USE OF GEOLOGIC MAPS AND THEIR DERIVATIVES IN CONSTRUCTING A 3D HYDROGEOLOGIC FRAMEWORK MODEL OF THE UPPER COLORADO RIVER BASIN OF COLORADO, WYOMING, UTAH, ARIZONA, AND NEW MEXICO

Numerical models of surface water and groundwater of the Upper Colorado River Basin require the development of a digital 3D hydrogeologic framework model (3D HFM) in order to simulate the physical system; the 3D HFM is constructed through sampling of surface and subsurface geologic maps and their derivatives. Geologic units within the 367,500 km² study area are aggregated into a soil layer and seven underlying hydrogeologic units (HGUs) that include Quaternary alluvium, subdivisions of Phanerozoic consolidated sedimentary and volcanic rocks, and crystalline Precambrian basement and younger intrusive rocks.

A digital elevation model (DEM) was defined at 1-km² resolution and edited to preserve stream elevation gradient and flow direction. A regional soil map is draped on the DEM; soil parameters that control water infiltration and storage were developed from SSURGO soil type and thickness data. Digital state-scale geologic maps were aggregated into HGUs based on age and regional stratigraphic correlation. Faults were compiled from the geologic maps for use during gridding of HGU surfaces. Geologic information for the HGUs is mapped to the centroids of 1-km² cells which are common to the 3D HFM and to the numerical model grid. Data on the subsurface elevation of each HGU was derived from published structure contour and isopach maps and from elevation of HGU tops from oil and gas wells.

All geologic map data were sampled as point data and combined with well data to create a dataset for each HGU with X, Y, and Z coordinates. These data were interpolated using faults as barriers during gridding; grids were evaluated by manual inspection and through statistical techniques to identify outliers in the input data that were reinterpreted or removed from the data set. Final grids are stacked in 3D space and evaluated to ensure that stratigraphic order and thickness are preserved, and unit extent is consistent with the geologic map relations. The resultant stack of surfaces defines the extent, elevation, and thickness of the seven HGUs throughout the 3D volume of the model. Discretized XYZ values for HGU tops are extracted using scripts for import into numerical modeling routines. Additional geologic detail may be added as necessary through map-based zones that define facies and lithology-based within-HGU heterogeneity.