by P. Allen Macfarlane, D.O. Whittemore, M.A. Townsend, J.H. Doveton,
V. J. Hamilton, W.G. Coyle III, and A. Wade
Kansas Geological Survey, The University of Kansas
and G.L. Macpherson
Department of Geology, University of Kansas
and R.D. Black
Agricultural Extension Service
Kansas State University
Kansas Geological Survey
Open-file Report 90-27
Previous investigations of the Dakota aquifer by State and Federal agencies have produced piecemeal and conducted without the benefit of an integrated research strategy, or have produced overly generalized results that are of little value to future planning and regulatory needs. In recognition of the need for an integrated program of research on the Dakota aquifer in Kansas, the Kansas Geological Survey has undertaken a long-term multi-agency program to assess the water-resources potential of this aquifer in order to meet the water-planning and regulatory needs of the future.
This report is a summary of the research findings and activities in progress as of the end of the program's first year (FY89). The report is subdivided into four parts. Part 1 contains introductory and background information on the Dakota aquifer and the Dakota aquifer research program being conducted by the Kansas Geological Survey. Part 2 discusses previous research efforts involving the geology, hydrogeology, and hydrogeochemistry of the Dakota aquifer. Part 3 summarizes the research findings of the first year of the program. Part 4 outlines the direction of future research in the Dakota aquifer program based on presently perceived research needs. The appendices to the report (1) deal with the development and documentation of data bases, (2) discuss the results of borehole and surface geophysical investigations aimed at collecting new data for the Dakota aquifer, and (3) discuss the results of a study of energy use in irrigation and other high-capacity well systems that withdraw water from the Dakota aquifer and the hydraulically connected Dakota-High Plains aquifer in Kansas. A number of plates are included at the end of the report that provide more detail on the various aspects of hydrogeology, geology, and water quality.
In central Kansas and widely separated areas of southwestern Kansas the strata that comprise the Dakota aquifer crop out at the surface, and the aquifer is unconfined or partially confined. Elsewhere in southwestern Kansas the aquifer subcrops beneath Cenozoic deposits and is hydraulically connected to the water-table aquifer. Where covered by the Upper Cretaceous aquitard, confined flow conditions exist.
Two primary ground-water flow corridors dominate the potentiometric surface map of the Dakota aquifer. Ground water moves along these corridors from recharge areas in southwest Kansas-southeast Colorado to discharge areas in the central part of Kansas. One corridor parallels the outcrop and subcrop areas of the aquifer and is heavily influenced by local sources of recharge and discharge. The other corridor directs flow northeastward through the confined portion of the Dakota aquifer and eastward into north-central Kansas discharge areas. The Dakota aquifer is primarily recharged by precipitation falling on the outcrop and the overlying water-table aquifer in southwest Kansas and by the underlying Cedar Hills aquifer where the units are hydraulically connected in the central part of the state.
The present configuration of the regional ground-water flow system is complex and is hypothesized to consist of both modern and relict components. The modern component of the system is responsive to changes in the water-table configuration that have been brought about by episodes of uplift, erosion, and deposition during the late Mesozoic (latest Cretaceous Period) and Cenozoic eras. The relict component consists of the deeper part of the flow system below the water-table aquifer. This part of the system is in a transient state and has responded much more slowly to changes in the configuration of the water table due to the less permeable nature of the hydrostratigraphic units below the water-table aquifer.
A map of the distribution of water-quality data across the state indicates that most of the information exists for a band along the eastern outcrop and subcrop of the Dakota Formation, with smaller amounts of data in southwestern Kansas. Contour maps of chloride and total-dissolved-solids concentrations show that freshwaters occur along the eastern outcrop, part of the eastern subcrop adjacent to the outcrop zone, and in the subcrop area of the Dakota aquifer in southwestern Kansas. The rates of change in the chloride and dissolved-solids contents with areal distance are greatest along the easternmost subcrop that trends in a southeast direction from Republic County through Cloud, Mitchell, Lincoln, Russell, and Ellsworth counties, to Barton County. The quality changes are related to regional and sub-regional flow paths that mix saltwater and freshwater.
The chemical water type was computed and mapped for each of the well locations for which a complete analysis existed for major dissolved constituents. Calcium-bicarbonate type water dominates in the outcrop zone of the Dakota aquifer due to solution of carbonate minerals and past flushing of saline water by freshwater recharge. Sodium-bicarbonate waters are common in the subcrop areas that extend away from the outcrop band; they were formed by the exchange of calcium and magnesium ions for sodium on clays in the Dakota sediments, coupled with the solution of carbonate minerals (mainly calcite) to increase bicarbonate concentrations.
Sodium-chloride type waters with high dissolved-solids contents predominate in the deeper portions of the aquifer in north-central, northwestern and central Kansas. The saltwater source has been geochemically identified in central Kansas using minor-element mixing curves and isotope chemistry. Both geochemical methods indicate that the saltwater is derived primarily from solution of halite (rock salt) in Permian rocks, and has flowed upward into lower Cretaceous strata and then into the Dakota to produce the present water mixture in the aquifer. The upward movement from the Permian due to the flow systems is affected by pinching out of confining layers and the presence of fractures and other geologic structures. Tritium determinations in well waters suggest that recent recharge to the Dakota aquifer in central Kansas has been minimal. Further minor element and isotopic work will be conducted to determine dissolved mineral sources and water mixtures, including recharge, in other areas of the Dakota aquifer.
Calcium-sulfate waters, primarily derived from gypsum dissolution or pyrite oxidation are scattered through portions of the outcrop zone. The small number of sodium-sulfate waters also scattered primarily in the outcrop zone may derive from the effect of ion-exchange on calcium-sulfate waters. Calcium-chloride waters, although small in number in comparison with other water types, suggest some oil-field brine contamination. The only conclusive oil-brine pollution identified by these methods and isotope geochemistry is in the lower Dakota aquifer at a monitoring-well site near where oil brines have been disposed into the underlying Permian Cedar Hills Sandstone in Russell County. The monitoring well in the upper Dakota aquifer at this site contained slightly saline water, but no detectable oil brine.
An assessment of the quality of water in the Dakota aquifer based on drinking-water standards was made using information in KWATCHEM. Waters from over 60% of the wells in the data base exceeded the recommended concentration of total-dissolved solids, while 28-29% of the wells exceeded recommended values for chloride and sulfate. Fluoride concentrations were above the drinking standard at 35% and nitrate contents above the standard at 7% of the well sites. High fluoride waters are common in areas of the Dakota aquifer where the water type is sodium-bicarbonate. Over half and nearly half of the well sites had iron and manganese contents, respectively, above the recommended levels. None of the samples in the data base exceeded the standard for toxic heavy metals or arsenic, but four sites had higher selenium values than the standard. However, the amount of data for toxic heavy and semi-metals, as well as radioactivity, is limited. Part of the additional work on water quality will include analyses of Dakota waters for constituents of concern for drinking and agricultural uses to allow improvements in the water-quality assessment. The source of the contaminants exceeding standards will also be determined.
Energy cost for pumping is one of the major variable costs in irrigated crop production in Kansas. About 3.5 million acres of land are irrigated annually with an estimated 5.12 million acre-feet of water. About 2/3 of this area is surface irrigated, primarily using furrows to conduct the water through the field, and the remainder is sprinkle irrigated using center pivots. The power sources for pumping are also variable. Natural gas engines predominate with 59% of the total. Electric motors are second with 19% followed by diesel engines at 15% and propane or LPG at 7%. Energy use is directly related to the depth to water in the well under static, non-pumping conditions. As a result, energy use is highest in southwest Kansas where the depth to water is greatest.
The overall plan of research has been designed to meet the needs of the program and those of the State and local regulatory and planning agencies, including the ground-water management districts. The primary purpose of this plan is to direct the course of future research efforts in the Dakota aquifer. The plan presented here is of necessity flexible in order to meet any unanticipated State needs. The original proposal submitted to the Water Steering Committee called for a fourteen-year effort to complete the program. With the completion of the first year of the program, Kansas Geological Survey estimates that the program can be completed in eight years (at the end of FY96), assuming full and continuous funding.
During FY90-92 the program will concentrate on areas where the aquifer is presently under development in southwest and central Kansas. In these areas, irrigated agriculture, public water supplies, and industry rely heavily on the Dakota and overlying aquifers. Up-to-date information is needed to fully evaluate the water resources potential of the Dakota aquifer for water planning and management. The potential impact of shallow underground injection of oil-field brines into the Cedar Hills Sandstone on the Dakota aquifer also needs to be addressed in southwest and central Kansas. With the completion of this phase the consequences of various water-management scenarios can be tested for the developed areas of the Dakota aquifer.
The emphasis of the program will then shift to the deeper subsurface of western Kansas in FY93. In this area readily available data are sparse and the depth to the top of the Dakota aquifer is considerable. As a result, most of the program's attention will be focused on those areas of potential aquifer utility adjacent to the present study areas, especially in Greeley, Hamilton, Kearny, Wichita, Scott, Lane and Gove counties in southwest and west-central Kansas and in Jewell, Smith and Phillips counties in northern Kansas. Concerns in the northwest Kansas area are primarily related to the quality of ground water in the Dakota aquifer. A preliminary analysis indicates considerable potential for water-resources development in Lane, Scott, Wichita and Gove counties.