Harrow and Hillingdon Geological Society

Bottled Water

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The Geology of Bottled Water

John Mather gave us an extremely interesting talk on bottled water. He explianed what happens to rain water as it moves through the ground, and how we can ascertain the source and age of water by the minerals to be found in it.

He also explained some of the problems with bottled water from other countries, and marketing techniques used by the manufacturers. There is a lot more to bottled water than Del Boy's Peckham Springs.

John very kindly supplied the following write up for us to use :-


The Geology of Bottled Water

As a hydrogeologist I am often asked about the benefits of drinking bottled water rather than water drawn directly from the kitchen tap. Apart from the fact that the smell of residual chlorine is absent from bottled waters is it worth either the money or the effort to stock up with water at the supermarket?

Most bottled waters are marketed in the UK as “Natural Mineral Water” or “Spring Water”, the use of both names being regulated by the European Commission. A “Natural Mineral Water” is defined as a microbiologically wholesome water, originating in an underground water table or deposit and emerging from a spring tapped at one or more natural or borehole exits. It must be clearly distinguishable from ordinary drinking water by its nature and original state. Both these latter characteristics will have been preserved because of the underground origin of the water, which has protected it from all risks of pollution. In other words a “Natural Mineral Water” is an untreated groundwater. A “Spring Water” comes from a source which has either not applied for recognition as a “Natural Mineral Water” or does not meet the conditions laid down. However, regardless of this it is likely to satisfy the requirements of the regulations which cover the quality of public water supplies. Again a “Spring Water” will be groundwater but might have been treated in some way. Waters marketed with neither of these titles may, for example, be desalinated waters or waters derived from Canadian glaciers, however, most of the bottled waters available in the UK originated as groundwater.

The ultimate source of most groundwater is atmospheric deposition, rain and snow melt, although deep groundwater may reflect contributions from other sources such as fluid inclusions or formation waters. Much of the chemical character of groundwater is then established within the soil and the unsaturated zone, above the water table. Although there may be little change in total mineralization within the saturated zone, below the water table, it is here that exchange reactions and other processes slowly modify groundwater chemistry.

The composition of rainfall is controlled by the dissolution of atmospheric gases, particularly oxygen and carbon dioxide and by the wash-out of components derived from the sea, land and pollution sources. Thus rain is essentially a dilute solution of carbonic acid and a sea-salt aerosol plus a variable mixture of sulphuric, nitric and hydrochloric acids. The acidic rainwater has low dissolved solids and rapidly dissolves carbon dioxide which occurs in the soil as a result of root and microbial respiration and the oxidation of organic matter. The carbon dioxide-charged water is very effective in dissolving minerals. The most common reaction involves the dissolution of calcite. When there are no carbonates the chemistry will be controlled by the dissolution of silicate and aluminosilicate minerals but these are relatively insoluble. Minerals such as gypsum are also dissolved and pyrite is oxidized putting sulphate into the groundwater system.

Within the saturated zone, if groundwater is not yet in equilibrium with carbonate, silica and aluminosilicate minerals, these will continue to dissolve proceeding towards equilibrium with those minerals available for dissolution. Soluble salts such as halite will dissolve and a sequence of redox reactions will occur along the flow system. The latter control the hydrochemistry of metals such as iron and manganese and carbon, nitrogen and sulphur species. Cation exchange reactions occur, the most important of which are those taking calcium out of solution and replacing it by sodium, reducing the hardness of water. The sodium generally comes from exchangeable sodium held within clay minerals.

The natural chemistry can be masked by impacts from urban and industrial developments and modern intensive agriculture. The latter is responsible for the major increase in nitrate concentrations seen in many groundwaters over the last three decades.

It is clear from the above that groundwater achieves its composition through a number of interacting factors. However, certain geochemical characteristics, such as the magnesium/calcium ratio, will be determined by the host rock; chloride concentrations may be controlled by atmospheric inputs and sulphate and nitrate concentrations by man’s activities. An understanding of these various factors enables the hydrogeologist to unravel the geochemistry of a bottled water and say something about its origin.

Waters derived from upland springs in say Cumbria or Dartmoor have relatively short flow paths and there is little opportunity for water/rock interaction to occur. In consequence such waters are low in dissolved solids and are often close to rainwater in composition. Waters from limestone terrains generally have high bicarbonate and calcium concentrations and are simply rainwater which has dissolved the host limestone. Elevated nitrate concentrations in such waters show that their source is intensively farmed agricultural land. Waters derived from tuffs and lavas in the Scottish Midland Valley have higher concentrations of magnesium and lower concentrations of bicarbonate In these areas rainfall has dissolved aluminosilicate minerals rather than carbonates. Some waters, such as those from Buxton, contain no nitrate suggesting that these waters have a deep-seated origin. In the East Midlands the waters at Woodhall Spa contain ammonia and are highly mineralized suggesting that these are old groundwaters with a long flowpath and plenty of time for extensive water/rock interaction to occur

In Britain bottled water is drunk because it is thought to be better or “safer” to drink than tap water. However, this is not the case in many overseas countries where bottled water is often consumed because of its medicinal properties. Many of these waters are high in dissolved solids and have diuretic properties. Thus British holiday-makers travelling abroad need to be careful to buy bottled waters which are comparable with the water which they drink at home, otherwise problems may ensue.

The question posed at the beginning of this summary was whether or not it was worth buying bottled water? Certainly if your local supply is over-chlorinated or the water smells musty because of algae growing in your storage tanks it is probably worth buying water for drinking or for diluting that after-dinner glass of whisky. However, in most cases the bottled water is unlikely to be any improvement on your local supply and certainly no safer.

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