Wyoming’s groundwater resources occur in both unconsolidated deposits and bedrock formations. The hydrogeologic units in the WBRB are variably permeable (aquifers) or impermeable (confining units) to the flow of groundwater. The heterogeneity at all scales in geologic or hydrogeologic units confounds their classification as aquifers or confining units. Figure 6-1 shows the great lithologic and grain-size heterogeneity measured in several typical hydrogeologic units in the Elk Basin (Bighorn Basin), and indicates how it is that virtually any geologic unit in the basin may yield at least small quantities of groundwater to wells. Even wells completed within a confining unit may produce water if the unit is saturated. For example, although the Cody Shale and Thermopolis Shale are classified as confining units, several domestic supply and stock wells have been completed in these units, according to records of the Wyoming State Engineer’s Office.
Detailed composite measured section of the lower Fort Union, Lance, Meeteetse,
and Mesaverde Formations and upper Cody Shale, Elk Basin (Bighorn Basin),
Wyoming, showing lithology and grain-size distribution.
This study covers the WBRB specifically; the conceptual models presented for WBRB recharge, discharge, storage, and flow may be applied generally to the major drainage basins in Wyoming. However, data on aquifer recharge, groundwater flow, aquifer discharge, subsurface inter-aquifer mixing, and total groundwater quantities available in the WBRB (and the other Wyoming basins) are sparse relative to the large area and stratigraphic and structural complexity of the basins. This sparsity of data precludes an accurate site-specific groundwater model for the WBRB. Computer or conceptual modeling of the stratigraphy, structure, groundwater flow, and water quality, quantity, and availability should allow for periodic updating and increasing reliability as data continue to be collected.
For the upper 1,000 feet of the interior basin areas of the Wind River and Bighorn River basins, the total amount of stored groundwater and the amount of available (producible) groundwater are estimated in this chapter. The stored and available groundwater in Quaternary unconsolidated deposits in the WBRB are also estimated. And, recharge to hydrogeologic units and groupings of hydrogeologic units is estimated using annual recharge mapped over the outcrop areas that together cover the basin.