WBRB groundwater volume

The groundwater volume estimates for the upper 1000 feet of the WBRB Q/T Aquifer were calculated for the entire stratigraphic volume rather than for individual hydrogeologic units, as adequate data for the more detailed estimates are not available. To account for uncertainty in the grain-size distribution (and associated average porosity), the volume estimates were calculated over a representative range of mixed sand (and coarser) and fine-grained porosities.

Groundwater quality generally deteriorates with depth, due primarily to longer residence time that increases the dissolution of minerals (primarily salts), especially in fine-grained, highporosity, high-surface-area lithologies that characterize much of the Q/T Aquifer strata in the WBRB. Water quality information for the Q/T Aquifer compiled for this study (Chapter 7) generally confirms this observation; however, there are exceptions. In some areas, shallow water has naturally high total dissolved solids (TDS) or is impacted by irrigation and other land uses; and deeper groundwater may exhibit better quality than shallow water in the same area if there is ample recharge with fresh water and vigorous flow some distance into the basin.

Figure 6-3
Estimated net annual aquifer recharge –
interior basin surface Q/T Aquifer
Figure 6-4
Estimated net annual aquifer recharge –
surface Mz/Pz Aquifer
Figure 6-5
Estimated net annual aquifer recharge –
surface Absaroka Volcanics
Figure 6-6
Estimated net annual aquifer recharge –
surface Symbol Aquifer

For this study a bottom depth of 1,000 feet was used for estimating the available volume of groundwater in the Q/T Aquifer, due to both the general decline in water quality with depth and the greater cost of accessing groundwater at greater depth. The cost of water from wells increases substantially with depth, because of the cost of drilling and constructing a well and the cost of lifting water from greater depth if the confining pressure is not adequate to bring the water close to the surface; however, the cost of extracting water from greater depth may be more than offset by adequate production.

The ranges of stored and available groundwater volumes in the upper 1000 feet of the WBRB Q/T Aquifer was estimated on the basis of the following parameters:

  • The surface areas of flat-lying, low-elevation Q/T Aquifers (Plate IV; Figure 6-3), totaling 5,975 square miles:
    • the Wind River Basin Q/T Aquifer, 2,885 square miles (80.435 × 109 ft2)
    • the Bighorn Basin Q/T Aquifer, 3,090 square miles
  • An assumed constant total saturated thickness of the Q/T Aquifer of 950 feet, extending from 50 feet below land surface to a depth of 1000 feet:
    • An assumed constant unconfined saturated thickness of 50 feet, extending from 50 feet below land surface to a depth of 100 feet
    • An assumed constant confined saturated thickness of 900 feet, extending from 100 feet below land surface to a depth of 1000 feet
  • For total groundwater volume estimates, assumptions on porosity based on published data and specific information compiled for this study for the lithologic characteristics of the geologic units that compose the Q/T Aquifer (Chapter 7, Plate IX):
    • 30 percent porosity for sand and coarser-grained lithologies (sand)
    • 35 percent porosity for fine-grained (silt, clay, shale) lithologies (fine)
  • For available groundwater volume estimates, assumptions on specific yield and storage coefficients based on published data and information compiled for this study for the lithologic characteristics of the geologic units that compose the Q/T Aquifer (Chapter 7, Plate IX): for unconfined groundwater,
    • a specific yield of 26 percent for sand and coarser-grained lithologies (sand)
    • a specific yield of 10 percent for fine-grained lithologies (fine) and for confined groundwater, a storage coefficient of 1 × 10-4 for sand and coarser-grained lithologies (sand)
    • a storage coefficient of 1 × 10-5 for fine-grained lithologies (fine)

Similarly, the range of stored and available groundwater volumes in the Quaternary Aquifer was estimated using the following parameters:

  • The surface areas of the Quaternary Aquifer (from Plate IV; Figure 6-2), totaling 3,198 square miles:
    • the Wind River Basin Quaternary Aquifer, 1,527 square miles
    • the Bighorn Basin Quaternary Aquifer, 1,671 square miles
  • Two assumed constant total saturated thicknesses of the Quaternary Aquifer, 10 and 20 feet
  • For the total stored groundwater volume estimates, assumptions on porosity based on published data and specific information compiled for this study for the lithologic characteristics of the geologic units that compose the Quaternary Aquifer (Chapter 7, Plate IX):
    • 30 percent for sand and coarser-grained lithologies (sand)
    • 35 percent for fine-grained (silt, clay, shale) lithologies (fine)
  • For available groundwater volume estimates, assumptions on specific yield based on published data and information compiled for this study for the lithologic characteristics of the geologic units that compose the Quaternary Aquifer (Chapter 7, Plate IX):
    • a specific yield of 26 percent for sand and coarser-grained lithologies (sand)
    • a specific yield of 10 percent for fine-grained lithologies (fine)

Additional information:

More info Stored and available groundwater estimates


Reference View complete Wind/Bighorn Basin Water Plan