Water Supply and Quality
6.1.3 Chemical contaminants
Chemical substances in water originate from a number of sources. They may occur nat-urally, originate from water treatment or supply technology, or be attributable to in-dustrial or agricultural wastes or operational chemical use. The reasons for controlling some of these substances may be either that they are health hazards or that they affect the acceptability of the water to the con-sumer, due to taste or smell.
It is rare that sudden, massive pollution occurs without rapid remedial action; in such cases the water is often undrinkable or tastes very unpleasant [1]. The guideline values for controlling risks from long-term exposure indicate tolerable concentrations, not target values; the latter should be as low as practicable in order to minimize exposure.
The guidelines are based on scientific evi-dence on routes of exposure, consumption of water, dose–response relationships (taking account of vulnerable groups), analytical data for the frequency of occurrence of con-tamination and magnitude of concentration, and the potential application of available control techniques [1]. For many sub-stances, tolerable daily intakes (TDI) have been calculated; the contribution of water to total exposure from a number of sources is important in setting the guideline for drink-ing-water. The TDI represents daily intake over a lifetime without appreciable health risk. For genotoxic carcinogens, when no
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threshold for effect is assumed, the guide-lines are set at the concentration in drinking-water for which an excess lifetime risk of 10–5 is estimated, i.e. one additional case of cancer per 100 000 population ingesting drinking-water containing the substance at the guideline value for 70 years [1].
There are obvious uncertainties attached to the guidelines. Extrapolation from non-human toxicological data for some sub-stances, together with a lack of information on overall exposure of humans to low levels and any associated effects, mean that a con-siderable safety margin is introduced be-tween exposure at the guideline value and ex-posure at which health effects may occur.
Where there is uncertainty but some evi-dence for health effects a provisional guide-line is given, as is also the case for sub-stances where the guideline would be below detection limits or below practical treatment possibilities. For a large number of sub-stances such as uranium, many pesticides and certain disinfectant by-products, not enough data are available to recommend guidelines. Continual progress is made in the study of health effects; on the basis of new information the revised WHO drinking-water guidelines cover a wider range of stances, and some of the guidelines for sub-stances covered in 1984 have changed.
Some substances are regulated by national standards on the basis of acceptability to the
consumer because of taste, smell or appear-ance. The aesthetic qualities of water are im-portant for public confidence in the overall quality of water. Aluminium, for example, does not have a WHO health-based guide-line value; at normally occurring concen-trations no health effects are observed. The EU Maximum Admissible Concentration (MAC) is 0.2 mg/l; intake at this level would be very small compared with normal food in-takes, reports of which vary between 3 and 160 mg/day [5]. The level of 0.2 mg/l is pri-marily associated with aesthetic problems, since at greater concentrations aluminium hydroxide floc can form with consequent consumer complaints. Excretion of alumin-ium takes place primarily through the kid-neys, with the result that people with im-paired kidney function accumulate excess aluminium. It is important that water used for renal dialysis does not contain significant concentrations of aluminium, since alumin-ium accumulation in the brain can lead to se-vere effects including impaired memory, aphasia, dementia, ataxia and convulsions [5].
There follows below a discussion of con-cerns in relation to some important sub-stances. Some recommended guideline values, together with information on the con-tribution of drinking-water to intakes, are shown in Table 6.1. (See also Chapter 10.) Arsenic is readily absorbed from water and
Table 6.1: Estimated percentage contribution to intake of some inorganic constituents from drinking-water at WHO guideline concentrations
widely distributed in various tissues. The most important effects of consuming ar-senic-contaminated drinking-water are the development of skin tumours on the hands and feet and peripheral vascular disturb-ances that can lead to gangrene. Thickening of the skin of the palms of the hands and soles of the feet, together with increased pig-mentation, is also characteristic [6]. These effects contrast with an increased risk of lung cancer in people who inhale arsenic at work.
The seriousness of the risk associated with arsenic concentrations in drinking-water has recently been evaluated by Smith et al. [7].
Estimates of cancer risk attributable to in-gested arsenic are currently based on skin cancer risks alone. This more recent study examines the possible risks from ingestion of arsenic in drinking-water, based on mortality from internal cancers. Excessive levels of ar-senic have been reported in natural ground-water in a number of the CCEE and NIS. In-dustrial sources may also be important.
The average daily intake of cadmium in Europe has been estimated to be 10–20 µg [8]. The contribution from water is of minor importance. Long-term ingestion of cad-mium leads to accumulation in the kidney, with possible damage and disturbance of cal-cium metabolism – with associated osteopo-rosis – when the critical level of 200 mg/kg is reached in the kidney. There is no reliable evidence for carcinogenicity in humans from ingested cadmium. The guideline of 0.003 mg/l in drinking-water aims to restrict intake so that long-term concentrations in the kid-ney will not exceed 50 mg/kg.
Fluoride levels over 1.5 mg/l in drinking-water give rise to dental fluorosis; over a 20-year period or more skeletal fluorosis may occur. The drinking-water concentrations at which fluorosis is observed are different in different regions, important factors being cli-matic conditions, water intake and food in-take. These conditions, although seen in western Europe in earlier years, are now mainly seen in certain republics of the former USSR such as Kazakhstan. In parts of Europe, fluoride is added to water
artifi-cially with the intention of reducing dental decay.
Lead contamination of water arises prin-cipally from reticulation through lead pipes, especially in areas where the water is plum-bosolvent, i.e. is “soft” with a pH below 8.
Guideline values for lead in drinking-water aim at minimizing exposure of pregnant women and young children, there being no defined tolerable weekly intake values. The aim of the current provisional WHO guide-line, now lowered to 0.01 mg/l [1] is to re-duce the risk of exceeding blood levels of 30 µg/dl from combined food and water intake.
The major health concern associated with lead relates to learning disabilities and be-havioural changes resulting from effects on the developing central nervous system. How-ever, there is increasing evidence that no threshold value for blood lead level can be assumed below which neuropsychological development is not impaired. Exposure of the fetus and young children should there-fore be reduced to the minimum possible.
Mercury in drinking-water is primarily in-organic and is poorly absorbed but, as for other toxic substances, guidelines aim to minimize total exposure from food and water.
The possible link between health and the hardness of drinking-water is an issue of con-tinuing importance, but there is as yet no agreement on whether there is a causal link between drinking-water with a low total hard-ness and increased incidence of heart dis-ease. The difficulty in identifying the link arises in part from problems in epidemiologi-cal studies to do with controls and the aggre-gation of data. Pocock et al. [9] describe a two-phase study in the United Kingdom as part of the British Regional Heart Study.
This study first examined the co-occurrence of heart disease with various factors in 234 towns in the United Kingdom. A negative as-sociation was found between total hardness and cardiovascular disease in the towns, allowing for climate and social and econ-omic factors. The second phase studied 7735 middle-aged men and found a negative as-sociation between ischaemic heart disease
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and alkalinity, calcium, lead, potassium and silicate levels, and the total hardness of the drinking-water, after taking account of per-sonal risk factors such as tobacco smoking and blood pressure. Despite this negative as-sociation, however, the authors suggest that personal risk factors were more significant in determining heart disease. WHO [1] con-siders that, although a number of epidemi-ological studies show a statistically signifi-cant inverse relationship between the hard-ness of drinking-water and cardiovascular disease, the available data are inadequate to establish a causal relationship. No health-based guideline for hardness of water has therefore been proposed.
Nitrate in drinking-water, predominantly arising from agricultural practices, may cause methaemoglobinaemia in infants, in which the oxygen-carrying capacity of the blood is impaired. It may prove fatal in ex-treme cases. This condition is now reported only in some of the CCEE. It has been sug-gested that increased levels of nitrate in drinking-water could cause stomach cancer.
Despite the existence of a theoretical basis for such an effect, most of the epidemiologi-cal evidence does not support such an as-sociation. WHO’s guideline is not based on carcinogenic effects. Exposure through veg-etables is more important than through water where concentrations in drinking-water are below 10 mg NO3/l. Water is the more significant source, however, where con-centrations exceed the guideline value of 50 mg NO3/l (based on prevention of me-thaemoglobinaemia). (See also Box 6.1.)
A large amount of toxicological data exists for pesticides and is primarily derived from laboratory studies examining short- and long-term exposure and toxic and carcino-genic effects. Some data also exist for expo-sure and effects in humans, including data from occupational exposure or self-induced poisonings. These data are the basis for the WHO guidelines. Concentrations in drink-ing-water are usually very low. Drinking-water standards for individual pesticides (such as atrazine) or total pesticides are often exceeded, but no related health effects
have been reported. Indeed, guideline values for drinking-water and exposures from this source are many orders of magnitude lower than exposures to the same compounds through food.
Levels of trihalomethanes (such as chloro-form and bromochloro-form) in chlorinated drink-ing-water, although low, may present a health risk. Chloroform and bromoform are carci-nogenic to laboratory animals at high doses, while other trihalomethanes are mutagenic.
Nevertheless, the risks of not chlorinating the water supply (to control microbial con-tamination) far outweigh potential health problems with chlorination. Epidemiological studies offer conflicting evidence of links be-tween chlorinated water supply and human cancers. A recent meta-analysis concluded that an increased incidence of bladder cancer and rectal cancer can be seen in populations drinking chlorinated surface drinking-water for many years [14]. Never-theless, a causal link has not been estab-lished [15] and the interpretation of the data is debated.
There is currently no basis for accurately comparing drinking-water quality on a pan-European scale. Data are not collected and organized in comparable systems, and moni-toring and management strategies are not similar for the whole Region. In addition, data on water-related diseases or effects of chemical contamination, while available to some extent, are certainly not in a form to allow valid comparison.
Because of public concern over the quality of drinking-water (often related to taste and smell) at locations in a number of countries, bottled water has assumed increasing im-portance as drinking-water. The rise in the sales of bottled water has, in some countries, been quite spectacular. Natural mineral water may be of various types, with “spring water” one of the common descriptions. The composition may vary considerably in terms of, for example, mineral salts.
Regulations in some countries place natu-ral minenatu-ral water within the food category and thus differ from the EU directive. Conse-quently fewer limits are set for natural
min-Box 6.1: The nitrate problem in European drinking-water sources
The release of nitrate from soil into surface water and groundwater is a natural process.
However, it is greatly increased by agricultural activities, especially the application of ni-trogenous fertilizers or organic manures in excess of crop requirements. Nitrate is re-leased by leaching from the soil and/or through runoff, and the rate of release varies with the type of crop and climatic conditions.
In Europe, rural populations are often the most at risk from exposure to nitrates in drinking-water. This is because many such areas are intensively agricultural and receive their water supply from shallow wells. Several of the CCEE (Bulgaria, the Czech Repub-lic, Hungary, Poland and Slovakia) are particularly affected; in this region of Europe a number of cases of methaemoglobinaemia have been reported.
The control of nitrate leaching is particularly difficult and is now recognized as a seri-ous problem, because the speed at which nitrate reaches groundwater varies considerably and present levels may reflect activity in the past, perhaps over several decades. Even when agricultural restrictions on the use of fertilizers are imposed, no immediate improve-ment in water quality results. Even in the shallow, relatively fast-responding aquifers typi-cal of continental Europe, it may take up to ten years for water to percolate down to the aquifer. Agricultural controls operating in Europe tend to be mainly aimed at ground-water protection, since approximately 70 % of drinking-ground-water in Europe comes from that source.
The seriousness with which the nitrate problem is now being considered in Europe has prompted a number of control measures and changes in practice in several countries. In order to conform to the European Union’s limit for nitrate in drinking-water of 50 mg/l, many member states are now imposing their own agricultural restrictions either nationally or regionally. These restrictions most commonly involve limiting the application of manure to land to levels set by livestock density and crop requirements. National restric-tions on the rate and timing of manure application are enforced in Denmark and the Ne-therlands, while regional restrictions are enforced in Germany [10]. In these countries also, water protection zones are designated to control land use around groundwater ab-straction points and, hence, to reduce nitrate leaching to aquifers.
Zones of “hygienic protection” have been established in the Czech Republic to protect both surface water and groundwater sources of drinking-water. Practical experience has shown, however, that the measures taken for resource protection are insufficient and that water quality has not improved. In a number of cases, such as the Zelivka reservoir where the nitrate concentration is increasing by 2–4 mg/m3 per year, water quality continues to deteriorate [11].
In the Czech Republic, the distribution of bottled mineral water, along with a well func-tioning warning system in the central Bohemian region, has considerably reduced the inci-dence of methaemoglobinaemia in infants. However, economic constraints in recent years have led to the abandonment of such government action because of its high annual cost.
It is planned to re-establish the distribution of bottled water on a commercial basis, es-pecially for babies and the sick.
During the period 1980–1988, the total use of nitrogenous fertilizers in the European Community increased by 50 % and available data suggest that, except in a few countries, the use of fertilizers in Europe is still increasing. Although in most countries the rate of in-crease slowed during the late 1980s due to the imposition of various control measures, in
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eral waters, and often there are no limits set for nitrate, nitrite, pesticides, polynuclear aromatic hydrocarbons (PAHs) or solvents.
Sources of mineral waters are usually not regularly monitored, but are subject to peri-odic checks.
In Europe, the quality and availability of data on exposure that can link drinking-water quality with health varies greatly be-tween countries. In western Europe, the quality and coverage of exposure data for a variety of chemical and microbiological agents range from “excellent” to “accept-able”. In eastern Europe, data are generally inadequate and health consequences often cannot be evaluated. The quality of studies on chronic disease epidemiology is inad-equate in this area, as are the data on numbers of people at risk at the local level from contaminants in water. As a conse-quence, the data are not reliable for policy-making except in a few situations. The prior-ity of drinking-water qualprior-ity is becoming greater as concern rises over contamination by substances such as nitrates and pesticides, as well as with pathogenic microorganisms.