Alluvial Aquifers in Kansas

Megan M. Flory

Spring 2008
ES 775 Advanced Image Processing
Dr. James S. Aber, Instructor

Table of Contents
Introduction
Site Characterization
Data Analysis
Surficial Alluvial Features
Results/Conclusions
References

Abstract

The availability of water has always been a major concern for Kansas residents and groundwater has often provided a solution to this problem. Alluvial aquifers present throughout the state are known for their ability to yield large amounts of groundwater, even during drought years. The goals of the project were to use a combination of GIS techniques and multiple datasets to analyze the boundaries of the Neosho River alluvial aquifer in a section between the Council Grove Reservoir and the city of Emporia, Kansas and to examine surficial features within the alluvium.

Alluvial Aquifer Extent in Kansas
Data downloaded from DASC and modified by author using ArcMap

Return to top of page.


Introduction

River systems are in a state of constant change. The continuous flow of water and sediment through a river’s banks alternates between erosion and deposition. Rivers periodically overflow their banks, creating large, flat floodplains and often change their course during a flood, leaving behind oxbow lakes and meander scars. As the river channel swings back and forth across this floodplain, point bar deposits of sand and coarse gravel are left behind on the inside of meanders as the river erodes away the bank opposite these deposits. In general, coarse gravels are deposited in the stream channel, sand and fine gravel form natural levees along river banks, and silt and clay come to rest on the floodplain. (Fetter, 2001) The term used for these unconsolidated sediments, including clay, silt, sand and gravel, deposited by flowing rivers is called alluvium. (Fetter, 2001, Macfarlane, 2000, and Buchanan and Buddemeier, 1993)

If the river is aggrading, which means the level of the river is increasing due to deposition of sediment, the surrounding alluvial deposits will thicken over time. Likewise, if the general land level is subsiding, alluvial deposits will also tend to thicken with time. These conditions represent overall periods of deposition throughout a floodplain. During times of significant deposition, it is more likely that meander scars and oxbow lakes will become buried under different types of sediment. Periods of significant erosion also happen within a floodplain. During these times, downcutting through previously deposited alluvial sediments occurs. These erosional and depositional processes form terraces along the sides of a floodplain. (Fetter, 2001) These terraces are often not symmetrical to the floodplain and can be quite complex.

Example of alluvial terraces in a geologic cross section across the Neosho River Valley
Taken from O’Connor, 1953

The described processes create a complex system of subsurface zones with varied hydraulic conductivities. In general, zones with sand and gravels will have higher hydraulic conductivity values and will therefore be more effective for holding and transmitting water. These zones contain what is known as groundwater, which Buchanan and Buddemeier (1993) define as “any water that can be pumped from a well,” and if the zones are large enough, they are referred to as aquifers, although the exact definition for the term aquifer is somewhat ambiguous. Buchanan and Buddemeier, (1993) define an aquifer as “a rock formation, either consolidated or unconsolidated, that is sufficiently porous enough to store water and permeable enough to yield significant amounts of ground water.” Fetter, (2001) suggests that an aquifer is “rock or sediment in a formation, group of formations, or part of a formation that is saturated and sufficiently permeable to transmit economic quantities of water to wells and springs.” Yet, both of these definitions contain somewhat imprecise terminology such as, “sufficiently porous” and “economic quantities of water.” These vague expressions allow flexibility in political and social applications, but add to confusion when attempting to use the definition for analytical purposes. Macfarlane (2000) attempts to quantify the term significant with the following statement, “An aquifer is considered to be significant if it yields water to wells at rates equal to or greater than 50 gallons per minute (gpm) or if it supplies water for industry, agriculture, or human consumption.”

Consequently, the definition of an alluvial aquifer is a combination of the term alluvium, which is rather clearly defined, and the term aquifer which is somewhat less definite. Macfarlane (2000) defines an alluvial aquifer as, “the water-bearing alluvium adjacent to the stream and hydraulically connected aquifers in terrace deposits and associated alluvial Quaternary deposits in paleodrainages.” Regardless of the exact definition of the term aquifer, the groundwater that is stored and extracted from alluvial deposits is one of the most important sources of water supply in many rural agricultural areas (Park, et. al. 2007) and the small cities near these regions. Emporia, Kansas is an example of one of these small cities surrounded by significant agricultural practices. The primary water source for Emporia and the surrounding areas is the Neosho River and its alluvial aquifer system. To successfully explore and manage our groundwater resources, a detailed knowledge of the spatial distribution of aquifer characteristics such as lithology, thickness and hydraulic properties is essential. (Park, et. al. 2007)

Return to top of page.


Site Characterization

Alluvium Geology:

There are two terraces that exist above the stream valley alluvium in Lyon County. These terraces are both Pleistocene in age and have been named the Wiggam terrace and the Emporia terrace. (O’Connor, 1953) The younger of the two is called the Wiggam terrace and has an elevation of about 5 to 20 feet higher than the corresponding parts of the alluvium valley. Although the age of these terrace deposits is probably Illinonian, they may be younger. The city of Emporia is built largely on the older terrace, which is located above the Wiggam terrace. This terrace is called the Emporia terrace and is Kansas in age. Both of these terraces are important sources of ground water. (O’Connor, 1953) The most effective way to show the relationships among these terraces, the valley alluvium and the rivers, is through geologic cross sections. Some of the cross-sections in O’Connor’s report (1953) are presented here.

Location map for geologic cross sections presented in the O'Connor (1953) report.
Geologic cross section across the Neosho River Valley upstream from Emporia. Taken from O'Connor, 1953.
Geologic cross section across the city of Emporia. Taken from O’Connor, 1953.
Return to top of page.


Data Analysis

To begin the project, the aquifer extent boundary was downloaded as a shapefile from the DASC website along with a generalized surface geology shapefile. These two shapefiles are compared for the study region in the following image.

Extent of alluvial aquifer and geologic units coinciding with aquifer boundary.
Data downloaded from DASC and modified by author using ArcMap.

From this image, it is evident that the alluvial aquifer extent boundaries were defined using only the surface geology. In other words, any geologic unit defined as alluvium, is also considered to be an alluvial aquifer. While geology is the primary factoring affecting the occurrence of groundwater (O’Connor, 1953), images like this give the false impression that below the surface there is a homogenous and continuous aquifer. As previously stated in the discussion of how alluvial aquifers form, this is definitely not the case. Additionally, the boundaries for the geology map are based largely on the Geologic Map of Kansas of 1964.

The alluvial deposits in this shapefile are separated into Tertiary, Kansan and post-Kansan deposits. The Emporia terrace is clearly defined in this image and is shown in gold. Most of the alluvial deposits are post-Kansan, which is expected, but this classification system does not allow a differentiation between the Wiggam terrace and the valley alluvium.

Return to top of page.


Surficial Alluvial Features

The term “alluvial” applies not only to subsurface geologic units, but also to surficial features. One of the most important aspects of the land surface is the soil. Soils present in alluvium tend to be unique in composition and are frequently excellent for agricultural purposes. Because of the shallow alluvial aquifers existing in these regions and the extensive agricultural practices, the way water moves through the soil is of particularly significance. If water moves quickly through the soil, it can carry contaminants with it to the shallow aquifer. Silts and clays tend to cover a river’s floodplain, as previously mentioned and often, these silts and clays can form confining units. Fetter (2001) describes a confining unit as a geologic unit having an intrinsic permeability of less than 10-2 darcy. These confining units are extremely important when considering aquifer recharge and contamination issues within the aquifer.

The following image contains Soil Survey Geographic (SSURGO) data, which was downloaded from DASC, clipped to the study area. Fetter’s definition of confining unit was employed along with his conversion table which suggests that 10-2 darcy units equal a hydraulic conductivity of 10-5 cm/sec. The SSURGO data includes a minimum permeability value for each soil type, given in inches/hour. These minimum permeability values, along with Fetter’s definition were used to classify confining units that overlie the aquifer. The results are shown in the following image.

Image showing the confining soil layers above the alluvial aquifer.
Data downloaded from DASC and modified by author using ArcMap.

This image indicates that much of the alluvial aquifer is overlain by soils that allow little to no water movement through them. However, Fetter (2001) mentions along with his definition of a confining layer, that the term is somewhat arbitrary and can vary according to site specific conditions.

In addition to analyzing the soils, imagery analysis can prove insightful for studying surficial alluvial features. A Landsat image from April was obtained for the study area. Bands 1,2,3,4,5, and 7 were all available so composites could be made to highlight features of interest.

A Landsat false-color composite highlighting the alluvial features.
Data modified by author using Idrisi.

This image shows the agricultural fields in golds and blues and the grasslands in light green. Water bodies appear black in this image, but there are also black sections in the grassland areas. These sections have been recently burned, which is a customary practice for these regions during this time of year. Emporia, which is at the bottom center of the image, appears in pinks and light blues. This image clearly shows evidence of the extensive agricultural practices occurring in the alluvial river valley.

Return to top of page.


Results/Conclusions

Rivers and their alluvial aquifer systems are dynamic and constantly changing. The alluvial aquifer systems provide a valuable source of groundwater to surrounding communities, while the surface alluvial features provide excellent soils for agricultural purposes. Because of the importance of these regions, monitoring them is essential, but as was demonstrated by this project, the data must be analyzed critically and carefully. A combination of GIS techniques along with the use of multiple government datasets can facilitate a more complete understanding of a study region.

Return to top of page.


References

Buchanan, R. and R.W. Buddemeier. 1993. Kansas Ground Water: An Introduction to the State's Water Quantity, Quality, and Management Issues. Kansas Geological Survey, Educational Series 10. Available at: www.kgs.ku.edu/Publications/Bulletins/ED10/index.html. Date Accessed: 5 May 2008.

Fetter, C.W., 2001. Applied Hydrogeology. 4th Ed. New Jersey: Prentice Hall Inc, 598p.

Macfarlane, P. A. 2000. Revisions to the Nomenclature for Kansas Aquifers. Kansas Geological Survey, Bulletin 244, part 2. Available at: www.kgs.ku.edu/Current/2000/macfarlane/macfarlane1.html. Date Accessed: 5 May 2008.

O'Connor, H.G. 1953. Geology, Mineral Resources, and Ground-water Resources of Lyon County, Kansas. Kansas Geological Survey Volume 12. Available at: www.kgs.ku.edu/General/Geology/Lyon/pt1_strat.html. Date Accessed: 5 May 2008.

Park, Y.H., S.J. Doh, and S.T. Yun. 2007. Geoelectric Resistivity Sounding of Riverside Alluvial Aquifer in an Agricultural Area at Buyeo, Geum River Watershed Korea: An Application to Groundwater Contamination Study. Environmental Geology. 58:849-859.

Return to top of page.