How Do Wells Work?Wells are drilled into a variety of different aquifer formations to supply water for many different uses. Specialized equipment is required to meet the construction standards for drilling a well, installing the well casing and screen, back filling the well hole, and test pumping the well for its intended purpose. Without proper well drilling and construction techniques, the reliability of the well and protection provided to the aquifer may be questionable. Water is removed from an aquifer using pumps. Pumps use mechanical energy supplied by a drive motor or engine to force water toward the land surface. Removing water lowers the water level in the well. The difference between the initial water level, or static water level, and the pumping water level causes water to move within the aquifer (Figure 4). Since the water level always is lowest in the well, water from the surrounding aquifer flows toward the well to replace the water being removed.
Figure 4. Water level changes associated with groundwater pumping. When pumping starts in an unconfined aquifer, most of the water is removed from very near the well. With continued pumping, water is removed further from the well, lowering the water level at a greater distance from the well. Drawdown decreases with the distance from the well until at some distance, the water level remains relatively unaffected by pumping. Drawdown in the well continues to increase slightly with pumping. After many hours of pumping the pumping water level nearly stabilizes. The resulting cone-like shape of the water surface is referred to as a cone of depression (Figure 4). The size and shape of the cone of depression is determined by the aquifer materials and the amount of water being removed from the aquifer. For example, domestic wells generally pump for short periods of time at rates of 5 to 20 gallons per minute. This results in small, poorly defined cones of depression. Even low-yield aquifers often can be developed for domestic uses. Irrigation and municipal wells typically have pumping rates that range from 100 to more than 1,500 gallons per minute, and operate for long periods. Aquifers must yield large volumes of water, and much larger and deeper cones of depression result. In some cases, the cone of depression may extend several hundred feet from the well and be up to 100 feet deep. Where there are many wells, like in the Platte River Valley, cones of depression for adjacent wells can overlap, increasing the depth and size of each well's cone of depression. |
