The Flint Hills Aquifer in KansasBy Robert Davidson
Emporia State University Geo 571 Hydrogeology
Dr. Marcia K. Schulmeister
Figure 1. Flowing spring, Chase County, Kansas. This spring house on an abandoned farm was built into an outcrop of limestone that contains part of the Flint Hills Aquifer. Photograph by author.
The Flint Hills Aquifer is a minor but locally important aquifer located in the east central part of Kansas. Used since the earliest days of European settlement (see figure 1 above) the aquifer continues to be an important source of water to many rural farms and ranches. In order to understand the Flint Hills aquifer in Kansas it is necessary to understand a little of the geology and physical geography of the Flint Hills themselves. The exact boundaries of the Flint Hills vary according to whose convention you follow, but in general they run along a north south line starting in Marshall County to the north and extending to Cowley County in the south, and then on into Oklahoma. See figure 2 below.
Figure 2. Extent of the Flint Hills Region in Kansas from Kansas Geological Survey
The Flint Hills were formed by the erosion of Permian age limestones and shales. The limestone layers contain numerous bands of chert or flint (see figure 3) and as the limestone weathered the less soluble flint was left behind in a clay matrix soil. Most of the hilltops in the Flint Hills are capped with this cherty/flint gravel. (Kansas Geological Survey) Because of this rock strewn soil and the numerous and extensive outcroppings of limestone in the hills (see figure 4), very little row crop farming was ever done in this region. However vast pastures of native tallgrass prairie grasses gave rise to an extensive cattle ranching industry. And because the prairie was never disturbed, the Flint Hills are one of the last and certainly one of the most extensive regions of tallgrass prairie left in the world.
Figure 3. Blue-gray chert nodules in the Florence Limestone member of the Barneston Limestone in Riley County, Ks (from the Kansas Geological Survey)
Figure 4. Limestone outcrop in Flint Hills, Chase County, Ks Photograph by authorClimate:
Climate is considered moderate, with average annual precipitation of about 30 inches (see figure 5) , and summer highs of 80-100 degrees (F). Winter lows vary from 0 to 20 degrees (F). (National Park Service Tallgrass Prairie Preserve) However, as in the rest of Kansas, the climate is extremely variable, with years of flood followed by drought, both severe and mild winters, and extremely hot summers with highs above 113F perhaps alternating with summers were the high only reaches into the 90’s. Humidity is one summer constant. Most of the rain from this region comes from the gulf coast, mainly during summer thunderstorms.
Figure 5. Normal Precipitation Chart from Kansas State University Oznet website Here you can see that most of the Flint Hills counties have average precipitation of about 32-37 inches. This will figure in the recharge of the Flint Hills AquiferThe Flint Hills Aquifer:
The Flint Hills aquifer almost exactly follows the extent of the Flint Hills themselves. (See figure 6). The Flint Hills aquifer consists of Permian limestones in the Chase and Council Grove groups (see figure 7) (Macfarlane, P.A. 2000) and more specifically the Noland, Winfield and Barneston formations within those groups (see figure 8)(Aber, J.S. 2004) In general the freshwater part of this aquifer is unconfined. Deeper waters may be extensively saline.
Figure 6. Map showing the extent of the Flint Hills Aquifer (Kansas Geological Survey)
Figure 7. Chart showing the general stratigraphy of all aquifers in Kansas (McFarlane, P.A. 2000)
Figure 8. Shown here in greater detail is the stratigraphy of the Chase and Council Grove Groups broken into separate formations. (Aber,J.S. 2004)
One of the important things to realize about the Flint Hills aquifer is that because it comes from a limestone formation, or rather a series of limestone formations, the nature of the aquifer is much different. Most of the water storage capacity of a limestone aquifer comes from bedding plane faults and vertical cracks (see figure 9). Some water storage also comes from secondary porosity as shown in the second photograph (see figure 10)
Figure 9. Here you can clearly see the possible water storage structures of this limestone outcrop. Photograph by author.
Figure 10.Secondary porosity in a block of Chase County limestone from the Clements Stone Arch Bridge Photograph by author
The primary porosity of limestones is variable and generally low compared to sandstones or other clastic rocks. Chemically precipitated rocks have very low porosity and permeability if they are crystalline but bedding planes can be zones of high primary porosity and permeability (Fetter,C.W. 2001) In the Flint Hills most of the limestone was chemically precipitated from the vast inland seas of the Permian period. Because of the transient nature of these bedding plane fractures, groundwater flow in the Flint Hills aquifer tends to be variable. Areas of high flow exist, with flows of up to 1000gallons per minute ( McFarlane P.A. 2000 ) Crystal Springs near Florence, Kansas (see figure 11) is one well known example of such high flow. This spring in the base of the Barneston Limestone is broadly and closely connected to nearby Martin Creek, which is thought to be the primary catchment for the spring. Numerous sinkhole type openings in the creek connect to the spring through conduits that may be up to a meter in diameter. Dye trace work shows that catchment basin water reaches the spring even in fairly dry periods in as little as 60 hours. (Kansas Geological Survey 2003)
Figure 11.Spring water storage structure for the City of Florence, Marion County, Kansas Photograph by authorGroundwater supply potential:
Estimating the ability of the Flint Hills aquifer to supply water for any use is difficult because of the transient nature of the aquifer itself. In general the aquifer is best suited for domestic water use on small farms, use in ranching to supply water to livestock and limited use as a water supply for small cities. It’s possible that in some instances it could be used by larger cities to supplement other sources during periods of drought or other limitations to supply. The aquifer was tapped from very early in the settlement period of Kansas to provide water to isolated ranches in the Flint Hills (see figures 11 and 12).
Figure 12. Jones Ranch Spring Hill Farm at Tallgrass Prairie National Park. Note the decorative fountain pool. The Flint Hills aquifer provided water for this farm and ranch operation, including cooling for the spring house and water for this fountain. Built in 1881. Photograph by author
Some work on analysis of the water bearing capacity of the Flint Hills aquifer has been done by the Kansas Geological Survey. Preliminary work is shown in figure 13. Several important assumptions were made about the aquifer during this study. One was that the average aquifer thickness is 40feet and the specific yield of the limestone strata is 15% . Both are believed to be conservative estimates. (McFarlane et al 2000 including chart below)
At least at a glance it looks like several counties could exploit this aquifer for substantial water. However work on sustainable yield has not been done, and would need to be done at a local level to be accurate, due to the variability of the aquiferWater Quality:
Water quality work has also not been done or at least not been published for most of this aquifer. Domestic water supplies restricted to a single home are not regulated by the state. Most counties require water sampling when wells are set up, but the results are generally unavailable to the public at large. Water testing has been done for the Florence water supply, and it does meet EPA federal and KDHE state standards for drinking water. Water contamination issues mostly relate to nitrate runoff from agricultural fields. Prairie grasses tend to be self supporting and are not generally fertilized at all, so nitrate runoff is not the problem it would be in other areas. The US EPA lists three superfund sites in the counties associated with the Flint Hills aquifer. One is the Pester Refinery site in Butler County. Clean up is complete there and groundwater is considered safe. The second site is the Tri-County Airport in Herrington, Ks in Morris County. This site is an old US Army airfield from WW2. Work here is preliminary and the groundwater is considered unsafe and unmitigated. The two main contaminants are TCE and DCE. The third site is Fort Riley in Riley and Geary counties, a huge sprawling and active US Army base. Groundwater mitigation has taken place and the groundwater in the general area is considered safe. However the pollutants are a real witches brew and include TCE, DCE, Benzene, Arsenic, Antimony, Beryllium, and Vinyl Chloride .Conclusions:
The Flint Hills Aquifer is a minor but locally important aquifer in Kansas. It has been studied on a preliminary basis but much work still needs to be done to fully characterize this resource. In general it is free of pollution and underutilized, as the area continues to be very rural in nature.References:
Fetter,C.W. 2001 Applied Hydrogeology Prentice Hall Upper Saddle River,NJ 598p.