Kansas Geological Survey, Open-File Rept. 96-1a
Proposed Management Areas--Page 6 of 16
The Dakota aquifer is not a uniform regional aquifer, but consists of hydraulically connected and isolated permeable sandstone bodies that are finite in size and encased in relatively impervious mudstone. Because the mudstones effectively do not yield water, the edges of these sandstone bodies are hydrologic barriers to flow depending on the rate and duration of pumping. This reduces the rate of recharge moving laterally toward the developing cone of depression or zone of influence. As a result, the zone of influence can extend out from the well along the length of these elongated sandstone bodies for several miles or more and may extend into other hydraulically connected sandstone bodies. However, in the direction perpendicular to the long axis of the sandstone body, this zone may only extend to the edge of the aquifer. In some cases this may be less than a mile away. These boundaries affect the well by accelerating the rate of drawdown increase with time (Figure 4). The lower water levels may necessitate lowering the pump in the well, which increases energy costs associated with water production.
Figure 4. The drawdown over time from pumping in the Dakota aquifer in comparison with the expected drawdown in an aquifer of infinite areal extent.
If two pumping wells in the Dakota aquifer are spaced even a few miles apart and are withdrawing water from the same sandstone body, their zones of influence will likely overlap and coalesce. When this occurs, the total recharge moving laterally into each well's zone of influence from the adjacent aquifer is reduced, creating an impairment. Impairment results when one well diminishes the supply of water available to another nearby well. The immediate impact of this impairment is to increase the drawdown in both wells. If both wells continue pumping for longer periods of time, it may cause local depletion of the aquifer. This is most likely to happen where the pumping wells are located far away from sources of recharge or a discharge area.
The pumping effect on hydraulic head is transmitted through an aquifer much faster than the actual movement of water. However, pumping does move water and can induce the movement of saline water in the aquifer into a supply well. Water withdrawn from a well will be preferentially derived laterally from the most permeable layers in a sandstone. Some water may also be derived vertically as leakage from overlying units. However, if pumping occurs at a great enough rate or over a long enough period of time, the stress may be sufficient to cause upward movement of saline water from the bottom of a sandstone below which there is a great salinity increase. Pumping stress can also move water laterally from near the edge of sandstone bodies where water in the confined aquifer can be more saline in finer grained sediments. In addition, long-term pumping near lateral zones of freshwater to saltwater transition could obtain higher salinity water from the transition zone. Decreasing the rate of withdrawal and increasing the distance between wells could decrease the amount of saline water drawn from below and laterally if leakage from above the sandstone is relatively important. However, upconing and lateral movement of salinity from nearby sandstone boundaries and saltwater transition zones could eventually cause slow increases in TDS concentrations if the leakage rate from above is relatively low.
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