Digital map of the configuration of the base of the principal aquifer in the Elkhorn-Loup Model area, north-central Nebraska

The U.S. Geological Survey (USGS), in cooperation with the Lewis and Clark Natural Resources District 
(NRD), Lower Elkhorn NRD, Lower Loup NRD, Lower Platte North NRD, Lower Niobrara NRD, Middle 
Niobrara NRD, Upper Elkhorn NRD, and Upper Loup NRD, have agreed to cooperatively study water 
resources from prior to the beginning of irrigation development to 2005 in the Elkhorn-Loup Model 
(ELM) area using a ground-water-flow model.  The ELM area covers approximately 30,800 square 
miles, and extends from the Niobrara River in the north to the Platte River in the south.  The western 
boundary of the ELM area coincides with the western boundary of the Middle Niobrara, Twin Platte, 
and Upper Loup NRDs; the eastern boundary coincides with the approximate location of the 
westernmost extent of glacial till in eastern Nebraska. The initial ground-water-flow model was 
constructed with a single layer vertically to represent the aquifers of the Tertiary-age Ogallala 
Group and Quaternary-age alluvial deposits, with a uniform node spacing of 2 miles. The model will 
be calibrated to measured ground-water levels and estimated ground-water discharge to streams for 
the pre-ground-water development period (approximately 1940) and the simulation of the 1940-2005 
period will be calibrated to measure ground-water level changes. The study results will assist Nebraska 
Department of Natural Resources and the NRDs in the ELM area to develop long-term strategies for 
managing hydrologically connected waters.
			
This dataset is one of three geospatial datasets that together revise previously published maps of the 
configuration of the base of the principal aquifer and of the geologic units that form the base of the 
principal aquifer in the study area; the revisions to the base-of-aquifer altitude are based on currently 
available or reinterpreted geologic logs of test holes and selected registered wells. The principal aquifer 
is the High Plains aquifer except in the northeast part of the model area, where the principal aquifer is 
an unnamed alluvial aquifer.
			 
This dataset consists of contour lines of the base-of-aquifer altitude above the vertical datum (National 
Geodetic Vertical Datum of 1929). The purpose of this dataset is to serve as the lower aquifer boundary 
in the ground-water-flow model of the Elkhorn-Loup Model area, north-central Nebraska. This dataset is 
not intended to be used at scales larger than 1:350,000.
			 
The density of registered wells and test holes with data about the depth to the base of aquifer varies 
greatly across the map area. The accuracy of the base-of-aquifer contours in a given area is related 
directly to the density distribution and availability of registered well and test-hole data with information 
about the depth to the base of aquifer in that area.

Tags: aquifer-base, aquifer-characteristics, brule-formation, chadron-formation, elkhorn-loup-model-area, geohydrologic-units, geoscientificinformation, high-plains-aquifer, inlandwaters, lower-tertiary, niobrara-formation, ogallala-aquifer, pierre-shale, upper-cretaceous, usgs-6d4df13e-660f-4a54-bba6-19028ac9e3d7, white-river-group
Organization: Department of the Interior