of nitrate is washed off into rivers and washed down into aquifers rising the nitrate concentrations above acceptable values.
Nitrate incorporated in human bodies can be reduced to nitrite
1.5 Sulphur（S）
Sulphur S is also a compound of biological relevance.It exists in water mostly as sulphate SO4and is normally dissolved from layers of anhydrit CaSO4or gypsum CaSO4*2 H3O or results from the oxidation of metal sulfides.In connection with magnesium it is purgative and therefore in drinking water only permitted below concentration limits.The limiting value for sulphate amounts to 240mg/L in Germany.
1.6 Fluorine（F）
Fluorine F in water dissolved from minerals such as fluorite CaF2and apatite Ca5[（F,Cl）（PO4）3]and from mica,is an element of biological importance.In small concentrations it protects teeth and bones against decay,in higher concentrations it causes fluorosis,a disease entailing weight loss,anaemia,brittle bones,joint stiffness and mottling of teeth.
Acute ingestion of excess fluoride-the ionic form of fluorine-results in salivation,nausea,vomiting,abdominal pain,and diarrhoea,followed by weakness,muscle spasm,and convulsions.Even death may occur by respiratory paralysis.
Chronic poisoning,developed from continued oral or respiratory intake with the ingestion of more than about 6 mg fluorine per day,results in fluorosis.Areas with a surplus of fluoride in water exist in the Ethiopian rift valley（1.2 to 36.0mg/L）,in Kenia（2mg/L）,Senegal（up to 7.4mg/L）,Japan（up to 7.8mg/L）,Sri Lanka,China,Mexico and in other areas.
The limiting value for drinking water in Germany amounts to 1.5mg/L F.
Despite the problems with fluorosis,fluorine does offer considerable protection against dental cavities.Once fluoride is incorporated into tooth enamel it reduces the solubility of the enamel and protects the tooth from decay.Fluorine is the only chemical that by some water-works is deliberately added to drinking water for therapeutic gain.The problem is that such a procedure is a mass medication and an infringement of civil liberty.In Germany,water is seen as foodstuff which has to be pure and as free of additional substances as possible !
1.7 Substances in water with healing properties
At some sites with special geological properties-often connected with metamorphic rocks,ores,joints and fissures filled with special minerals,sometimes in connection with re-cent volcanism-deep groundwater rising up through natural fault systems in the rocks or extracted by borings contain elements or substances in concentrations higher than usual.Such groundwater,often called as water with healing properties,is drunk or inhaled under medical control in order to act against special diseases.To be recognised medically it must contain at least:
iron.containing water 20mg/L Fe,
iodine-containing water 1mg/L I,
sulphur-containing water 1mg/L H2S,
fluorine-containing water 1mg/L F,
radon-containing water 18nCi/L,
carbon dioxide-containing water 1000mg/L CO2.
2 Heavy metals
Heavy metals,67 metallic elements with specific weights greater than 5g/cm3,are natural constituents of rocks,enriched in ore deposits.In low concentrations they are present in the atmosphere,in soils,in river_,lake_,sea_，…and groundwater.
The presence of heavy metals in water is not only caused by dissolution out of minerals,but also by leaching out of waste and sewage,because heavy metals are used in industry and handicraft in enormous quantities.
As chemical elements heavy metals are not degradable by natural decay.They can precipitate especially as hydroxides（e.g.Fe（OH）3）,oxides（e.g.Fe3O4）,and sulphides（e.g.FeS2）.Many minerals,especially clays,metal oxides,and humic matter,can adsorb and accumulate metal ions on their surfaces.But if the chemical environment changes,in particular into the acid or very alkaline range,the adsorbed metal ions can be desorbed and dis-solved by water.In this regard it must be emphasised that in many areas around the world-especially also in the eastern parts of China-acid rain decreases the pH-value of soils and water and contributes to the dissolution of heavy metals.
Since most of the heavy metals are toxic against human beings it is of great importance to know their concentration in drinking water.
Heavy metals can be incorporated into human beings via the biological cycle,and above individually specified concentration levels they may cause acute and chronic illnesses.On the other hand it has to be taken into consideration that several heavy metals are essential trace elements for humans e.g.copper,selenium,and zinc for metabolic processes.
2.1 Arsenic（As）
Arsenic（As）is present in the lithosphere most often as arsenic sulphide As2S3and metal arsenates e.g.lead arsenate PbHAsO4.Arsenic is an extremely toxic substance but used for several industrial applications（processing of glass,pigments,textiles,paper,wood preserva-tivs,pesticides,feed additives,pharmaceuticals,etc.）.Consequently,arsenic is accumulated in waste materials.Shallow groundwater normally has concentrations of less than 0.01mg/LAs.In mineral waters and thermal waters rising up from deep situated zones very high concentrations may occur.
The health of millions of people is highly endangered in areas where arsenic is dissolved in shallow groundwater.Particularly areas in India,Bangladesh,Taiwan of China,United States,Chile,and Argentina are concerned.
In Germany the limiting value for arsenic in drinking water amounts to 0.01mg/L.
Intoxication include abdominal pain,vomiting,diarrhoea,muscular pain,skin and lung cancer.
2.2 Copper（Cu）
Copper,wide spread in the lithosphere,is a constituent of many minerals.If the oxidation potential in the chemical environment is high,copper is soluble in the form of copper sulphate CuSO4or copper ions.For plants,animals and human beings copper is a vital trace element.On the other hand,in higher concentrations it acts as a strong poison.
Copper pipes of drinking water supply systems become a source of dissolved copper ions if the water becomes acid.By this effect acid rain can cause the death of babies,as was seen in Germany 1988 when the water of a well contained 10mg/L copper.The limiting value defined in Germany for drinking water is 2mg/L Cu.
2.3 Cadmium（Cd）
Cadmium is a rare element but comparatively enriched in zinc ores.As a result of the widespread technical use of zinc and cadmium（e.g.in Ni-Cd-batteries,in alloys and anti-corrosion covers of metal,in high temperature resistant paints,in fertilizers,and as stabilizer in PVC）it is enriched in waste.
Cadmium is a non-essential element for human beings.It affects especially liver and kidneys and is carcinogen.Cadmium caused e.g.the so called“Itai-Itai”poisoning in Japan.There,cadmium was contained in drinking water taken from a river which was polluted by mining activities and by-products of zinc refining.The disease makes bones fragile by exchanging calcium for cadmium.Cigarettes are also cadmium sources（1 cigarette delivers 0.1～0.2μg/L Cd into the human body）.
The limiting value for drinking water amounts to 0.05mg/L Cd in Germany.
2.4 Mercury（Hg）
There are natural sources for mercury,especially volcanic eruptions（approx.1,000t/a）and man-madereleases,especially caused by industry（approx.10,000t/a world-wide）.Mercury is used in the electrical and chemical industry,as toxic compound in plant protective agents,etc.It has been found that nowadays the very toxic methyl mercury CH3HgCl is enriched in the tissues of fishes and so is ultimately incorporated in the bodies of human beings.
In South America,especially in Brazil,for gold extraction out of river sand mercury is applied with the effect that many rivers are highly contaminated by mercury.
It was on the Japanese island of Kyushu where more than 80 tons of mercury,contained in the sewage of a chemical plant,had been discharged since 1932 into the Minamata Bay.The methyl mercury,formed by the activities of micro-organisms,contaminated fish and inflicted in the 1950s thousands of people-about 100 of them died-by disfiguring paralysis or slow death,called Minamata disease.Later on,harbor dredging threatens to send mercury up the food chain once again.
In Germany the limiting value for drinking water is 0.001mg/L Hg.
2.5 Lead（Pb）
Historically,lead has been widely used for water pipes.Water,especially acid water,can dissolve lead out of such pipes.Sources of contamination may also be tin-lead solder,installation parts of water distribution networks and the dissolution out of alloys such as brass and bronze（containing 5% to 6% lead）.Another source of lead is the coating of galvanized pipes.
To avoid high concentrations of lead in potable water lead pipes and lead containing parts within the water distribution system have to be removed.Moreover,lead can be released into surface-and groundwater from several substances,e.g.pesticides,fuel additives,and special paints.There is also an enrichment of lead in cigarettes.Lead is very poisonous for human beings,affecting the lungs,especially in children.
In Germany the limiting value for drinking water is 0.1mg/L Pb.
3 Organic substances
The organic chemicals include a wide range of different compounds such as proteins,vitamins,drugs,disinfectants,petroleum products,herbicides and pesticides.Many organic chemicals have significant physiological effects,often at very low doses.Some are needed by human beings,e.g.vitamins,others are only poisonous.
3.1 Phenol（C6H5OH）
Phenol is an aromatic compound,used as one of the first antiseptics in modern medicine.Water which is contaminated by phenolcauses diarrhoea,nausea,vomiting,abdominal pain,headache,rash,malaise,etc.
3.2 Trichloroethylene（TCE=C2HCl3）
Trichloroethylene which is one of the widely used organic chemicals in industry can affect the central nervous system.
4 Outlook
Chemical contamination of drinking water supplies is of great concern to consumers.But in many cases the overall impact of chemical contaminants in drinking water on human health re-mains unclear.There is a real need for detailed research into the health problems of water and environmental contamination in many countries,especially in dry,and in subtropical and tropical regions.
References
Yang G.and Zhou R.Further observations on the human maximum safe dietary selenium intake in a seleniferous area of China.J.Trace El.Electrolytes Health Dis.,1994.
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