Hydrogeology
The Middle Arkansas River and three distinct aquifers make up the Middle Arkansas River subbasin. The aquifers include the Middle Arkansas alluvial aquifer, the Ogallala-High Plains aquifer and the Dakota aquifer.
The permeable alluvium is well defined along most of the river. Its thickness varies, but its composition of sand and gravel is relatively the same. The alluvium ranges in thickness up to 50 feet. Some of the coarse alluvial deposits are capped by finer sediments. There is little high-permeability material below the Arkansas River alluvium except in Barton and eastern Pawnee County.
The alluvium is generally relatively thick -- a few tens of feet in most of Edwards County. Typically, a substantial thickness of mostly fine-grained deposits underlies the alluvium. There is some permeable material below the fine-grained interval. It is unclear whether this material is laterally continuous. It does not appear to exceed a couple tens of feet in thickness. The alluvium thins near the Edwards-Pawnee County line, as does the entire sequence of unconsolidated deposits. The alluvium is underlain by an interval of almost entirely fine-grained deposits. There does not appear to be a water-producing horizon below the fine-grained sediment.
The total sediment thickness increases in the central part of Pawnee County where the river crosses over a paleochannel near the confluence of the Pawnee and Arkansas rivers. Here the sediments beneath the Arkansas River can reach nearly 160 feet. The alluvium appears to thicken in this vicinity. A deeper permeable aquifer zone is present at the base of the paleochannel. A thick layer of fine-grained material separates the Arkansas River alluvium from the deeper permeable zone. East of the paleochannel, throughout eastern Pawnee and Barton counties, the thickness of the unconsolidated sediments below the river remains fairly uniform, up to 140 feet. The amount and percentage of coarse-grained materials increases in Barton County. The matrix is composed of mostly sand and gravel with interspersed clay layers and lenses. The shallow alluvium is difficult to differentiate from deeper deposits in parts of Barton County. The aquifer thins in extreme east Barton County nearing an outcrop of the Kiowa Formation.
The High Plains aquifer has sufficient saturated thickness to yield water over most of the subbasin. Low permeability sediments do not consistently underlie the alluvial deposits in the subbasin. These would separate alluvial deposits from the main part of the High Plains aquifer. Therefore, it is difficult to characterize the transition from the alluvial aquifer to the main High Plains aquifer.
The Dakota aquifer underlies the High-Plains aquifer throughout most of the Middle Arkansas subbasin. It is mostly an unconfined aquifer in the basin. However, for areas in northeast Pawnee County and areas of southeast Rush County into Barton County, it is a confined aquifer.
Source: Numerical Model of the Middle Arkansas River Subbasin
Kansas Geological Survey, June 2006