Info

Evaporation

Fresh water Ground water Lakes, Streams

FIGURE 9-1 The water cycle. Volcanoes also add a small contribution to the atmosphere.

composition; usually, however, they contain small amounts of dissolved substances, biological materials, and frequently much suspended matter. These water sources are most heavily used for agricultural, industrial, and domestic purposes. Ice, in the Arctic and Antarctic ice caps, makes up the largest reservoir of fresh water.

Subsurface or ground waters, held in sand, gravel, or porous or fractured rock, are normally quite free of suspended and organic materials because of their percolation through soil and rock formations, but may contain dissolved inorganic material. These waters also are used extensively in some locations; about 20% of the U.S. freshwater usage comes from this source. A great deal of groundwater is used for irrigation in the western states, and over half the population relies on groundwater as a drinking water source. The term "aquifer" refers to a permeable geological formation that is saturated with water and capable of producing a significant flow at a well or spring.1 One type of aquifer formation consists of fractured rock: blocks of more or less solid rock separated by cracks and fissures. Much of the water in this type of aquifer is in the fissures, but some is in pores in the rock itself. The latter typically is released very slowly in comparison to the former. A second type of aquifer consists of more finely divided material such as sand; flow in this will follow the rules of flow through porous media.

1The water-saturated region of soil or porous rock is referred to as the phaedose zone; the upper bound of this is called the water table. The region above this, where the pores are unsaturated, is the vadose zone. In the region between them (the capillary zone), water rises upward through capillary action.

Freshwaters are classified as hard when they contain significant amounts of calcium or magnesium salts in solution. Hardness is easily demonstrated through the precipitate formed with soap (Section 7.2).

Cycling between the atmosphere and the reservoirs of liquid water obviously involves evaporation and condensation (precipitation), but also transpiration through plants. Vegetation can play a significant role in this transport, either increasing or decreasing water loss from land surfaces, depending on the circumstances.

Cycling involving the groundwater reservoir is much more complex. Some aquifers (confined aquifers) are isolated by relatively impervious rock or clay so that input from rain may be extremely slow. Since extensive withdrawal of water from these is not always matched by recharge, a groundwater supply, unlike surface waters, may not be a renewable resource on the time scale of interest to the users. One estimate suggests that these aquifers are being depleted by 160 billion cubic meters of water annually, most extensively in India, North Africa, China and the United States. Removal of water may be accompanied by ground settling (e.g., Mexico City, Venice, Bangkok) or, if near an ocean, inflow of salt water. The Ogallala aquifer that underlies a large portion of the Great Plains of the United States is one example of a major groundwater source that is heavily used for domestic, industrial, and agricultural purposes. Indeed, much of the agriculture in this area has depended on this source of irrigation water. In many locations, withdrawal has exceeded recharge by two or three orders of magnitude, with the result that flow rates are decreased, deeper wells are needed, and pumping expenses are higher. There is real concern about the continued availability of water from this source, but little positive conservation action.

Contamination of a deep aquifer is, for all practical purposes, an irreversible event. Such contamination may arise from leaching from waste dumps or agricultural activities if there is exchange with surface waters, or from deep-well disposal that inadvertently penetrates an impervious layer. Other aquifers (unconfined) are near the surface and are readily recharged by rainfall. Because they are accessible, they are also easily contaminated.

Cleanup of contamination from a localized pollution source may be possible in some cases because distribution of the contaminants is normally slow and follows the flow of the water in the porous medium. Therefore, if caught in time, the contamination may be restricted to a relatively small volume of the total aquifer. Pumping out of water, destruction of the contaminants in situ by biological or other means, or construction of impermeable barriers around the contaminated zone are examples of remedial methods that may be applicable in particular cases (see Chapter 12). None is cheap.

0 0

Post a comment