Wyoming Framework Water Plan - Living Rivers Home Page

4.0 RESOURCES
In summary, optimum conditions for the development of groundwater reflect the conjunction of
favorable formations, favorable permeability conditions within those formations – primarily coarsegrained
zones and/or fractures, and recharge conditions that provide suitable groundwater quality.
4.4.3 Historical Aquifer Performance
Economically developable groundwater has been found across much of the Wyoming landscape.
Figure 4-10 displays all water wells for which the permitted yield is greater than zero, excluding coalbed
methane (CBM) wells (discussed below). Thinly populated areas of the map reflect either a lack of
demand such as in the Red Desert or a lack of even small quantities of water from some of the “major
aquitards” such as northwest of Casper. In some areas of Wyoming, useful groundwater is simply not
present.
Highlighted on Figure 4-10 are all wells with permit yields of 500 gpm or greater. The
distribution is a function of aquifer performance. Most conspicuous is the coincidence with the alluvial
aquifer, reflecting both the commonly favorable conditions of permeability, depth, and quality, and the
concentration of agricultural and municipal demands in Wyoming’s stream valleys.
4.4.4 Groundwater Quality
Included on Figure 4-9 are general ranges of total dissolved solids (TDS) reported for the various
formations. As with yield, the broad ranges of aquifer TDS result from widely varying conditions across
basins. TDS is a generalized parameter of water quality, reflecting the total presence of dissolved
minerals. For example, a TDS concentration of 500 mg/l is considered the limit of aesthetically
acceptable human drinking water; 2,000 mg/l is a common maximum for irrigation water; and 5,000 mg/l
is considered the limit for livestock use. Maximum groundwater TDS values of 200,000 mg/l have been
reported from deep, oil field water wells.
The alluvial aquifers primarily receive recharge from an overlying stream (or irrigation
applications) and/or the surrounding geologic materials. Where the former dominates, groundwater
quality is generally good. The aquifer sands and gravels tend to filter sediment and bacteria from the
surface source to produce water that is clean and of low salinity. Where there is substantial inflow to the
alluvial aquifer from bedrock, alluvial groundwater quality will reflect that of the surrounding formations.
This water will commonly be higher in salinity than the surface water and may render the alluvial aquifer
of limited value for many applications.
Bedrock aquifers receive recharge through the infiltration of rainfall, snowmelt, and streamflow
although discharge from groundwater to streams is more common than the other way around.
Groundwater developed close to the areas of recharge may be of relatively high quality, regardless of the
host formation. As water moves deeper, it generally becomes more mineralized. Near outcrops,
groundwater is most commonly of a calcium bicarbonate type. Deeper, toward the basin centers, sodium
commonly increases relative to calcium, and sulphate and chloride dominate over bicarbonate.
In general, groundwater quality tends to be better in the more productive aquifers because of the
more active groundwater circulation and less soluble minerals. While even the “major” aquifers of Figure
4-9 may yield poor quality (particularly in deeper, central basin locations), they are more likely to provide
acceptable quality than the “marginal” aquifers or aquitards. The latter group has notoriously poor
quality, due to the coincident deposition of fine-grained materials (shale) and soluble minerals. An
exception is the crystalline rocks (last entry of Figure 4-9) in which quality is generally good due to the
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