York County, an area of 575 square miles, is situated on an upland plain in southeast Nebraska. Although tributaries of the Big Blue River have eroded valleys into this plain, much of the original surface is still intact and is characterized by broad shallow undrained depressions. The economy is based almost wholly on agriculture, and corn is the major crop. More than 111,000 acres of cropland was irrigated in 1964 with water pumped from 1,240 wells. The upland plain is underlain to depths of 100-450 feet by unconsolidated deposits of Quaternary age. The upper part of this depositional sequence consists largely of wind-deposited clayey silt, and the lower part consists of stream-deposited sand and gravel. In part of the county, the sequence includes some glacial till also. The unconsolidated Quaternary deposits mantle the eroded surface of marine strata of Cretaceous age.
The lower unconsolidated rocks of Quaternary age are saturated and constitute a highly productive aquifer throughout much of the county. Replenishment to this aquifer, derived principally from precipitation, is believed to average about 1.5 inches per year. As the quantity of ground water pumped per year greatly exceeds the average annual quantity of recharge, most of the. water used for irrigation is from storage. Consequently, water levels have been trending downward. A comparison of 1964 water levels in wells with water levels measured in 1953 shows that the water table declined more than 10 feet beneath 42 square miles.
The ground water is of the calcium bicarbonate type, and, though hard, is chemically suitable for irrigation use on most soils in the county.
