Page 296 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

260 part II The Water, Weather, and Climate Systems
Precipitation is the main source of groundwater, percolating downward as gravitational water from the soil-moisture zone. This water moves through the zone of aeration, where soil and rock are less than saturated (some pore spaces contain air), an area also known as the unsaturated zone (Geosystems in Action 9.1, number 1).
Eventually, gravitational water accumulates in the zone of saturation, where soil pore spaces are com- pletely filled with water (GIA 9.1, number 2). Like a hard sponge made of sand, gravel, and rock, the zone of satu- ration stores water in its countless pores and voids. It is bounded at the bottom by an impermeable layer of rock that obstructs further downward movement of water. The upper limit of the zone of saturation is the water table, the point of transition between the zone of aera- tion and the zone of saturation (note the white dashed lines across Figure GIA 9.1). The slope of the water table, which generally follows the contours of the land surface, drives groundwater movement toward areas of lower elevation and lower pressure (GIA 9.1, number 3).
Aquifers and Wells As discussed earlier, permeable rock or materials conduct water readily, while imperme- able rock obstructs water flow. An aquifer is a subsurface layer of permeable rock or unconsolidated materials (silt, sand, or gravels) through which groundwater can flow in amounts adequate for wells and springs. The blue under- ground area in Figure GIA 9.1 is an unconfined aquifer; note the water wells on the left-hand side. An unconfined aquifer has a permeable layer above, which allows water to pass through, and an impermeable one beneath (GIA 9.1, number 4). A confined aquifer is bounded above and below by impermeable layers of rock or unconsolidated materials (GIA 9.2, number 9). The solid, impermeable layer that forms such a boundary is known as an aqui- clude. An aquitard is a layer that has low permeability but cannot conduct water in usable amounts. The zone of satu- ration may include the saturated portion of the aquifer and a part of the underlying aquiclude (GIA 9.1, number 7).
Humans commonly extract groundwater using wells that are drilled downward into the ground until they penetrate the water table. Shallow drilling results in a “dry well” (GIA 9.1, number 6); drilling too deeply will punch through the aquifer and into the impermeable layer below, also yielding little water. The water in a well drilled into an unconfined aquifer is not under pressure and so must be pumped to rise above the water table (GIA 9.1, number 5). In contrast, the water in a confined aquifer is under the pressure of its own weight, creating a pressure level called the potentiometric surface to which the water can rise on its own.
The potentiometric surface can be above ground level (GIA 9.2, number 10). Under this condition, artesian water, or groundwater confined under pres- sure, may rise in a well and even flow at the surface without pumping if the top of the well is lower than the potentiometric surface (GIA 9.2, number 11). (These wells are called artesian for the Artois area in France,
where they are common.) In other wells, however, pres- sure may be inadequate, and the artesian water must be pumped the remaining distance to the surface.
The size of the aquifer recharge area, where surface water accumulates and percolates downward, differs for unconfined and confined aquifers. For an unconfined aquifer, the recharge area generally extends above the en- tire aquifer; the water simply percolates down to the water table. But in a confined aquifer, the recharge area is far more restricted. Pollution of this limited area causes groundwa- ter contamination; note in Figure GIA 9.2, number 12, the pollution caused by leakage from the disposal pond on the aquifer recharge area, contaminating the nearby well.
Groundwater at the Surface Where the water table intersects the ground surface (GIA 9.1, number 8), water flows outward in the form of springs, streams, lakes, and wetlands. Springs are common in karst environments, in which water dissolves rock (primarily limestone) by chemical processes and flows underground until it finds a surface outlet (karst discussion is in Chapter 14). Hot springs are common in volcanic environments where water is heated underground before emerging under pres- sure at the surface. In the southwestern United States, a ciénega (the Spanish term for spring) is a marsh where groundwater seeps to the surface.
Groundwater interacts with streamflow to provide base flow during dry periods when runoff does not occur. Conversely, streamflow supplements groundwater dur- ing periods of water surplus. Geosystems in Action 9.2 illustrates the relationship between groundwater and sur- face streams in two different climatic settings. In humid climates, the water table is higher in elevation than the stream channel and generally supplies a continuous base flow to a stream. In this environment, the stream is ef- fluent because it receives the water flowing out from the surrounding ground. The Mississippi River is a classic example, among many other humid-region streams. In drier climates, the water table is lower than the stream, causing influent conditions in which streamflow feeds groundwater, sustaining deep-rooted vegetation along the stream. The Colorado and Rio Grande rivers of the Amer- ican West are examples of influent streams.
When a water table declines so that the bottom of a streambed is no longer in contact with it, streamflow seeps into the aquifer. In Central and Western Kansas, the Arkan- sas River channel is now dry as a result of overuse of the High Plains Aquifer (see the photo in Figure GIA 9.2).
Overuse of Groundwater
As water is pumped from a well, the surrounding water table within an unconfined aquifer might experience drawdown, or become lowered. Drawdown occurs if the pumping rate exceeds the replenishment flow of water into the aquifer or the horizontal flow around the well. The resultant lowering of the water table around the well is a cone of depression (Figure GIA 9.1, left-hand side).