by Donald O. Whittemore, P. Allen Macfarlane, John H. Doveton,
James J. Butler, Jr., Tyan-ming Chu,
Rod Bassler, Martin Smith, James Mitchell, and Alan Wade
Kansas Geological Survey
Open-file Report 93-1
In recognition of the need for an integrated research program, the Kansas Geological Survey (KGS) began in FY89 to conduct and coordinate a long-term multi-agency program to assess the water-resources potential of the Dakota aquifer. Several water-quantity and water-quality issues associated with long-term development of the aquifer are being studied: (1) water availability, (2) sources and amounts of recharge and their effects on water quality, (3) impact of water withdrawals from the Ogallala and Dakota aquifers in southwestern Kansas on future water-supply availability, (4) effect of shallow disposal of oil brines in central Kansas, (5) delineation of usable zones in the aquifer, and (6) effect of saltwater discharge from the Dakota aquifer on water quality in central Kansas stream-aquifer systems.
For FY90-93 the overall objectives of the Dakota aquifer program are to characterize subregionally the water-resources potential of the Dakota aquifer where it is shallowest and is presently being used, to develop conceptual models of ground-water flow and chemistry, and to begin digital simulation of ground-water flow and quality. Results obtained in FY92 provide important advances in the understanding of the distribution of aquifer units, the ground-water flow system, and the water quality of the Dakota aquifer system. Although the focus of many of the diverse projects was on southwestern Kansas, other projects were conducted in central and north-central Kansas.
Work on the geologic framework of the Dakota system in FY92 focused on geologic maps and cross sections based largely on analysis of geophysical logs. A series of geologic maps nearing completion for southwestern Kansas comprise contoured surfaces and thicknesses for different stratigraphic units in the Upper Cretaceous aquitard overlying the Dakota aquifer, in the Dakota aquifer system, and in the Permian underlying the Dakota. A colored image for a cross section through southwestern to central Kansas was generated using computer analysis of geophysical logs to better illustrate distribution characteristics of aquifer sandstone units with distance and depth. Geostatistical analysis of sandstone and shale distributions was initiated in Hodgeman County where a high-density distribution of geophysical logs have been digitized.
Geohydrologic investigations included (1) determination of aquifer characteristics and aquifer interactions at monitoring sites in southwestern Kansas and (2) simulations of ground-water flow both aerially and vertically across the Dakota aquifer system at different scales and in different areas. New monitoring sites were constructed in Stanton and Gray counties. Hydrologic and/or water-quality tests were conducted at the Finney, Hodgeman, and Gray County monitoring sites. Ground-water flow simulations ranged from a regional steady-state model in western Kansas and southeastern Colorado to models for subregional and local areas. Extension of the Dakota study area into southeastern Colorado is important because the area is the source of much of the recharge for a substantial portion of the aquifer in Kansas. A report on the ground-water flow systems and water-resources potential of the Dakota aquifer in parts of Republic, Cloud, Clay, and Washington counties has been completed and is being reviewed. A steady-state model of ground-water flow in southeastern Colorado and southwestern Kansas is being developed in conjunction with the larger scale regional model but at a more detailed scale that will form the basis for a transient-flow model for assisting water-resources management in southwestern Kansas. A flow model completed along a cross section from southeastern Colorado to central Kansas shows the effect of hydrostratigraphy and topographic relief on the steady-state flow in the Dakota aquifer system. The model considers the interactions between geologic strata above, within, and below the Dakota aquifer and explains the generation of subnormal pressures in the aquifer. In addition, simulated flow in the cross section characterizes the water budget for local flow systems in the recharge and discharge areas of southwestern Colorado and the eastern outcrop belt of the Dakota system in central Kansas and the regional flow in the confined aquifer from western to central Kansas.
The geochemical framework of the saltwater-freshwater transition zone in the Dakota aquifer was characterized in north-central Kansas in preparation for simulating chemical changes during ground-water flow. Another geochemical project identified sources of salinity in ground waters within and discharging from the Dakota aquifer. No oil-field brine was found in the slightly saline ground waters from test wells drilled for the development of supplemental municipal water supplies for the city of Hays. Although no detectable oil brine was found in low flows of the Saline River where the Dakota aquifer discharges saline water, oil-brine contamination was found to contribute to natural saline discharge to the Smoky Hill River in southern Russell County. A study to geochemically characterize aquifer interactions and recharge to the Dakota aquifer from atmospheric precipitation and adjacent aquifers was begun in southwestern Kansas and southeastern Colorado. Water-quality distribution maps for the Dakota aquifer were modified to incorporate the new data, and maps of selected constituent concentrations were corrected for the effect of local nitrate contamination of the aquifer. A water-quality assessment based on drinking-water supply use was updated based on additional data and reassessment for selected constituents for which new state and federal standards took effect in 1992. The wide range in fluoride concentrations in Dakota ground waters is explained by water-rock interactions associated with natural softening, which in turn is related to differences in the chemistry and relative flow rates of recharge from the overlying Upper Cretaceous and underlying Permian rocks that mix with waters flowing through Dakota sediments.
In addition to the research just described, the KGS worked with and provided information on the Dakota aquifer to several federal, state, and local agencies and the public in FY92. Research was conducted with the U.S. Geological Survey through a cooperative arrangement between the two agencies, including working with Groundwater Management District No. 3 in southwestern Kansas. Lawrence Livermore National Laboratory and the Texas Bureau of Economic Geology cooperated with the KGS on research on the geochemistry of recharge and flow in the Dakota aquifer system and interactions with other aquifers. The KGS provided information on the Dakota aquifer to Hays as a part of the city's search for additional supplies of ground water for municipal use. Investigations for salinity source determination were conducted in cooperation with the Kansas Corporation Commission and the Kansas Water Office. Information transfer in FY92 included a symposium organized by the KGS and held at the University of Kansas to present results and plans of the Dakota program. The symposium provided a useful forum for communication of accomplishments and receipt of needs and suggestions for proposed work. Throughout FY92, program research staff answered questions and provided data in response to many requests for information on the Dakota aquifer. Other research communications included reports and presentations; the annual report of the FY91 Dakota aquifer was completed and published as KGS Open-File Report 92-1.
The primary emphases during FY93-95 in the Dakota program are (1) to integrate geologic, geohydrologic, and geochemical results to form conceptual models of the aquifer system and useful products for water-resource evaluation and (2) to develop three-dimensional digital models of the Dakota and adjacent aquifers that can be used to simulate the flow of water, transport of solutes, and chemical reactions along the flow path. Development and the use of the quasi-three-dimensional models to simulate the hydrology are being carried out in phases. Phase 1 work is reported in this FY92 report. Work on the regional and subregional steady-state simulations of the Dakota aquifer will continue through FY93. In the latter part of FY94 work will begin on developing a transient-flow model that will be completed in FY95 for southwestern Kansas to allow simulation of the effect of pumping. The models will be used to assess the effect of various water-management options in FY95. Late in FY94 research will also shift to the deeper subsurface of west-central, northwest, and north-central Kansas. In this area readily available data are sparse, the depth to the top of the Dakota aquifer is considerable, and salinities are commonly high. The main focus will be on the areas adjacent to the region under present development and on areas predicted to have usable water quality.