Ground water is withdrawn from wells to provide water for everything from drinking water for the home and business, to water to irrigate crops, to industrial processing water. When water is pumped from the ground, the dynamics of ground water flow change in response to this withdrawal.

When a well is installed in an unconfined aquifer, water moves from the aquifer into the well through small holes or slits in the well casing or in some types of wells, through the open bottom of the well. The level of the water in the well is the same as the water level in the aquifer. Ground water continues to flow through and around the well in one direction in response to gravity.

When pumping begins, water begins to flow towards the well, in contrast to the natural direction of ground water movement. In response, the water level in the well falls below the water table ,in the surrounding aquifer. As a result, water begins to move from the aquifer into the well. As pumping continues, the water level in the well continues to increase until the rate of flow into the well equals the rate of withdrawal from pumping. The movement of water from an aquifer into a well results in the formation of a cone of depression. The cone of depression describes a three dimensional inverted cone surrounding the well that represents the volume of water removed as a result of pumping. Drawdown is the vertical drop in the height between the water level in the well prior to pumping, and the water level in the well during pumping.

This information is used in a number of ways:

knowledge of the drawdown helps to ensure a continuous supply of water; drawdown that reaches to the bottom of an aquifer could result in a "dry well" knowledge of the lateral, or sideways, extent of the cone of depression helps in identifying the overlying land area to be managed for ground water protection. A spill, for example, occurring in this area could percolate into the ground water and be "pulled in" by the pumping of the well pumping can result in a change of the ground water's source. For example, water that was once discharging into a stream may now be "pulled in" to the well. Surface water quality generally is more apt to be contaminated; in addition, the regulatory and monitoring standards for drinking water originating from surface water bodies are often different than those originating from ground water sources.

The knowledge of natural ground water flow and the impact of pumping on flow is important in the strategic placement of wells (geographically and vertically) and the design of appropriate pumping rates and frequencies. This is important for a number of reasons:

• to ensure the specific source of water is known, be it a specific aquifer or a nearby surface water body,
• to pump out contaminated ground water so that it can be treated on the surface, such as by an air stripper. An air stripper remediates ground water contaminated by volatile organic compounds (VOCs). The illustration on the right shows water being treated using an air stripper.
• to manipulate local ground water flow so that contaminated ground water flows away from a drinking water source.

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