HYDROGEOLOGIC MAP – WIND RIVER BASIN AND BIGHORN BASIN, NORTHWEST WYOMING: A NEW APPROACH TO RIVER BASIN GROUNDWATER PLANNING

Geologic maps depict the outcrop of stratigraphic units, and the common use of geologic maps in groundwater assessment and planning implies that hydrostratigraphy is tied to stratigraphy. However, a hydrogeologic unit – aquifer or confining units – may coincide with a stratigraphic unit, part of a stratigraphic unit, a group of stratigraphic units, or parts of stratigraphic units. Therefore, a hydrogeologic map, that depicts outcrop of hydrogeologic units, will allow greater accuracy in groundwater planning and aquifer protection endeavors.

Our hydrogeologic map depicts the hydrostratigraphy of Wyoming’s Wind River and Bighorn Basins. The map is based on regional hydrogeologic investigations conducted from 1963 to 2007. The accompanying chart combines hydrostratigraphy of the basin with the geologic unit compilation of Love and Christiansen (1985) and Love, Christiansen, and Ver Ploeg (1993). We assign an aquifer code to each hydrogeologic unit. On the map, the hydrogeologic units are displayed geospatially highlighting outcrop/recharge areas at a scale 1:380,000. Forty-two geologic units are grouped into twenty-five hydrostratigraphic units. The chart shows the three aquifer systems that contain most of the hydrogeologic units. This classification defines roughly ninety percent of the surface outcrop area in the two basins.

Mapping the outcrop of the various aquifers and confining units to define the three-dimensional hydrogeologic framework of the basins will help in calculating basin-wide recharge and will help in proactive planning for potential aquifer contamination. Recently this approach has been adopted for use in the Wyoming Water Development Commission’s “Available Groundwater Determination Technical Memorandum: Wind/Bighorn River Basin Water Plan”.