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Kansas Geological Survey, Public Information Circular (PIC) 1
A User's Guide to Well-spacing Requirements for the Dakota Aquifer in Kansas--Part 4 of 5


What Happens When the Pump Is Turned On?

When the pump is turned on, the water level drops in the well and in the aquifer adjacent to the well being produced. The drawdown is the decline of water level observed in wells screened in the aquifer being pumped. The amount of drawdown is at a maximum at the pumping well and diminishes to zero some distance away. The region affected by drawdown from pumping is called the cone of depression (figure 5). The size of the cone of depression and the drawdown will increase until there is a balance between the pumping rate and the flow into the well from the surrounding aquifer. Once the pump is turned off, the cone of depression diminishes in size and water levels will recover to near pre-pumping levels as flow continues to move into that portion of the aquifer affected by drawdown.

Figure 5--Cone of depression resulting from drawdown from pumping.

The size of the cone of depression and the amount of drawdown depend on the pumping rate and the ability of the aquifer's material to transmit water to the pumping well. The aquifer's ability to transmit water to the well is directly related to its permeability (the capacity of a porous material for transmitting a fluid) and total thickness. Aquifer materials that are more permeable and have greater thicknesses allow larger volumes of water to flow toward the pumping well.

What Happens in the Dakota Aquifer When the Pump Is Turned On?

Sandstones of the Dakota aquifer in the western part of the state consist of cemented, fine to very fine sand grains. In general, this aquifer material is 50-100 times less permeable to the flow of water than the uncemented, coarser sands and gravels in shallower aquifers such as the Ogallala. These permeability differences indicate that a pumping well in the Dakota aquifer will produce a significantly greater amount of drawdown than would a well in the shallower Ogallala aquifers being pumped at the same rate. In some parts of central Kansas, the permeability of the thicker river-deposited sandstones of the Dakota aquifer may approach the permeability of these shallower aquifers.

Sandstone aquifers in the Dakota are also much smaller in extent and thickness than the Ogallala aquifer. The Dakota aquifer can be thought of as a complex natural plumbing system consisting of sandstone bodies, some of which are connected to each other and which transmit water for considerable distances. If all the sandstone bodies in a local area are connected, then water flows from all these bodies toward the well (figure 3A).

The shape of the cone of depression depends on the shape of the sandstone bodies that are connected to the well. Depending on the rate and duration of pumping, the cone of depression of a river-deposited sandstone can extend along the length of the sandstone body for several miles and may extend into other sandstone bodies (figure 3A). This exaggeration of the cone of depression along the sandstone body occurs because the relatively impervious shaly rock surrounding it contributes no water to the pumping well. The result is a cone of depression with a linear or irregular shape. In the sheetlike shoreline sandstone bodies, the shape of the cone of depression is usually more circular (figure 3B).


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Kansas Geological Survey, Geology Extension
1930 Constant Ave., Lawrence, KS 66047-3724
Phone: (785) 864-3965, Fax: (785) 864-5317
bsawin@kgs.ku.edu

Web version Nov. 1995
http://www.kgs.ku.edu/Publications/pic1/pic1_4.html