Chugwater–Dinwoody aquifer and confining unit
The Chugwater–Dinwoody aquifer and confining unit is composed of the Lower and Upper Triassic-age Chugwater Group or Formation and the Lower Triassic-age Dinwoody Formation. The unit is part of the lower and middle Mesozoic aquifers and confining units hydrogeologic sequence. The Chugwater and Dinwoody Formations are overlain by the Sundance and Nugget Formations and underlain by the Goose Egg and Phosphoria Formations (Plate II). The Chugwater Group contains the Chugwater aquifer and is composed of four lithostratigraphic units – the uppermost unit, the Popo Agie Formation, composed of reddish siltstone, shale, and silty sandstone, ranging in thickness from 0 to 300 ft; the next lower unit, the Crow Mountain Sandstone, composed of red to orange, fine- to coarse-grained sandstone and siltstone, ranging in thickness from 0 to 130 ft; the next lower unit, the Alcova Limestone, composed of hard, finely crystalline limestone, ranging in thickness from 0 to 30 ft; and the lowermost unit, the Red Peak Formation, composed of interbedded reddish siltstone, shale, mudstone, and fine-grained silty sandstone, ranging in thickness from 900 to 950 ft (McGreevy et al., 1969; Richter, 1981, table IV-1, and references therein). The Dinwoody Formation comprises the Dinwoody confining unit and is composed of interbedded sandy dolomitic sandstone, calcareous sandstone, and thin dolomite and limestone (Richter, 1981, table IV-1, and references therein). The Popo Agie Formation, Alcova Limestone, and Dinwoody Formation are confining units; the Crow Mountain and Red Peak Formations are aquifers or subaquifers (Plate II; Richter, 1981, table IV-1). Consequently, the Chugwater–Dinwoody aquifer and confining unit may be considered a sequence of rocks that functions as both confining or leaky confining unit and aquifer (Plate II). The Crow Mountain and Red Peak aquifers/ subaquifers may yield small quantities of water to wells and springs at some locations in the WRB. Little hydrogeologic information is available describing the Chugwater–Dinwoody aquifer and confining unit, but well-yield and spring-discharge measurements and other hydraulic properties for both formations composing the hydrogeologic unit are summarized on Plate IX.
Groundwater-quality data are presented and described for the Chugwater aquifer and Dinwoody confining unit that compose the Chugwater–Dinwoody aquifer and confining unit. Groundwater-quality samples from the Chugwater aquifer generally were available or assigned only for the Chugwater Group, and not for the four stratigraphic members composing the group in the WRB.
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