Hydrogeology
The Arkansas River and three distinct aquifers -- the Arkansas River alluvial aquifer, the Ogallala-High Plains aquifer and the Dakota aquifer -- make up the Upper Arkansas River subbasin. The shallow Arkansas alluvial aquifer is located along the Arkansas River. It overlies the deeper Ogallala-High Plains aquifer, which is located from western Kearny County into eastern Ford County. The Dakota aquifer is located below the Ogallala-High Plains aquifer throughout the subbasin.
Upper Arkansas River Alluvial Aquifer
The Arkansas River corridor consists of coarse-grained highly porous sediments which make up the alluvial aquifer. The alluvium is composed of sand, gravel and lesser amounts of silt and clay. In general the impermeable clay units along the base of the alluvium tend to get thicker towards the eastern counties of Ford and Gray. In addition, clay units are located north and south of the alluvium along the outer fringes of alluvial deposits, but they are more prominent north of the Arkansas River.
The Arkansas River alluvium is recharged primarily by infiltration of streamflow from the Arkansas River and secondarily by recharge from precipitation. Leakage from irrigation canals within the corridor does provide some additional recharge. Discharge from the alluvium to the Arkansas River is practically non-existent except directly after high flow events when subsequent discharge of alluvial bank storage occurs.
West of the Bear Creek fault, alluvial sediments overlie Cretaceous bedrock. The impermeable nature of this bedrock allows for minimal to no infiltration of alluvial groundwater.
The thickness of the alluvium ranges from about ten feet southwest of Hartland to over 80 feet in parts of Finney and Hamilton Counties. Thickness of alluvial sediments in Hamilton County is between 30 and 80 feet. In Kearny County, alluvial thickness is between ten and 60 feet. Finney and Gray County alluvial thickness is between 20 and 80 feet while Ford County alluvial thickness is between ten and 60 feet.
Bear Creek Fault
The western edge of subsidence that created the Hugoton Embayment is known as the Bear Creek Fault. The fault is located south of the Arkansas River in Hamilton County and travels north, northeastward towards the former town of Hartland in Kearny County. Near Hartland, the fault turns north, intersects the Arkansas River, and generally follows the length of the Amazon/Great Eastern irrigation canal to the north/northeast. West of the fault, the Arkansas River alluvium is underlain by Cretaceous bedrock. East of the fault, the alluvium is underlain by undifferentiated deposits that are generally referred to as the Ogallala-High Plains aquifer.
Ogallala-High Plains Aquifer
The second main aquifer within the Upper Arkansas subbasin is the Ogallala-High Plains aquifer. It is composed of thick layered unconsolidated sediments, which underlie most of Kearny, Finney, Gray and Ford Counties. These sediments are heterogeneous and are composed of intervals of course-grained sands and gravels. The Ogallala and overlying Arkansas alluvial aquifers are hydraulically connected throughout most of Kearny, Finney and the western portion of Gray County. The absence or discontinuity of impermeable clay units at the base of the alluvial aquifer within these counties and the similar lithology of both aquifers facilitates the connectivity.
The Ogallala-High Plains aquifer consists of alternating layers of fine and coarse grained sediments; therefore, groundwater movement may be retarded vertically and laterally. Extensive fine grained sediments are common north of the Arkansas River from Kearny County to the east. Here, these fine grained impermeable sediments slow the rate of downward migration of water. In contrast, south of the Arkansas River, particularly in Kearny and Finney Counties, fine grained impermeable sediments are common but do not retard the downward movement of water to the extent that it occurs north of the Arkansas River.
Thickness of the Ogallala-High Plains aquifer sediments increases from north to south, with the greatest thickness of more than 500 feet located in southeast Kearny and southwest Finney Counties. West of the Bear Creek Fault, the High Plains aquifer is absent. Generally, the thickness of the High Plains aquifer is relatively thin north of the river compared to south of the river.
The Ogallala-High Plains aquifer is primarily recharged from precipitation. Where the Ogallala underlies and is hydraulically connected to the alluvial aquifer, downward migration of alluvial groundwater provides additional recharge. In areas of Kearny and Finney Counties where irrigation canals are present, canal leakage can provide a substantial amount of additional recharge. In addition, up to 15 percent of applied irrigation water on cultivated fields can infiltrate below the soil zone and act as an additional source of recharge to the Ogallala aquifer.
Groundwater flow direction in the Ogallala-High Plains aquifer is generally from northwest to southeast. Local deviations in flow direction are common where substantial declines or cones of depression are present as a result of the additive effects of groundwater pumping in the area. Generally, groundwater trends in the Ogallala-High Plains aquifer within the Upper Arkansas subbasin have been on a steady decline since the 1960s when groundwater withdrawal has exceeded annual recharge.
Dakota Aquifer
The Dakota aquifer lies beneath the High Plains aquifer throughout the subbasin. The bedrock is composed of Cretaceous rocks with alternating layers of sandstones and shales. The Dakota aquifer is the principal aquifer in use only in areas where the High Plains aquifer is thin or absent including northeastern Finney, Hodgeman, easternmost Ford, and parts of Hamilton, Greeley and Wichita Counties. The Upper Cretaceous Graneros Shale acts as an aquitard and confines the Dakota bedrock aquifer from the overlying High Plains aquifer in the northern and east-central part of the subbasin.