a spring, along the side of a hill, produced by a perched water table. A perched water table is one that lies above the normal water table due to an impermeable layer that blocks the normal downward movement of groundwater. WATER TABLE FIGURE 10.61 A Perched Water Table Water infiltrating the ground from rain or melted snow, moves downward through the pore spaces within a permeable layer. If it encounters an imper- meable layer, that movement is momentarily halted, and produces a perched water table, i.e. perched above the normal water table. The water in the perched water table continues to move laterally and flatten. In this example, the water moving to the right creates a spring or seep, depending on the volume of water flowing at the surface. The photograph illustrates a spring created from a perched water table. This is one of many at Thousand Springs State Park in Idaho. Illustration and photo by Stan Celestian IMPERMEABLE LAYER SPRING PERCHED WATER TABLE
The water above the impermeable layer, which could be a layer of clay, shale, or even a basalt lava flow, produces a dome (as seen in Figure 10.62). This dome is a wave of water, which gravity is driving lower. The water moves laterally and flows off the edge of the impermeable layer. In this diagram, the water flowing off the left side will drop down to join the main water table below. The water flowing to the right, encounters the surface to produce a seep or a spring. A perched water table, being closer to the surface, may be easier to access than the main water table, as shown in Figure 10.62. However, depending on the climate and the recharge rate of the water table, it may not be as reliable a producer of water as a deeper well that accesses water from the main water table. Applications of Darcy ’ s Law Around an actively pumping well, the water table is depressed into a cone of depression . This depressed area is the result of water moving out of the pore spaces of the rock and into the well. Darcy ’ s Law predicts how steep the cone of depression will be. For example, in a very permeable, unconsolidated sand, the cone of depression will be very steep. For a moderately well - cemented sandstone, the cone of depression will be very gradual. Figure 10.63 is an example of the cone of depression from a well on a ranch. When the well is not pumping, the water level in the well pipe is at the water table level. When the pump is active, water is drawn out of the well and the downward curvature of the water table (cone of depression) is created by the water moving out of the permeable material and into the well. This lowering of the water table is called drawdown . The size of the cone of depression is dependent upon the flow rate of the water through the permeable sediments as determined by Darcy ’ s Law. If k , the hydraulic conductivity is high, meaning that the sediments are very permeable, the cone will be fairly steep. This is due to the rapid replacement (ease of flow) of the water in the sediments near the well. If the k value is low, the cone of depression will be larger. If the well pumps at a higher rate, the cone of depression will also be larger.
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