water and water types Water, the essence of life, is also a fundamental component of the dynamic Earth system; water participates in nearly every geological process. It alters, sculpts, and creates landforms while wearing away mountains. Water is abundant in the atmospheric surrounding the Earth, on the Earth's surface, and within the Earth's crust. More than two-thirds of the Earth's surface is covered with water, as oceans, glaciers, rivers, and lakes. Within the Earth, water occurs as a discrete phase in the void spaces of rocks, is absorbed on to mineral surfaces, is incorporated into mineral structures, and becomes trapped in growing minerals as minute fluid inclusions. Although the depth to which water occurs in the Earth is still debated, deep drill holes encounter zones of aqueous fluids coming from even greater depths. Water may even be carried down into the mantle in subduction zones.

The water molecule

Water, H₂O, is involved in a plethora of Earth processes because of its unique nature and unusual properties. The remarkable qualities of H₂O are exhibited, in part, by its existence in three physical states at the Earth's surface: as solid (ice and snow): liquid (often referred to simply as ‘water’); and gas (water vapour, steam). Each state is stable over a range of pressure (P) and temperature (T) conditions, as shown by the phase diagram for the H₂O-system (Fig. 1). For a given pressure–temperature (P–T) regime within the region labelled ‘liquid’, water exists only in the liquid state: similar considerations apply to regions of solid and gas. The solid region is subdivided because solid water is a mineral, ice, and crystallizes into different structures according to the P–T conditions. Separating these major regions are phase boundaries that mark the abrupt transition from one state to another and delineate the marked change in physical properties of H₂O. At P–T conditions on a boundary, two phases coexist in equilibrium. At the surface of the Earth, H₂O is commonly observed along a two-phase boundary: water vapour over ice on glacial fields traces the solid–gas boundary; water ponding on a frozen lake lies along the liquid–solid boundary; and water vapour hanging over a lake represents the liquid–gas boundary (Fig. 1). All three states coexist in equilibrium at the triple point for H₂O near 0.01 °C (273K), 0.006 bar (0.0006 MPa) (Tr. pt, Fig. 1). Cirrus clouds form at triple-point conditions. At °C and surface pressure, liquid water freezes to become solid. Along the solid (ice I–liquid) boundary, increasing pressure causes liquid water to freeze at a lower temperature. Conversely, ice skaters glide effortlessly over the ice as their weight (P) induces melting; beneath their blades a two-phase region develops: a thin film of water over ice. Along the vapour–liquid boundary, water vaporizes at higher temperatures as pressure increases. Increasing T to 100 °C at surface pressure causes water to boil as is crosses the phase boundary from liquid to gas. An example of this transition is commonly seen when steam rises from kettles. Liquid water has a narrow range of thermal stability at the Earth's surface. The two-phase liquid vapour boundary continues until the critical endpoint is attained. For pure H₂O, the critical point is at about 374 °C (647K), 220 bar (22 MPa) (Fig. 1). Extreme values in many thermodynamic and transport properties of water occur at this point. In natural systems, thermal springs in the mid-ocean ridges emanate fluids near critical-point conditions. At higher temperatures, a discrete vapour and liquid phase no longer exist; instead phase properties vary continuously. Water in this P–T region is referred to as a supercritical fluid; aqueous fluids typical of metamorphic and igneous environments.

The P–T stability of water is also an indication that it has unique bonding characteristics. This uniqueness is derived from the nature of the water (H₂O) molecule, which consists of two hydrogen atoms (H) bonded to one oxygen atom (O) in a V-shaped configuration, with the oxygen atom at the apex (Fig. 2). The two H are separated by an angle of about 104.5°. Although the molecule is symmetrical, it has an asymmetrical charge distribution. The oxygen side of the molecule has a local negative charge whereas the hydrogen side has a local positive charge. This non-coincidence of charge centres imparts the property of polarity to H₂O: water is a polar molecule. Liquid water is associated because the positive hydrogen atom of one water molecule attracts the negative oxygen atom of another. This polar association is responsible for many of the properties of water, including: low vapour pressure, high boiling-point, and high heats of fusion and vaporization. High energy is required to break the hydrogen bonds holding the polar molecules together. Its capacity to bind both positively and negatively charged ions is also a consequence of it polarity. Liquid water is a universal solvent, behaving as an acid and a base, although it is a poor solvent for non-polar molecules. Water has an abnormally high dielectric constant, which is responsible for its ability to form complexes with other substances.

As mentioned above, solid H₂O, ice, is also a mineral. The most common form of ice (ice I, Fig. 1) crystallizes in the hexagonal crystal system (Fig. 3). This is readily apparent on examination of individual snowflakes. No matter how small and intricate, these ice crystals have symmetrical sixfold shapes. In the crystal structure of ice, each oxygen atom is surrounded by four hydrogen atoms attached by hydrogen bonds and two by electron-pair bonds (Fig. 3). This arrangement results in an open structure and explains the low density of ice. H₂O is also unusual in that this solid form is less dense than its liquid form. On melting, the hydrogen bonds are broken, the open structure collapses, and the water molecules move more closely together because of the intermolecular forces of polarity. This closer packing results in liquid water being nearly incompressible and having a higher density than ice. Thus, ice floats in water (there is a two-phase boundary). The maximum density for liquid water is at about 4°C, the temperature of ocean bottom waters. Water vapour is much less dense than liquid, and occupies a volume about a thousand times greater than liquid.

Water in geological processes

Erosion, weathering, and alteration processes caused by water at the Earth's surface are observed in every climatic regime. The erosional capacity of water is expressed in features ranging from smooth water worn stones and sculpted rock faces, to enormous sedimentary deposits at the base and sides of mountains. V-shaped valleys carved by moving water are transformed into broad U-shaped valleys by the movement of glacial ice. Silt deposits from floods and boulders far from their source attest to the transport capacities of water. Water is a primary modifier of landscape.

Within the Earth, water is found in all rock-types, as a constituent of minerals or as interstitial or pore water contained within pore space between mineral grains until the void space becomes vanishingly small at depth. Groundwater is a general term used for water contained in the pore spaces of rocks, typically in the upper portion of the Earth's crust. Although this water is a minor component within the crust, it contributes significantly to the chemical, mechanical, and thermal development of the crust because of its superb ability to transfer heat and solutes and to do mechanical work. Water dramatically enhances rates of chemical reactions and mineral transformations. Water is an excellent inorganic solvent. It reacts with both metals and with nonmetals to dissolve rocks. All minerals exhibit some degree of solubility in water. When water flows, it carries the dissolved constituents in solution and redeposits them elsewhere. Ore deposits are the remnants of former metal-bearing water-rich fluids. The migration, emplacement, and destruction of hydrocarbons depend on the movement and presence of aqueous pore fluids. Water alters the mechanical properties of rocks; it reduces rock strength and the effective stress but enhances the ductility of the rock.

Water can also be dissolved within magmas. Upon crystallization, the magmatic water is expelled as bubbles which later form vescicles or microlitic cavities in the rock, or it infiltrates into the surrounding country rocks. These hot igneous intrusions also produce hydrothermal water by heating pore fluids contained in adjacent rocks. Because water expands when it is heated, water pressure may increase, if the water is in a confined area, to overcome the effective strength and fracture the rock. Hydrofracturing is a method by which water does mechanical work (fractures rock) in response to a driving force and creates passageways through which it can migrate. Hydrothermal waters are excellent transporters of heat and mass, and are responsible for much of the chemical alteration of the crust. If large convection cells of the thermally heated water find pathways to the surface, geysers erupt or the fluids emanate at the surface as hot springs.

Structural water (H₂O) or hydroxyl ions (OH) may form an integral portion of the structure of a mineral and be necessary for its formation. In some minerals, water occurs in structural cavities and is only loosely bound to the crystal structure. It readily enters and leaves the mineral, as in some swelling clays and zeolites. The ‘fire’ in opals is due to the reflection of light from water residing in the interstices of the silica spheres. If these minerals are heated, they will dehydrate and lose their water of hydration. During metamorphism, as the rocks are subjected to higher temperatures and pressures, hydrous minerals (e.g. muscovite, amphiboles) are exposed to conditions outside their stability range and are converted to more stable anhydrous phases. Water and hydroxyl ions bound within the crystal are released, forming metamorphic aqueous fluid as the minerals devolatilize and dehydrate.

Water types

In addition to the physical state in which it exists, water is described and classified by several features. One of the prevalent classification schemes is based on chemical attributes of water, as suggested by J. I. Drever.

All natural waters contain some quantity of dissolved substances. The total amount of dissolved solids, TDS measured in milligrams per litre (mg l⁻¹), provides one basis for the chemical description of water types. (TDS is determined by evaporating a specific quantity of water and measuring the material remaining). The ultimate source of fresh water is unpolluted rainwater. Meteoric water, derived from the atmosphere, originates and falls to the Earth as precipitation. Some of this precipitation infiltrates into the ground through porous sediments and becomes groundwater, the most desirable source of drinking water. Pure water is a tasteless, odourless, colourless substance and is not for human consumption. Fortunately, the water molecule has an atomic configuration that solvates rock-forming elements and contaminates itself on the continents into a compositional range on which we thrive. Fresh water contains small quantities of solids, less than 1000mgl⁻¹ TDS, and most is potable. Minor amounts of dissolved gases and salts in the water provide the taste. Small quantities of organic compounds can produce an unpleasant taste, and a few hundred parts per million (ppm) of salt will cause water to taste salty. As the TDS increases to c. 1000–20 000 mg l⁻¹ TDS, brackish water becomes too saline for consumption by humans. These waters have lower salinities than sea water, which has an average salinity of 35 000mgl⁻¹TDS or more. Brines are chemically variable but contain extremely high concentrations of TDS, at least ten times the salinity of sea water. Sea water and sedimentary brines are volumetrically more important that fresh waters, but are unfit for human consumption. In areas of active volcanism, some lakes have water which is extremely acidic because of dissolved gases. The range of water composition on which humans can survive is a narrow one, and that range approximates that of average river water.

As water moves and chemical reactions occur, the chemistry of the water changes and its properties are altered. Incorporation of dissolved ions makes water a better conductor of electricity. Continued contamination by metals makes the ores on which we depend.

Specific chemicals dissolved in water or constituting the water also give rise to descriptive terms. Heavy water results when the heavy isotope of hydrogen, ²₁H, deuterium, D, combines with oxygen to form D₂O. These waters occur naturally and are also produced in atomic energy plants, where they are used to slow nuclear chain reactions. Hard water, which leaves an insoluble residue and cleans poorly, results from an abundance of dissolved calcium (Ca) with lesser magnesium (Mg). Conversely, soft water has lower concentrations of calcium ions and gives the sensation that it cannot be washed off the skin.

The elements of which water is composed, hydrogen and oxygen, both have stable isotopes. The origin of a water sample can thus be determined by use of oxygen and hydrogen isotopes. Meteoric, magmatic, metamorphic, formation, and exotic waters all have definite isotopic signatures that vary in a systematic fashion, as summarized by S. M. F. Sheppard. Formation water resides in the pore spaces within deeply buried sediments. Exotic water is a catch-all term which indicates that water has been introduced into a system and was externally derived. Organic water is water that results from the transformation of organic materials. The use of isotopes as tracers has become important in understanding fluid flow, contaminant transport, and alteration of the crust.

Water holds a special place in several respects. It is used as a standard for the determination of numerous physical constants. It has profoundly influenced the evolution of the Earth. It is the substance that is required to sustain life. It also provides a source of power and for recreation. As our reserves of fresh water diminish and hazardous wastes become increasingly abundant, water quality is now of the utmost concern. The groundwater on which so much of the world's population depends is essentially a non-renewable resource. In most areas infiltration rates are so slow that consumption far outstrips replacement. As demands increase, more and more stress is placed on our planet's ever-dwindling supplies of potable water.

Barbara L. Dutrow

Bibliography

Bentley, W. A. and and Humphreys, W. J. (1962) Snow crystals. Dover Publications, New York.
Drever, J. I. (1988) The geochemistry of natural waters. Prentice Hall, New York.
Fyfe, P. and and Thompson, A. B. (1978) Fluids in the crust. Elsevier, New York.
Hunt, C. A. and and Garrells, R. M. (1972) Water: the web of life. W.W. Norton, New York.
Sheppard, S. M. F. (1986) Characterization and isotopic variations in natural waters. Reviews in Mineralogy, 16, pp. 165–84.