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Kansas Geological Survey, Open-file Report 88-39
Great Plains and Cedar Hills Aquifers--Page 24 of 25

10.0 Preliminary Conclusions

The preliminary conclusions of this hydrogeologic and water quality investigation consist of descriptions of the ground-water flow system and water chemistry of the Great Plains and Cedar Hills aquifers in the study area. These aquifers are widely used in the study area: the Great Plains aquifer for water supply and the Cedar Hills aquifer for the shallow disposal of oil-field brines.

The framework of the Great Plains aquifer consists of the Dakota Formation, Kiowa Shale, and Cheyenne Sandstone. Preliminary analysis of a diverse set of geophysical logs from a borehole drilled in northern Ellis County indicates that geophysical logs can be used to determine paleoenvironments and clay mineralogy in these rocks, and, consequently, stratigraphic boundaries between units. This aquifer system has been subdivided further into upper and lower portions based on the sedimentary architecture of the rock units themselves, that is, the distribution and nature of the lithofacies represented in these rocks.

The upper portion of this aquifer system consists of thick, lenticular, alluvially deposited, channel and tabular, flat-bedded, delta-front sandstones, and interbedded mudstones, claystones, and thinly bedded sandstones of the Dakota Formation. The lower portion of the Great Plains aquifer consists of strata belonging to the Kiowa Formation and Cheyenne Sandstone. These units consist largely of non-marine to shallow marine or marginal marine, lenticular sandstones and interbedded clay shales, siltstones, and thinly bedded sandstones. Lenticular, alluvially deposited sandstones occur sporadically near the base of the Lower Cretaceous. Shallow marine, off-shore bar and distributary sandstones are developed in the Kiowa Formation, primarily east of the study area.

The ground-water flow system in the Great Plains aquifer consists of two components. A component of flow originating in the Western Kansas basin moves west to east across the study area. Another northeastward- moving component of flow moves parallel to the Dakota outcrop and enters the study area from the southwest into Rush and Barton counties. These flow systems converge in the study area along T15S.

The Cedar Hills aquifer consists of the Lower Permian Cedar Hills Sandstone. The thickness of the Cedar Hills aquifer ranges from featheredge in western Russell County to over 275 feet in Rush and Trego counties where it has not been truncated by pre-Lower Cretaceous erosion. It is estimated that on the average, 5035 acre-feet of oil-field brines were injected annually into the Cedar Hills aquifer through 460 shallow disposal wells in the study area from 1975-1983.

The Cedar Hills aquifer ground-water flow system can be subdivided into two portions based on the configuration of the potentiometric surface. West of R19W fluid pressures have been affected by injection of oil-field brines under confined conditions and, as a result, the potentiometric surface is a series of isolated highs and lows. East of R19W, the aquifer is hydraulically interconnected with the Great Plains aquifer and the configuration of the Cedar Hills potentiometric surface is subparallel with the potentiometric surface for the lower part of the Great Plains aquifer.

Hydraulic head data from the monitoring sites and areal surveys of water levels show that the vertical component of ground-water flow is directed upward from the lower part of the Great Plains aquifer to the upper part. East of R19W, the Great Plains aquifer is hydraulically interconnected with the Cedar Hills aquifer, and saline waters from the Cedar Hills aquifer are moving upward into the Great Plains aquifer. This upward movement has affected the water chemistry of the lower part of the Great Plains aquifer. Upward movement of these waters may be facilitated by fractures associated with minor structural features on the Central Kansas uplift. West of the subcrop, water levels are lower in the Cedar Hills aquifer than in the lower part of the Great Plains aquifer except where injection has increased fluid pressures in the Cedar Hills aquifer. Little site specific data are available to determine the more precisely the relationship between the Cedar Hills and Great Plains aquifers west of R19W.

Fluid pressures in the Cedar Hills and Great Plains aquifers are subhydrostatic across the study area. This suggests that these aquifers are relatively well confined and isolated from the near surface hydrologic system. However, the occurrence of relatively fresh waters in the deeper part of the system is problematic if this system is relatively isolated. These fresh waters may be very old or possibly may have come from the Ogallala through fractures in the overlying rocks.

Contamination of the Cedar Hills and Cheyenne aquifers by oil-field brines has been identified at the Gorham monitoring site. The brine contamination has not affected the quality ground water in the overlying aquifer of the upper Dakota at this location. These preliminary conclusions have important implications for the use of the Great Plains aquifer as a water supply and the Cedar Hills aquifer for the disposal of oil-field brines. Upward movement of saline waters from the Cedar Hills into the Great Plains aquifer in the area where they are interconnected, may be accelerated by the continued use of this aquifer for the disposal of oil-field brines. West of the Cedar Hills subcrop, high injection pressures may cause hydraulic fracturing of the media and as a result allow the upward movement of salt water into shallower zones, including the Great Plains aquifer.

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Kansas Geological Survey, Dakota Project
Original document dated December, 1988
Electronic version placed online April 1996
Scientific comments to P. Allen Macfarlane
Web comments to webadmin@kgs.ku.edu