Chapter 6.9, Page 2 of 4

Hydrogeologic Units

The High Plains aquifer is composed principally of the unconsolidated to semi-consolidated sands, gravels, clays, and silts of the Miocene-aged Ogallala Formation (Table 6.9-1). Quaternary-age alluvial, valley-fill, dune sand, and loess deposits are also considered a part of the High Plains aquifer where they are hydraulically connected to the underlying Ogallala Formation.

Table 6.9-1 Hydrogeologic Units of the High Plains Aquifer.

Modified from Gutentag and others, 1984

The High Plains aquifer in Colorado ranges from less than 50 feet thick along the western, eroded outcrop edge to more than 500 feet thick in the paleo-river valleys of Washington County. Thicknesses average about 250 to 350 feet in the northern counties and 50 to 150 feet in the counties south of the Arkansas River. Erosion and downcutting have removed the High Plains aquifer along the main channels of the Arkansas and South Platte rivers, effectively separating the northern and southern portions of the aquifer in Colorado.

Currently, the saturated thickness ranges from zero to greater than 250 feet in the northern portion of the aquifer. The average saturated thickness ranges from 25 to 200 feet in the north and 25 to 75 feet in the south.

Chapter 6.9, Page 2 of 4