Hydrogeology

The Ogallala-High Plains aquifer underlies portions of eight states. The portions of the aquifer that lie in western Kansas are an important source of water primarily used for irrigated agriculture. This section will discuss the hydrogeology of the High-Plains aquifer in Kansas as divided into three regions. The three regions are northwest, central and southwest.

Northwest Region

The northwest region of the Ogallala-High Plains aquifer consists mainly of silt, sand, gravel and beds formed by the cementation of sediments with calcium carbonate. Lens-shaped beds of sorted sand, gravel, bentonite and volcanic ash also occur. A few feet of dense, sandy, algal limestone top the Ogallala in this region. Streams that flowed eastward deposited the sediments.

In general, the saturated thickness of the northwest region declines as you head from the west to the east, as illustrated on the map below. This is due to the Cretaceous bedrock underlying the aquifer that generally dips to the west. The thickness is less than 50 feet in northern Phillips County. It is greater than 160 feet in parts of Sherman County. The overall trend is that saturated thickness in the Ogallala-High Plains aquifer increases from north to south as shown on the map below.

Central Region

This region of the Ogallala-High Plains aquifer is composed of thick layers of unconsolidated sediments. They include course-grained sand, gravel and lesser amounts of clay. Groundwater movement may be retarded vertically and laterally due to the alternating layers of coarse-grained sediments.

Saturated thickness is 20 feet or less in portions of Lane and Greeley counties. On the other hand, it is over 200 feet in central portions of Kearney and Finney counties. In general, the central region has less overall saturated thickness when compared to the northwest and southwest regions.

Groundwater flow direction in the Ogallala-High Plains aquifer is generally from northwest to southeast. Local deviations in flow directions are common where substantial decline or cones of depression are present. These are due to groundwater pumping effects.

Southwest Region

In the southwest region, the Ogallala-High Plains aquifer mostly overlies Lower Cretaceous bedrock in the northern portion. It overlies the Permian bedrock in the southern portion. The aquifer is composed of undifferentiated Quaternary and Tertiary sediments. They consist of clay, silt, sand and gravel. Streams that flowed eastward from the Rocky Mountains deposited the sediments. Water movement within the aquifer also generally follows an eastward direction. The movement is about one foot/day.

Type of material, thickness and layer continuity varies widely within the High Plains aquifer. In general, the individual beds are not continuous. Even within short distances, they may grade laterally or vertically into material of different composition. In general, the aquifer is unconfined. Some local areas may be semi-confined and confined.

Unconsolidated sediment thickness varies greatly mainly due to the uneven bedrock surface below. The greatest thicknesses are between the Bear Creek and the Crooked Creek-Fowler Faults in the lower half of Finney County. Thickness may vary as much as 250 feet across the fault zones because of subsidence. Stevens and Seward counties also have greater thicknesses. Saturated thickness in the southwest region is variable and can range up to 500 feet.