Water is the most common liquid on our planet, vital to all life forms. It is the dispersion medium for all biochemical reactions of the living process and takes part in many of these reactions. It is the most abundant compound on the surface of the earth and is distributed in the oceans (97.13%), the polar ice caps (2.24%), the groundwater (0.61%) and the surface water (0.02%) (Eisenberg and Kauzman, 1969; Franks, 2000). Naturally occurring water is a solution containing not only water molecules but also chemical matter (inorganic ions, dissolved gases and dissolved organics), solid matter (colloids, silts and suspended solids) and biological matter (bacteria and viruses). The structure of water, while inherently simple, has unique physicochemical properties which have practical significance for water supply, water quality and water treatment engineers (Crittenden et al., 2005).
Water is a major factor in shaping our landscape. Through the processes of erosion and sediment transport, water forms many surface features such as valleys, flood plains, deltas and beaches as well as subsurface features e.g. caves.
Surface waters include streams, rivers, lakes, reservoirs and wetlands. Because surface waters are on the land surface, they are easily developed for use. Surface waters and their associated ecosystems provide habitat to many plant and animal species.
Stream flow varies in response to climatic factors and human activities. Some streams have a small annual discharge for the large size of their drainage area, such as the Colorado River in the USA, and some have a greater demand for their water than they can supply without reservoir storage.
Streams are a dynamic part of the environment and are good indicators of what is happening in a watershed. Stream flow in a watershed includes all water contributed from headwater areas, stream banks, channels, flood plains, terraces, connected lakes, ponds, wetlands, and groundwater (Figure 1).
Figure 1: Illustration of a drainage basin
A drainage basin is the land area drained by a stream. The term watershed commonly refers to the whole drainage basin. The physical characteristics of a watershed (land use, soil type, geology, vegetation, slope and aspect) and climate control the quantity and quality of water that flows from them. Changes to any of these characteristics can affect water quantity and quality. For example, the removal of vegetation by natural causes such as fire can change the water storage and infiltration characteristics of a watershed. Because burned areas contain less vegetation to slow runoff and hold soil in place, the rate and quantity of water that runs off the surface to streams increases, and so does erosion. During heavy rains, the increased runoff and erosion can result in increased chance of flooding, mudslides and impaired water quality.
Water seeks the path of least resistance. As water flows through a watershed, it picks up and deposits sediments, soil and rock particles, creating stream corridors. These corridors, which consist of stream channels, banks and flood plains, are affected by natural and human activities that occur within watersheds. The physical processes of sediment transport and deposition are critical to the formation of the stream corridor. The transport of sediment within and from a watershed is one of the major processes that help shape the surface of the Earth. Sediment particles are classified by size, with smallest being clay and the largest being boulders. Smaller particles are usually carried in suspension while the larger materials are moved along the channel bottom by rolling, sliding, or bouncing.
Groundwater occurs almost everywhere beneath the land surface. Although surface water is currently the most commonly used water source, groundwater provides about 50 % of the drinking water in the United States. Groundwater is also used for irrigation purposes.
The availability of groundwater as a water source depends largely upon surface and subsurface geology as well as climate. The porosity and permeability of a geologic formation control its ability to hold and transmit water (Figure 2).
Figure 2: Porosity and permeability of a geologic formation
Porosity is measured as the ratio of voids to the total volume of rock material and is usually described as a percentage. Unconsolidated sands and gravels make some of the most productive aquifers because they have many internal voids that are well-connected. If the grains of sand or gravel that make up an aquifer are all about the same size, the water-filled voids between the rock grains account for a larger portion of the volume of the aquifer than if the grains are of varied size. Therefore, an aquifer with uniform grain size usually has a higher porosity, than one with grains of varied size.
Permeability is a measure of the ability of fluids to move through geologic formations. Geologic formations with a high permeability can be the best aquifers. For water to move through an aquifer, the internal voids and fractures must be connected. Geologic formations can have significant porosity and not be good aquifers if the voids are not connected, or if they are very small.
Some sedimentary rocks, such as sandstone and limestone, can also be good aquifers. Permeability in limestone is commonly provided by fractures and by openings caused by water dissolving the rock. These areas are called “karst” areas, in which landscapes are characterized by sinkholes, caves and underground drainage.
Most igneous rocks su ch as granite and metamorphic rocks such as quartzite, have very low porosity and make poor aquifers unless they have interconnected fractures. Water moves through an aquifer from areas of recharge to areas of discharge. Recharge of groundwater occurs from precipitation that infiltrates soils or that seeps from the bottom of surface water bodies such as lakes and streams. Discharge areas include streams, lakes, wetlands, coastal areas, springs or where the groundwater flow is intercepted by wells.
Natural events, such as floods, droughts and changes to water quality, may cause problems for humans. Many human water uses require changes to the natural flow of water through the construction of dams, canals and by the pumping of groundwater. These changes bring benefits to people, but they also affect natural environments. Municipal, industrial, or agricultural uses of water may degrade water quality and cause environmental problems.
- Crittenden J., R.R. Trussell, D.W. Hand, K.J. Howe and G Tsobanoglous (2005). Water Treatment: Principles and Design, 2nd Edition, John Wiley & Sons, Inc., Hoboken, New Jersey, ISBN: 978-0-471-11018-7.
- Eisenberg, D. and W. Kauzman (1969). The structure and properties of water, Oxford University Press, New York and London, ISBN 0-19-857026-0.
- Franks F. (2000). Water: A Matrix of Life, 2nd ed. Royal Society of Chemistry, Cambridge.
- South African Water Quality Guidelines (1996), Department of Water Affairs and Forestry Second edition, ISBN 0-7988-5338-7.
- Vandas S.J., T.C. Winter and W.A. Battaglin (2002). Water and the environment. American Geological Institute in cooperation with Bureau of Reclamation, National Park Service, U.S. Army Corps of Engineers, USDA Forest Service, U.S. Geological Survey, ISBN: 0-922152-63-2.