Groundwater - Groundwater Flow
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Water is always in motion. Groundwater generally moves from recharge areas, where water enters the ground, to discharge areas, where it exits from the ground into a wetland, river, lake, or ocean. Transpiration by plants whose roots extend to a point near the water table is another form of discharge. The path of groundwater movement may be short and simple or incredibly complex, depending on the geology of the areas through which the water passes. The complexity of the path also determines the length of time a molecule of water remains in the ground between recharge and discharge points. (See Figure 4.2.)
The velocities of groundwater flow generally are low and are orders of magnitude less than the velocities of stream flow. Groundwater movement normally occurs as slow seepage through the spaces between particles of unconsolidated material or through networks of fractures and openings in consolidated rocks. A velocity of one foot per day or more is a high rate of movement in groundwater. Groundwater velocities can be as low as one foot per decade or one foot per century. Stream flows, on the other hand, are generally measured in feet per second. A velocity of one foot per second is about sixteen miles per day. The low velocities of groundwater flow can have important implications, particularly in relation to the movement of contaminants.
The age of water (time since recharge) varies in different parts of groundwater flow systems. Groundwater gets steadily older along a particular flow path from an area of recharge to an area of discharge. In shallow, local scale flow systems, groundwater age at areas of discharge can vary from less than a day to a few hundred years. (See Figure 4.2.) In deep, regional flow systems with long flow paths, groundwater age may reach thousands or tens of thousands of years.
An aquifer is a saturated zone that contains enough water to yield significant amounts of water when a well is sunk. The zone is actually a path of porous or permeable material through which substantial quantities of water flow relatively easily. The word "aquifer" comes from the Latin aqua (water) and ferre (to bear or carry). An aquifer can be a layer of gravel or sand, a layer of sandstone or cavernous limestone, a rubble zone between lava flows, or even a large body of massive rock, such as fractured granite. An aquifer may lie above, below, or in between confining beds that are layers of hard, nonporous material (clay or solid granite, for example).
There are two types of aquifers: unconfined and confined (artesian). In an unconfined or water table aquifer, precipitation filters down from the land's surface until it hits an impervious layer of rock or clay. The water then accumulates and forms a zone of saturation. Because runoff water can easily seep down to the water table, an unconfined aquifer is very susceptible to contamination.
In a confined aquifer, the confining beds act more or less like underground boundaries, discouraging water from entering or leaving the aquifer, so that the water is forced to continue its slow movement to its discharge point. Water from precipitation enters the aquifer through a recharge area, where the soil lets the water percolate down to the level of the aquifer. The ability of an aquifer to recharge is dependent on various factors, such as the ease with which water is able to move down through the geological formations (permeability) and the size of the spaces between the rock particles (porosity). Figure 4.3 illustrates natural and artificial aquifer recharge.
Usually, the permeability and porosity of rocks decrease as their depth below the surface increases. How much water can be removed from an aquifer depends on the type of rock. A dense granite, for example, will supply almost no water to a well even though the water is near the surface. A porous sandstone, however, thousands of feet below the surface can yield hundreds of gallons of water per minute. Porous rocks that are capable of supplying freshwater have been found at depths of more than 6,000 feet below the surface. Saline (salty) water has been discovered from aquifers that lie more than 30,000 feet underground.
Aquifers vary from a few feet thick to tens or hundreds of feet thick. They can be located just below the Earth's surface or thousands of feet beneath it. An aquifer can cover a few acres of land or many thousands of square miles. Any one aquifer may be a part of a large system of aquifers that feed into each other. Figure 4.4 shows the principal aquifers of the United States.
The Ogallala or High Plains Aquifer is one of the world's largest. It is located in the United States and covers 156,000 square miles, stretching from southern South Dakota to the Texas panhandle. Figure 4.5 shows the aquifer's location. The Ogallala's average thickness ranges from 300 feet to more than 1,200 feet in Nebraska. For more than fifty years, the Ogallala has supplied most of the water for irrigation and drinking to the Great Plains states. Approximately 200,000 wells tap into this vast aquifer and extract huge quantities of water. The Environmental Protection Agency (EPA) has designated the Ogallala Aquifer a sole source aquifer. This means that at least 50% of the population in the area depend on the Ogallala Aquifer for its water supply.
The U.S. Geological Survey (USGS) reported in its 1996 publication Ground Water Atlas of the United States, Oklahoma, Texas, HA 730–E that because the Ogallala Aquifer was being pumped far in excess of recharge, the USGS and the Texas Department of Water Resources projected an increasing shortage of Ogallala aquifer water for future irrigation needs. The projections suggested that the irrigated acreage in the High Plains of Texas (69% of irrigated Texas cropland) would be reduced to one-half of its present acreage by 2030 unless an effective water conservation plan was implemented.
In 1999 Texas oil tycoon and corporate raider T. Boone Pickens formed Mesa Water, Inc., to market water from part of the Ogallala Aquifer to large Texas cities for municipal use. About 100 landowners and 200,000 acres of land in the Texas Panhandle were involved in Pickens's plan as of 2005. The project sparked controversy in Texas, where the hundred-year-old "rule of capture" was still in effect. The rule of capture, which was at one time standard doctrine in much of the United States, says that the owner of land that sits upon an underground water source can pump out unlimited amounts of water regardless of the impact on surrounding property owners.
Concerned that the already rapidly draining water source would become depleted even further, residents of the surrounding Panhandle area protested, arguing that their essential source of water should not be pumped hundreds of miles away. Many Texans began campaigning for tighter government regulations on water rights. However, comprehensive water legislation that would have modified rule of capture failed to reach a vote in the Texas state senate in May 2005, and in June 2005 Pickens announced that Mesa Water would begin building a system of wells and pipelines to sell the water to major urban centers throughout Texas ("Private Water Group Preparing to Pump the Ogallala Aquifer and Sell Groundwater to Far-Away Texas Cities," http://www.argentco.com/htm/f20050607.614464.htm, June 2005).
A spring is a natural discharge of water at the Earth's surface from a saturated zone that has been filled to overflowing. Springs are classified either according to the amount of water they produce or according to the temperature of the water (hot, warm, or cold). Giant Springs in Great Falls, Montana, is the largest freshwater spring in the United States and is the source of water for the Missouri and Roe Rivers. Giant Springs removes 7.9 million gallons per hour from underground reserves and maintains a constant temperature of 54°F. Its flow accounts for approximately one-sixth of the water flowing downstream in the Missouri River, and its water has been carbon-dated to be about 3,000 years old.
Thermal springs have water that is warm or, in some places, hot. They are fed by groundwater that is heated by contact with hot rocks deep below the surface. In some areas, water can descend slowly to very deep levels, getting warmer the farther down it goes. If it rises faster than it descended, it does not have time to cool off before it emerges on the surface. Well-known thermal springs are the Warm Springs in Georgia and the Hot Springs in Arkansas. Geysers are thermal springs that erupt periodically. Old Faithful in Yellowstone National Park is perhaps the most famous and spectacular geyser in the world. It erupts at intervals of thirty to ninety minutes.