Water can be both unsafe to drink (unpotable) and unacceptable to drink due to taste, colour and feel (unpalatable).
The various activities generating water supply contaminants are agriculture, mining, and the production of household, commercial and industrial wastes.
While certain solid and liquid agricultural wastes occur in concentrated forms, low-level ground-water contamination in rural areas occurs as a result of the widespread leaching of excess nutrients applied to both arable and pastoral land as inorganic and organic fertilizers. The principal contaminant is nitrate, derived both from fertilizers and as a result of the transformation of organically bound nitrogen in the soil to inorganic forms by bacteria (mineralization) following the ploughing of established and temporary grasslands. The potential quantities of nitrate released for leaching by ploughing may be large, exceeding the total annual quantities of nitrogen normally applied as fertilizers. Increased sulphate and chloride concentrations derived from the use of ammonium sulphate and potassium chloride fertilizers may also occur in drainage water from farmland. The residues of pesticides and herbicides may be leached from the soil and examples of ground-water contamination have been reported.
The principal solid mining wastes that have a potential to pollute ground water come from coal and metal ore exploitation. In the former case, the waste rock, generally sandstone, causes oxidation of the disseminated sulphides leading to the formation of sulphate ions, a lowering of the pH of interstitial fluids, the mobilization of iron and manganese and the physical disintegration of the shale. This produces a leachate high in sulphate, iron and dissolved and suspended solids and low in pH. Drainage waters from collieries characteristically possess both high suspended and high dissolved solids, particularly iron and sulphate ions resulting from the oxidation of ferrous sulphides in the host rocks. Chloride concentrations may also be high because of connate water trapped within the sedimentary rocks. Discharge of such waters on to the ground surface may result in widespread contamination of previously potable ground-water resources.
The risk of infection of ground-water supplies by faecal bacteria from free ranging livestock is generally slight when livestock densities are low, as a result of the complete natural degradation of faeces and urine by soil bacteria when wastes are deposited in a disseminated manner. However, the problem is more serious where animals congregate in large numbers, for example around water holes or in stock yards, in which case large volumes of liquid and semi-solid faecal matter may readily infiltrate to the water table. Even in areas where wells are lined, the rate of production and the highly polluting nature of farmyard slurries and especially silage effluents, necessitate either their collection for treatment or confinement to avoid ground-water contamination.
Household wastes contain a high proportion of putrescible matter which is broken down by biodegradation, leading to an initial temperature rise within the waste mass and the generation of carbon dioxide and methane gases. The leachate contains high concentration of total organic carbon (TOC), of which often more than 80% is in the form of volatile fatty acids (acetic, butryric, etc) and give waste disposal sites their characteristic odour. As the wastes age, the organic carbon component of the leachate changes to higher molecular weight substances such as carbohydrates. The period of this change is between 5 to 10 years in humid, temperate climates, but probably a shorter period in warmer climates. Under arid conditions the rate of bacterial degradation may be limited by lack of moisture.
The composition of industrial wastes varies with the source, ranging from cyanide wastes from metallurgical operations, through sulphite-rich paper and pulp manufacturing wastes, mercury-rich materials from the electrical industry, to solid residues, such as polychlorinated biphenyls (PCBs), pesticide or herbicide residues and phenol-rich tar wastes, from the petrochemical industries. In many cases toxic substances are present which, if solubilized, may present major threats to ground-water quality. In some cases the total quantity of waste occurring is relatively small and co-disposal may afford a method of using the absorption and biodegradation processes in landfills to attenuate the leachate from the industrial materials.
When chlorine is added to the water it may reacts with pollution creating more dangerous chemicals, like chlorophenols. The water pipes in old buildings may contain lead, cadmium and mercury.
Over 1.5 billion people in developing countries are without safe drinking water, and even more do not have basic sanitary facilities. Tens of millions of women and children, in particular, have to walk miles and spend half each day fetching water, often polluted, and carrying heavy buckets of water home. Although the percentage of people without safe water is falling the total number is increasing.
In developing countries, unsafe water is responsible for 80% of diseases and 33% of deaths. In 1991, WHO reported that 253 million people in developing countries are without safe drinking water.
In many developing countries, rivers downstream of large cities are little cleaner than open sewers. Levels of suspended solids in Asia's rivers, for example, almost quadrupled since the late 1970s and rivers typically contain four times the world average and 20 times OECD levels. The faecal coliform count in Asia's rivers is 50 times higher than the WHO guidelines. People using such water for washing, bathing or drinking are at high risk. In Latin America as a whole, only about 2 per cent of sewage receives any treatment. Worldwide, polluted water is estimated to affect the health of about 1200 million people and to contribute to the death of about 15 million children under five every year (ICWE 1992).
The USA Environmental Protection Agency considers pollution of drinking water to be one of the four greatest environmental risks, causing as many as 1,000 cancer cases a year and stunting the mental development of as many as 240,000 children. In UK it it estimated that annual current spending of £10 million would be needed in 20 years time to keep nitrate concentrations in all public water supplies within EEC/EU-agreed levels. In 1996, dropping the effort to reach an amicable deal, the European Commission finally announced it would take the UK to court for not enforcing European Union minimum standards on the quality of drinking water that became compulsory in 1985. Britain did not meet water standards notably on pesticides levels and especially in the Greater London area.
Most public water supplies are routinely monitored, but private supplies may not be subject to the same quality standards. In the Russian Federation, half the population uses water that fails to meet quality standards. In Latvia, 55% of water samples from shallow wells fail to meet microbiological standards. Yet half the rural population relies on these wells as a source of drinking-water. Some 50% of Armenian supplies also fail to meet quality standards.
An outbreak of parasites giardia and cryptosporidium occurred in Sydney's water system during 1998. water in only 8% of Sydney's households was declared fit to drink. The cause of the contamination is unclear but run-off is thought to be to blame. In 2022, tap water in more than 500 remote Australian indigenous communities isn’t regularly tested and often isn’t safe to drink. In some communities, drinking water contained unacceptable levels of uranium, arsenic, fluoride and nitrate.
The financial resources needed to improve water supply and sanitation in developing countries have been estimated by the World Bank at US$ 30 billion per year during the period 1981-1990. This price is far less than what is now being paid in terms of human suffering and economic loss.
"If we persist with intensive agriculture, industrialization and overexploitation of our water resources, it will become increasingly difficult to ensure a supply of safe water in many parts of Europe, including a number of big cities," warns Günter Klein, Director, Environment and Health at WHO's Regional Office for Europe.