Kansas Geological Survey, Open-file Report 88-39
Great Plains and Cedar Hills Aquifers--Page 23 of 25
Determination of the potential for upward movement of ground waters below the water table can be shown most effectively using information gathered about the ground-water environment along vertically separated measurement points (Freeze and Cherry, 1979; Javandel, 1983). In this regard, it is sufficient to show hydraulic head differences between points in the ground-water system. Evidence of movement can also be developed from an analysis of the water chemistry along the vertical direction to show mass transport or from the calculation of the vertical component of ground-water velocity using the material properties of the rocks.
Ideally, many of these determinations should be made over the study area. However, in this investigation, only three such determinations have been made from the multiple-completion monitoring wells and the results are being generalized to cover an extensive area. Fortunately, the locations of the monitoring sites have been reasonably well distributed and representative of the hydrogeology of the Great Plains and Cedar Hills aquifers in the study area. This has allowed for some degree of predictability about the hydrologic system.
9.2 Geographic Areas of Potential Upward Movement of Saline Waters into Shallow Freshwater Aquifers
Aquifers in the Cheyenne and Cedar Hills Sandstones are the primary sources of salt water in the shallow subsurface of the study area. Fresh water is not known to occur in the Cedar Hills aquifer in the study area. As a result the Cedar Hills aquifer is being used as a disposal zone for oil-field brines. In several areas of Russell and Ellis counties, the Cheyenne was also used as a disposal zone for oil-field brines. However, the occurrence of fresh water in the Cheyenne, Kiowa, and lower Dakota (middle and lower portions of the Great Plains aquifer) in a well in western Graham County suggests that the Cheyenne has been locally flushed of salt water at least by fresher recharge waters. This well is located in an area of relatively low ground-water that were salinities mapped using SP resistivity methods. At the Hill City monitoring site, the vertical direction of ground-water flow appears to be upwards from the lower to the upper part of the Dakota Formation (upper part of the Great Plains aquifer). In this part of the study area, more than 350 feet of younger Permian and Jurassic sedimentary rocks separate the Cedar Hills aquifer from the Great Plains aquifer.
Farther east, these younger Permian and Jurassic rocks are not present and the Cedar Hills and Great Plains aquifers are interconnecting aquifer systems (Figure 6). In this part of the study area, significant hydraulic head differences between the Cedar Hills and overlying Great Plains aquifers indicate that saline ground waters from the lower aquifer have the potential for moving upward into shallower zones of the Great Plains aquifer. An analysis of the ground-water geochemistry data shows that waters in the Dakota aquifer east of the Cedar Hills aquifer subcrop are sodium chloride- type waters and generally contain higher chloride levels than those west of the subcrop. This indicates that upward movement between the Cedar Hills and Great Plains aquifers is occurring in the area where the lower aquifer subcrops beneath the Great Plains aquifer. The coincidence of zones of sodium-chloride waters in the upper part of the Great Plains aquifer with geologic structures suggests that some of this salt water may be migrating upward through fractures rather than through interconnecting lenses of sandstone. However, insufficient data are available to determine the flow rates.
These findings indicate that the Cedar Hills aquifer may not be suitable for the continued use as a disposal zone for oil-field brines in the subcrop area. The most important consideration in the disposal of wastes using shallow injection is containment in the injection horizon in order to prevent contamination of fresh and usable surface and ground-water supplies (Warner and Lehr, 1980). Based on the preliminary findings from this study continued use of the Cedar Hills as a disposal horizon will increase the discharge of naturally occurring saltwaters present in the Cedar Hills as the fluid pressure in the injection zone increases. A comparison of Cedar Hills injection well fluid-level data collected by the Kansas Department of Health and Environment in the 1970's with data collected in this study for the same area indicates that some fluid levels may have risen as much as 100 feet since the time of the original survey. Hydrologic testing of the Cedar Hills aquifer should be conducted to determine the effect of injection on the disposal zone and the overlying Great Plains aquifer during injection, especially in those areas where high injection pressures are used to inject the oil-field brines into the disposal horizon.
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