Introduction

Water supplies are uniquely vulnerable to terrorism, whether it is aimed at humans, livestock or crops. Access to water supplies, and treated water, via service reservoirs and the distribution network places water at particular risk (Denileon, 2001). The key actions that can be taken against supplies are (1) physical damage to water treatment and distribution systems, or the computer systems used to operate them, interrupting the supplies or preventing adequate treatment; (2) deliberate chemical contamination; and (3) bioterrorism using either micro-organisms or biotoxins.

Chemical contaminants are not very effective due to the volume of chemical required and the relative toxicity of most chemicals. In contrast, biological agents have been widely developed for warfare but rarely employed (Hawley and Eitzen, 2001). Although most are airborne many of these organisms and toxins are equally effective via water. The infective dose of the disease agent varies significantly as does the effect on the target organism or population. Microbial agents are infectious, in some cases can be subsequently spread from person to person or via contaminated food, are stable within the environment, colourless and odourless, and have delayed response times unlike chemical contaminants that cause an effect in the target organism relatively quickly.

Biological agents are classed into two categories by the US Center for Disease Control and Prevention (Rotz et al., 2002). Category A micro-organisms posing the highest risk with high morbidity and mortality rates include smallpox, anthrax, plague and botulism, while category B agents pose a much lower risk and rates of mortality and morbidity such as brucellosis, typhus fever and cholera. There is a third group, category C, for emerging biological agents such as hantaviruses and tickborne haemorrhagic fever viruses. A wide range of bacteria, fungi and algae produce toxins, mostly potent neurotoxins, posing a particular risk to water supplies. These include a flatoxins, botulinum toxins, microcystins, ricin and saxitoxin (Section 11.2).

Water treatment and disinfection are the front-line protection for targeted water supplies and most treatment plants employ continuous pollutant and/or toxicity detectors in some form.

Aluminium

Aluminium is a widespread and abundant element that is found as a normal constituent of all soils, plant and animal tissue. It is especially common in food, resulting in a typical daily intake of between 5 and 20 mg depending on individual variations in eating and drinking habits. In the UK the mean dietary aluminium intake is approximated at 3.9 mg d−1 (Ysart et al., 2000) (Table 15.1). It is recommended that the dietary intake should not exceed 6 mg d−1 if potential toxicity problems are to be avoided (Soliman and Zikovsky, 1999). Diet is a major factor in aluminium uptake. For example, aluminium is taken up in large amounts by tea plants, so that drinking tea significantly enhances aluminium uptake (Flaten and Ødegård, 1988). In fact, tea may contain anything from 20 to 200 times more aluminium than the water it is made with. Aluminium can also be leached from cooking utensils (Jagannatha and Valeswara, 1995), and cooking acidic foods such as citric fruits, rhubarb or tomatoes can lead to enhanced leaching from aluminium pots and pans. Enhanced leaching has also been reported for spinach and other green vegetables. Aluminium leaching also occurs from coffee percolators made from aluminium. In new percolators coffee contains on average 4.1 mg Al 1−1, of which 85% comes from aluminium leached from the metal pot. This reduces as the percolator ages, although more than 70% of the aluminium in the coffee will still be leached from the pot. Elevated aluminium concentrations have also been noted in soft drinks (pH < 3.0) when supplied in aluminium cans (Table 15.2) (López et al., 2002). While aluminium packaging materials are normally coated with lacquers to reduce leaching, aluminium cartons and packaging can contribute to the amount of aluminium in the diet. It is difficult to estimate the exposure to aluminium from containers and cooking utensils, and in most cases they are likely to be small compared with the total dietary intake.

Source of odour and taste problems

Most odour and taste problems occur at the water treatment stage and are linked to chlorination (Levi and Jestin, 1988). Chlorine itself has a distinctive odour with a reported taste threshold of 0.16 mg 1− at pH 7 and 0.45 mg 1− at pH 9. Although consumers generally accept a slight chlorine odour as a sign that the water is microbially safe, excessive concentrations of chlorine can make the water most objectionable. In recent years a number of outbreaks of diarrhoeal diseases have occurred when water companies have reduced the level of disinfection due to complaints of chlorinous odours. Clearly a balance must be struck between the protection of public health and wholesomeness in terms of taste and odour. This is an operational problem which can be solved in a number of ways, such as using more sensitive disinfection equipment at the treatment plant, disinfecting within the supply zone or distribution network or by using alternative disinfectants.

Ammonia reacts with chlorine to produce three chloramines (monochloramine, dichloramine and trichloramine or nitrogen trichloride). These compounds are more odorous than free chlorine and become progressively more offensive with increasing numbers of chlorine atoms, with trichloramine by far the worse. The proportion of each compound formed depends on the relative proportions of chlorine and ammonia present, the chlorine demand exerted by other substances and the pH (Montgomery, 1975). This is usually avoided by using breakpoint chlorination in which a high chlorine to ammonia ratio is used (i.e. > 7.6:1 at pH 7–8) so that the residual chlorine present is mainly in the free form (Section 14.2). It is not only ammonia that reacts in this way to produce odorous compounds, although the reactions are considerably slower and continue within the distribution system (Section 21.1).

Phenolic compounds have already been mentioned in Chapter 8. They produce odorous compounds during chlorination. Phenol itself has very little odour, but its chlorinated forms monochlorophenol and dichlorophenol have an intense odour and are difficult to remove by subsequent treatment.

Introduction

Water treatment plants must be able to produce a finished product of consistently high quality regardless of the demand. With the exception of particularly pure groundwaters, all water supplied for drinking requires purification. Although, in theory, the dirtiest water can be purified to drinking water quality, in practice treating even relatively pure water to produce finished water of consistent quality, and of sufficient volume, is technically very difficult. Water treatment consists of a range of unit processes that are usually operated in series (Stevenson, 1998). These are listed in Table 14.1, although it is unusual for all of them to be used at any one particular plant. The cleaner the raw water, the smaller the number of steps or unit processes that are required, and hence the overall cost of the water is less. The most expensive operations are sedimentation and filtration in conventional treatment, whereas specialized processes for softening water or removing specific contaminants such as nitrates or pesticides using membrane processes or activated carbon can be very expensive (Parsons and Jefferson, 2006).

Groundwater is generally much cleaner than surface waters and so does not require the same degree of treatment, apart from aeration and disinfection, before supply. Naturally occurring substances, which may need to be reduced or eliminated in groundwaters, include iron (Section 9.1), hardness (if in excess of 300 mg 1−1) (Chapter 10) and carbon dioxide. Substances originating from humans that are becoming increasingly common in groundwaters and that need to be removed include nitrates (Chapter 5), pathogens (especially bacteria and viruses) (Chapter 13), trace amounts of organic compounds such as pesticides (Chapter 6) and endocrine-disrupting compounds and PPCPs (Chapter 7). The water industry tries to obtain the cleanest water possible for supply, although the volume and consistency of supply are the major factors in the selection of a resource.

Introduction

Unlike distilled or deionized water all drinking water supplies will contain a range of dissolved chemical compounds. Even rainwater, which is the purest water found naturally, contains a wide range of ions and cations. In the early days of chemistry, water was known as the universal solvent as a result of its ability to slowly dissolve into solution anything it comes into contact with, from gases to rocks. So as rain falls through the atmosphere, flows over and through the Earth's surface, it is constantly dissolving material, forming a chemical record of its passage from the clouds. Therefore, water supplies have a natural variety in quality, which depends largely on the source of the supply. All our water comes from the water cycle and it is this process that controls our water resources.

The water cycle

Water is constantly being recycled in a system known as either the water or more correctly the hydrological cycle. Hydrologists are the people who study the chemical and physical nature of water and its movement on and below the ground. While the total volume of water in the world remains constant its quality and availability varies significantly. In terms of total volume, 97.5% of the world's water is saline with 99.99% of this found in the oceans, the remainder making up the salt lakes. This means that only 2.5% of the volume of water in the world is actually non-saline. However, not all of this fresh water is readily available for use by humans. About 75% of this fresh water is currently locked up as ice caps and glaciers, with a further 24% located underground as groundwater, which means that less than 1% of the total fresh water is found in lakes, rivers and the soil. Therefore, only 0.01% of the world's water budget is present in lakes and rivers, with another 0.01% present as soil moisture but unavailable to humans for supply.

Organic micro-pollutants

Under normal circumstances food intake of pesticide residuals is about 103 to 105 times higher than that induced by drinking water or inhalation (Margni et al., 2002). However, when organic contaminants enter water resources then drinking water becomes an increasingly important factor in human toxicity. Organic compounds found in drinking water fall into three broad categories: (1) naturally occurring organics; (2) synthetic (man-made) organic compounds; and (3) organic compounds synthesized during water treatment (disinfection by-products). The last category is dealt with in Chapter 18, while the first two categories are discussed below.

A large number of organic compounds are found naturally as well as synthesized for industrial uses (as solvents, cleaners, degreasers, petroleum products, plastics manufacture and, of course, their derivatives) and for agricultural use (mainly pesticides). Since the 1940s tens of thousands of new chemicals have been developed and are being used not only by industry and agriculture, but by the service and domestic sectors. In a survey carried out by the Water Research Centre in the UK in the late 1970s, 324 organic compounds were identified in drinking water samples (Fielding et al., 1981).

Organic micro-pollutants, including pesticides, are similar to nitrates in that their toxicity cannot be attributed solely to drinking water and that many of them percolate into the soil and accumulate in aquifers or surface waters. They can contaminate drinking water sources via many routes: agricultural runoff to surface waters or percolation into ground-waters, industrial spillages to surface and ground waters, runoff from roads and paved areas, industrial wastewater effluents, leaching from chemically treated surfaces, domestic sewage effluents, atmospheric fallout, carried in rainfall, and as leach ate from industrial and domestic landfill sites.

The Water Research Centre survey showed that a high proportion of the British public is exposed to minute amounts of a wide range of organic chemicals and for most of these there are insufficient toxicological data to make an accurate assessment of their safety.

Formation

Since the beginning of the twentieth century chlorine has been used to disinfect drinking water. Although it has provided an effective barrier to the spread of waterborne diseases, chlorine is also very reactive towards the natural compounds present in water. Humic acids, which give peaty water its clear brown colour, and ammonia interfere with the disinfection process, whereas other compounds such as phenol react with chlorine to affect both taste and odour (Section 14.2).

It was not until the development of new analytical methods such as gas chromatography coupled with mass spectrometry, which can identify the organic compounds present in water, in the early 1970s that another problem related to chlorination was identified. This new technology revealed that water could contain hundreds of natural and man-made organic compounds, most of which were present only at very low concentrations of less than 1 μg 1−. It was discovered that some of these organic compounds could react with the chlorine during the disinfection process at the treatment plant to form new, complex and often dangerous chemicals known generically as disinfection by-products (DBPs). Chloroform and other trihalomethanes (THMs) were first identified in chlorinated drinking water taken from the River Rhine (Rook, 1974). Trihalomethanes (THMs) and haloacetic acids (HAAs) are the commonest DBPs in drinking water although other groups and compounds are also formed including haloacetonitriles (HANs), haloketones (HAKs) chloral hydrate (CH) and chloropicrin (CP) (Nikolaou et al., 1999).

Trihalomethanes are simple, single-carbon compounds that have the general formulae CHX3, where X may be any halogen atom (i.e. chlorine, bromine, fluorine, iodine, or a combination of several of these). They are all considered to be possible carcinogens and are therefore undesirable in drinking water. There are four common THMs found in drinking water: chloroform (CHCl3), bromodichloromethane (CHBrCl2), dibromochloromethane (CHBr2Cl) and bromoform (CHBr3). The THMs and other chlorinated by-products are only found in raw or treated waters that are disinfected using chlorine.

Introduction

The main source of radiation is from naturally occurring radionuclides. In fact, the combined effect of all these natural sources accounts for, on average, 87% of the total radiation exposure to a person over their lifetime. So man-made sources only account for 13% of radiation exposure, of which 11.5% is from medical sources, 0.5% from fallout, 0.4% due to occupational exposure and 0.1% from nuclear discharges, with various other miscellaneous sources accounting for the remainder. Of the natural sources, radon accounts for 32% of this total exposure. The majority of water supplies have very low levels of radionuclides that represent no health risk. Those that are a cause for concern are due to naturally occurring radioactivity. Many types of rock contain mildly radioactive elements, which are known as parent radionuclides, that decay producing other radioactive contaminants known as daughter radionuclides. Depending on their chemical properties both can accumulate in water sources, often at dangerous concentrations. In the environment the parent radionuclides may behave differently from their daughter radionuclides resulting in very different patterns of occurrence in water resources. For example, groundwater with high radium levels tends to have low uranium levels and vice versa, even though uranium238 is the parent of radium-226.

The intensity or activity of a radionuclide was originally measured by the curie (Ci), which is still in use in the USA. Elsewhere this has largely been replaced by the becquerel (Bq). In practice the Ci is too large for use with normal environmental monitoring so the picocurie (pCi) is used, so 1 pCi = 10−12 Ci. Conversion of the two units is 1 Bq = 2.7 × 10−11 Ci or 1 Ci = 3.7 × 1010 Bq.

Uranium-238 series radionuclides are the major contributors to natural radiation associated with drinking water, the most important being radium-222 (radon), which is also released from water into the air. Other long-lived radionuclides include uranium-234, radium-226, lead-210 and polonium-210, which are mostly alpha-emitters, as well as their shorter-lived progeny that also emit beta and gamma radiation.

Introduction and definitions

The use of cosmetics and prescribed drugs, including antibiotics and synthetic hormones, continues to increase each year. Collectively they are known as pharmaceutical and personal care products (PPCPs), and comprise a wide spectrum of compounds. The PPCPs are either ingested or applied externally and used for both human and veterinary purposes. Antibiotics, growth promoters and other pharmaceuticals are also widely used as food additives for a wide range of livestock. These compounds are excreted in urine and faeces either as the active ingredient or an intermediate metabolite with their fate dependent upon a number of factors (Figure 7.1).

While some PPCPs end up in surface waters due to runoff from land, or directly into groundwater from septic tank leachate, the majority of these chemicals end up in sewage that is normally centrally treated at a wastewater treatment plant. Unfortunately PPCPs are only partially removed by conventional wastewater treatment processes, with adsorption onto suspended solids the main mechanism. This results in approximately 60% of the PPCPs being incorporated into the sludge, with significant quantities of the active ingredient and any metabolic breakdown products lost in the final effluent, which ends up in either surface or ground waters. There is increasing concern that PPCPs are finding their way into drinking water due to increasing surface and ground water pollution, and the reuse of sewage effluents in areas of water scarcity. Among the more commonly recorded PPCPs are antibiotics, painkillers, beta-blockers, lipid-reducing drugs and sex steroids from birth control and hormone replacement therapies (Table 7.1). While pharmaceutical drugs are designed to induce specific biological effects in the target organism for a limited time, most of these compounds retain their active ingredient even after wastewater treatment and subsequent discharge to the environment. Almost nothing is currently known about the risk associated from long-term exposure to these drugs at trace concentrations, or indeed exposure to cocktails of these drugs, on either human or environmental health.

List I (black list)

Organohalogen compounds and substances which may form such compounds in the aquatic environment

Organophosphorus compounds

Organotin compounds

Substances, the carcinogenic activity of which is exhibited in or by the aquatic environment (substances in List II which are carcinogenic are included here)

Mercury and its compounds

Cadmium and its compounds

Persistent mineral oils and hydrocarbons of petroleum

Persistent synthetic substances

List II (grey list)

The following metalloids/metals and their compounds:

Zinc, copper, nickel, chromium, lead, selenium, arsenic, antimony, molybdenum, titanium, tin, barium, beryllium, boron, uranium, vanadium, cobalt, thallium, tellurium, silver

Biocides and their derivatives not appearing in List I

Substances which have a deleterious effect on the taste and/or smell of products for human consumption derived from the aquatic environment and compounds liable to give rise to such substances in water

Toxic or persistent organic compounds of silicon and substances which may give rise to such compounds in water, excluding those which are biologically harmless or are rapidly converted in water to harmless substances

Inorganic compounds of phosphorus and elemental phosphorus

Non-persistent mineral oils and hydrocarbons of petroleum origin

Cyanides, fluorides

Certain substances which may have an adverse effect on the oxygen balance, particularly ammonia and nitrites

Introduction

Polycyclic (polynuclear) aromatic hydrocarbons (PAHs) are organic compounds with two or more fused aromatic rings of carbon and hydrogen atoms. They predominantly enter the atmosphere from combustion processes and while PAHs are ubiquitous in the environment they are not generally found in high concentrations in water resources. Normal concentrations in uncontaminated groundwater resources are in the range of 0.5 ng 1−1. Due to PAHs having low solubilities but a high affinity for particulate matter they are not found at high concentrations in natural waters (Section 6.4). Drinking water can become contaminated by PAHs where coal-tar and bitumen linings have been used to protect distribution pipes from corrosion.

In the UK, 60% by length of the distribution pipes are ductile iron. These were coated internally by immersing the pipe sections in a batch of coal-tar preparation held at a high temperature. The coal tar is intended to protect the pipe. Asbestos cement pipes are also occasionally coated internally with bitumen materials, as are water storage tanks used by the water companies. Finished water is often distributed over considerable distances through ductile iron mains lined with bitumen and related coal-tar products, which are derived from petroleum sources. This means that the water may have a long contact time with such tar-based materials between treatment at the water treatment plant and being used by the consumer. The length of contact may be significantly increased where the water remains static or flows very slowly towards the end of spur mains.

The coal-tar lining usually applied to iron pipes contains massive levels of PAHs and these readily leach into the water supply by diffusing into the water itself. Fluoranthene is the most soluble PAH compound and so comprises the bulk of the total PAHs present in water in contact with coal-tar linings. These compounds can also be found in drinking water due to small fragments of the lining becoming detached because as coal-tar pitch ages small particles can be shredded from the lining by the flow.

Introduction

The development of water treatment towards the end of the nineteenth and beginning of the twentieth century arose from the need to control the primary pathogens that caused infectious diseases such as cholera, typhoid and paratyphoid. The control of waterborne and water-related diseases remains the primary objective of water treatment today. However, in the 1950s and 1960s the need to control chemical contaminants in drinking water was also realized. This arose from two factors, the greater understanding of the causative toxic effects of some metals such as lead, and the increasing development of organic chemicals and their widespread use in industry and agriculture. The severe impact of some of these organic chemicals on wildlife observed during the 1960s quickly raised fears about their potential toxicity and carcinogenic properties in relation to humans. While pathogens cause a rapid response in consumers (24 – 48 hours), chemical contaminants in drinking water are more likely to have medium- (1–5 years) to long-term (> 10 years) toxic effects, including cancer. This has led to the widespread development and adoption of drinking water quality standards that include microbial, chemical and physical parameters, rather than previously employed microbial-based hygiene standards (Section 2.5). Another milestone in drinking water quality has been its use to promote public health through the addition of fluoride to water to prevent dental caries within the community; although the use of drinking water for community mass-medication has given rise to considerable controversy.

In this book, contamination of drinking water is classified by the point at which individual contaminants enter the supply chain, with each dealt with in depth. Problems arise from the resource due to leaching from natural rocks and soils or from chemicals used by man that subsequently enter the hydrological cycle (Part II), from the treatment of surface and ground waters (Part III), as the water travels from the treatment plant to the consumer via the distribution network (Part IV) or within the household plumbing system (Part V) (Table 3.1).

Introduction

Many impoundment and storage reservoirs are rich in nutrients from farming and the discharge from sewage treatment plants upstream (Chapter 5). Given adequate light, algae can quickly establish themselves and become a problem. Algal development is worse in the summer due to thermal stratification, with the lower zones of the reservoir (i.e. hypolimnion) becoming anaerobic. This causes ammonia, phosphorus and silica, all algal nutrients, to be released from the bottom sediments, which encourages even more algal growth (Sections 4.3 and 5.5).

Problems associated with algae

There are two specific problem areas for the water supply engineer associated with algal blooms. The first is an operational problem. The physical presence of the algae makes water treatment more difficult because the algae must be removed from the finished water. If small algae do get through the filter, then algae will enter the distribution system and cause several problems. The water will be coloured and turbid, the algae will decay causing taste and odour problems, and finally the algae will either become the source of food for micro-organisms growing on the walls of the supply pipes, or the source of food for larger animals infesting the supply system (Chapter 24). In practice, algae reduce the rate of flow through the treatment plant by blocking microstrainers; the larger algae blocking rapid sand filters and the smaller species blocking the finer slow sand filters. In severe cases the supply may be drastically reduced or stopped altogether.

The second problem area is one of metabolism. The algae do two things. They all produce carbon dioxide during respiration, and this can cause a severe alteration of the pH of the water. This is most often caused by large crops, known as blooms, of blue-green algae. These changes in pH severely disrupt the coagulation process, resulting in the loss of coagulant, often aluminium sulphate, into the finished water and a reduction in quality in terms of colour and turbidity.

Introduction

After treatment the water has to be conveyed to the consumer. This is done by a network of distribution pipes, also known as water mains, which are laid underground normally under roads and pavements. There is, however, much more to water distribution than just pipes; there are service reservoirs and booster stations as well.

Service reservoirs

Service reservoirs are needed primarily because the water resource, and often the treatment plant, is usually a considerable distance from the centre of population. Where groundwater resources are used, pumps need to be capable of pumping water at peak demand rates, rather than at the average daily demand rate. The service reservoir ensures that all peak demands for water are met, while smaller pumps and trunk mains can be used to cope with the average daily flow-rates rather than the peak demand rates, which may be 50–80% greater.

Like electricity, the demand for water varies during the day (diurnal variation). Although the peaks (high) and troughs (low) in demand can be calculated fairly adequately from experience, service reservoirs and water towers are needed to ensure that these demands are fully met (Figure 20.1). They also have other important functions such as providing a reserve storage capacity in case of problems at the treatment plant or with the trunk mains. They also compensate for any variation in water quality; for example, when water comes from more than one source.

Although the flow to the service reservoir remains constant, the level in the reservoir will rise and fall according to demand. The size of the reservoir varies according to the size of the area served, although enough storage for 24 or 36 hours is usually selected. While generally constructed of concrete, smaller reservoirs are often of steel or brick. An important design feature is that the tanks should be watertight, not only to prevent water loss, but to prevent contamination from outside the tank.

Introduction

The water supply industry is vitally important not only to maintain the health of the community, but for the sustainability of industry, business and agriculture. Without adequate water supplies our present society would never have evolved, and our lives today would be unrecognizable. Our dependence on treated water is now incalculable, and threats to that supply are comparable to the worst natural and man-made disasters. The volumes of water consumed each day by agriculture, industry and the public are vast, requiring an enormous infrastructure to satisfy the demand. Like the other service providers, electricity, telephone and gas, the water utilities deliver their product to the home, which requires a network of distribution pipes to service each household, but unlike the other utilities these are stand alone local or regional networks, rather than integrated national supply networks.

In England and Wales there are 26 private water companies that together supplied 52.7 million consumers in 2004/5 with 15 807 million litres (Ml d−1) of water each day. Sixty-eight per cent of this came from surface waters and the remainder (32%) from groundwater. It requires 1344 plants to treat this volume of water, which is supplied to consumers via 326 471 km of distribution mains. When this is broken down by region, the greatest demand is in the south-east and north-west regions, which have the largest populations. However, the areas of highest demand do not normally correspond to the areas where adequate water resources are to be found, so shortages occur. The current demand for potable water in England and Wales has stabilized and is currently at 91% of the peak demand recorded in 1990/1 (Table 1.1).

Water consumption

Water demand varies significantly between countries due to differences in culture, climate and economic wealth (Smith and Ali, 2006). The demand for water also varies over the 24-hour period.

The nature of asbestos

Asbestos is the name given to a class of naturally occurring fibrous silicate minerals containing iron, magnesium, calcium or sodium. Its name is derived from the Greek meaning unquenchable or inextinguishable, which refers to the fact that it cannot be destroyed by fire. It is a widely used material, with over 5 • 106 tonnes mined annually during the 1980s, although over the past 15 years demand for asbestos has been slowly declining. Among its important properties are its resistance to fire, its thermal and electrical insulation properties and its binding capacity, which provides strength and stability when it is incorporated with cement or other materials or used in the formulation of brake linings and even floor tiles (Commins, 1988).

There are a number of different types of asbestos; the most important are listed in Table 22.1. Those most widely used are chrysolite (white asbestos) (Figure 22.1), amosite (grey) and, in the past, crocidolite (blue). These are the forms most commonly used in asbestos cement products including water pipes.

Asbestos occurs naturally worldwide, although the largest mined deposits are found in Canada, Russia, Africa, Australia and the USA. In Britain outcrops of chrysolite and tremolite are found in Snowdonia, Caernarvonshire, Cornwall, Hereford and Worcester, Aberdeenshire, Banffshire, Inverness-shire, Skye, Sutherland, Jersey and Guernsey. Although none of these are mined commercially, where asbestos is mined on a large scale up to 1200 • 106 fibres 1−1 can find their way into drinking waters. In the UK the major outcrops occur at Lochnager in Scotland and on the eastern slope of Moel Yr Ogof in Snowdonia, North Wales. Neither of these areas drain into water supply catchments, and so no studies have been carried out to determine if there is any significant natural runoff of asbestos.

Asbestos fibres in water supplies originate from a number of sources; for example, from the dissolution of natural asbestos mineral, industrial effluents, atmospheric fallout and the effects of aggressive and corrosive water on asbestos cement water distribution pipes and storage tanks.