Hydrogeology
The alluvium consists of sand, gravel, silt and clay. Thick deposits of silt and clay limit the rate and quantity of groundwater recharge from precipitation. Beneath the silt and clay is a thick deposit of gravel and sand. This makes up the primary water supply to irrigation wells in the Pawnee-Buckner subbasin. The alluvium of the Pawnee Valley is up to three miles in width in part of Pawnee County. It is only two miles in Hodgeman and Ness counties. The maximum thickness of the alluvium in Pawnee County is 138 feet. In Hodgeman and Ness counties, it reaches a maximum of 100 feet.
The terrace deposits consist primarily of clay and silt. They also contain some caliche deposits interbedded with the sand and gravel. The sand and gravel deposits are thick and adequate for higher capacity irrigation wells. Terrace deposits overlie the Cretaceous rocks on both sides of the Pawnee River throughout Pawnee County and extend into Ness and Hodgeman counties. Thick terrace deposits also occur along the north side of Sawlog and Buckner Creeks. These extend to a few miles upstream of Hanston. The average thickness of the terrace deposits is 60 feet.
The Ogallala-High Plains aquifer is present in the western portion of the subbasin. It can also be found along the alluvial interfaces of the Pawnee-Buckner alluvium. It consists of silt, sand and gravel-containing caliche. In general, it is highly dissected. The formation caps the plateau-like uplands. They make up about a third of the area of Ness and Hodgeman counties. It creates the divide between Buckner and Sawlog Creeks. The aquifer thickness ranges from 66 to 250 feet. Well production is generally limited to smaller irrigation use and stock watering.
In Hodgeman County, the hills surrounding the alluvial valleys include the Ogallala Formation. They retain any water that infiltrates beyond the root zone. As water moves through the formation, seeps and springs release water to Buckner Creek, Sawlog Creek and the Pawnee River. Therefore, recharge that is retained in the western part of the subbasin slowly releases after an initial recharge event to the alluvial aquifer system.
The Dakota Aquifer lies beneath the Ogallala Formation. It is in most of the western two-thirds of Kansas. The aquifer consists of discontinuous bodies of sandstones interspersed in relatively impervious shales. The Dakota Formation outcrops in isolated areas in Pawnee County. It also outcrops in large areas south of Buckner and Sawlog Creeks in Hodgeman County. It has a maximum thickness of 200 feet in Pawnee County and 300 feet in Hodgeman and Ness counties. The Dakota Aquifer is both a confined aquifer and an unconfined aquifer in the subbasin. The confined Dakota is located in the western portion. The unconfined is primarily isolated in the eastern parts of Hodgeman County. It also continues into Pawnee County. The Dakota and alluvial aquifers hydraulically connect in areas of unconfined Dakota along the Buckner Creek, Sawlog Creek and Pawnee River. Water flowing from the unconfined Dakota sustains the water levels in the confined Dakota. This same thing occurs between the confined Dakota and the Ogallala in the western part of the subbasin. The Dakota Aquifer discharges to the Buckner and Sawlog Creeks in Hodgeman County.
A number of wells located between Buckner Creek and Sawlog Creek pump water from the confined Dakota Formation. Along the alluvial valleys of the Buckner and Sawlog Creeks is a confining shale layer of the Dakota Aquifer that narrows between Range 24W and 23W. There are wells screened in both the alluvium and the Dakota Aquifer. It creates an artificial connection.
