The Tensleep aquifer is a major aquifer in the BHB. The aquifer is used primarily as a source of water for domestic, stock, and (rarely) irrigation purposes along the eastern margin of the BHB, where the hydrogeologic units composing the aquifer are exposed at land surface (crop out) or are at shallow depths (Libra et al., 1981; Cooley, 1986a; Doremus, 1986; Plafcan et al., 1993). Large volumes of water also are withdrawn from the numerous oilfields developed, mostly on anticlines, throughout the basin (Western Water Consultants, Inc., 1982a,b; Cooley, 1986a; Doremus, 1986).
Most wells flow at land surface under artesian pressure, except near some outcrops associated with anticlines, where unconfined conditions exist (Horn, 1963; Cooley, 1984, 1986a; Hinckley et al., 1982a). Flowing wells located along the BHB margin may yield large and dependable supplies of potable water, especially those located near anticlines, but reported well and spring yields vary considerably (Plate IX). Water in the aquifer generally is under artesian pressure, with wellhead pressures generally less than 50 pounds per square inch (psi) in the Ten Sleep area (Cooley, 1986a). Jarvis (1986, Figure 9, and references therein) reported the same wellhead pressures as Cooley did in outcrop areas and wellhead pressures ranging from 200 to 5,000 psi in the basin interior.
The Tensleep aquifer comprises the Pennsylvanian-age Tensleep Sandstone and Ranchester Limestone Member of the Amsden Formation (Cooley, 1984, 1986a; Jarvis, 1986; Spencer, 1986a) (Plate III). Some investigators also include the overlying Permian and Lower Triassic-age Phosphoria and (or) Goose Egg Formations and equivalents (Park City Formation) as part of the aquifer, but note that hydraulic connection between the hydrogeologic units is dependent on the absence of local confining units (low permeability lithologies) and on the extent of fracture-enhanced permeability associated with folds and faults (Western Water Consultants, Inc., 1982a,b; Doremus, 1986; Jarvis, 1986; Spencer, 1986a) (Plate III). Thus, the upper boundary (and associated upper confining unit) of the aquifer is subject to different interpretations among the various investigators. The Tensleep aquifer is defined herein as being confined above by overlying low-permeability shales and siltstones of the overlying Goose Egg-Phosphoria aquifer and confining unit hydrogeologic unit (Plate III); however, the overlying Phosphoria and (or) Goose Egg Formations and equivalents (Park City Formation) may be in hydraulic connection with (and considered part of ) the aquifer at some locations where folding and faulting and associated fracturing allows for hydraulic connection of these units (Western Water Consultants, Inc., 1982a,b; Doremus, 1986; Jarvis, 1986; Spencer, 1986a). The aquifer is confined below by shales of the underlying Horseshoe Shale Member of the Amsden Formation (Cooley, 1984, 1986a; Jarvis, 1986; Spencer, 1986a) (Plate III). The Horseshoe Shale Member corresponds to the Amsden aquifer, as described in a following section.
The Tensleep Sandstone is composed of predominantly tan, cross-bedded, well-sorted, fine- to medium-grained sandstone cemented with carbonate and silica (Todd, 1963; Libra et al., 1981; Cooley, 1986a; Doremus, 1986; Love at al., 1993; Plafcan et al., 1993). Cherty dolomite in the upper part and discontinuous marine limestone and dolomite (carbonates) in the lower part of the formation were reported by Moore (1984). Porosity and permeability in the Tensleep Sandstone are primarily intergranular, and decrease with secondary cementation and recrystallization, both of which increase with burial depth (Todd, 1963; Bredehoeft, 1964; Lawson and Smith, 1966). Fractures and solution processes (in carbonaterich zones) may enhance intergranular sandstone permeability (Stone, 1967; Lowry et al., 1976). Interstitial porosity decreases from the eastern basin margin to the basin interior (and with increasing depth) due to precipitation of dolomite and silica (Todd, 1963; Bredehoeft, 1964; Lawson and Smith, 1966; Fox et al., 1975). Secondary fracture porosity and permeability are common in folds and faults in the BHB, and these locations have the best potential for groundwater development (Western Water Consultants, Inc., 1982a,b; Jarvis, 1986; Spencer, 1986a).
The Ranchester Limestone Member of the Amsden Formation is composed primarily of limestone. The geologic unit is not permeable everywhere; Jarvis (1986a, p. 37) noted that the unit is “probably impermeable except where limestones have been dolomitized.”
Numerous potentiometric surfaces of the Tensleep aquifer in the BHB have been constructed because of its importance as a petroleum reservoir and its utility as a model of artesian basins in the Wyoming foreland. Predevelopment potentiometric surfaces of the aquifer have been constructed by Todd (1963), Bredehoeft and Bennett (1972), and Haun (1984). These investigators based their potentiometric surfaces exclusively on hydraulic-head data from the Tensleep Sandstone and did not include hydraulic-head data from the underlying Ranchester Limestone Member of the Amsden Formation or the overlying Phosphoria and (or) Goose Egg Formations and equivalents (Park City Formation) (as described above, these last geologic units are considered part of the aquifer by several investigators). The potentiometric surface constructed by Bredehoeft and Bennett (1972) has been widely reproduced and discussed since publication (and is reproduced herein on Plate XII). The potentiometric surfaces of Todd (1963) and Bredehoeft and Bennett (1972) are fairly similar, and both neglect the effects of faulting; both maps show groundwater flowing uniformly from outcrops along the basin margins (recharge areas) to the deep, central parts of the basin. The potentiometricsurface map constructed by Haun (1984) differs from that of Bredehoeft and Bennett (1972). In addition to hydraulic head, Haun utilized oilfield tilts from Zapp (1956) and, more importantly, included faults as barriers to lateral groundwater flow. Consequently, the interpretations of the predevelopment potentiometric surfaces of the Tensleep aquifer (Figure 7-10) are substantially different (Bredehoeft et al., 1992). Bredehoeft et al. (1992, p. 535) noted that Haun (1984) minimized higher outcrop elevations as control, and also noted that hydraulic potentials were several hundred feet lower than those of Bredehoeft and Bennett’s (1972) potentiometric surface. By examining the various hydraulic factors that affect construction of potentiometric surfaces, Bredehoeft et al. (1992) concluded that major faults can act as both barriers and vertical conduits for regional flow, and that major fault zones control regional Paleozoic aquifer groundwater flow in the BHB. This led them to conclude that the hydraulic heads presented in both Tensleep aquifer predevelopment potentiometric surface maps are likely “too low over much of the basin” (Bredehoeft et al., 1992, p. 545).
The potentiometric surface of the Tensleep aquifer in the northeastern BHB was mapped by Western Water Consultants, Inc. (1982a, Plate 4; 1982b, Plate 4; 1983c, Plate 4). These maps were based on hydraulic-head data from the Tensleep Sandstone and overlying Phosphoria Formation. (The Phosphoria Formation and equivalent Park City Formation are considered part of the Tensleep aquifer by several investigators.) The general direction of groundwater flow is from outcrop areas along the eastern basin margin toward the basin interior. Large cones of depression were associated with the Byron, Garland, Sage Creek, Deaver, and Frannie oilfields (Western Water Consultants, Inc., 1982a,b). Drawdown at the Manderson and Bonanza oilfields was hypothesized (Western Water Consultants, Inc., 1983c). Discontinuity of the potentiometric surface near the Manderson anticline was noted and attributed to a fault acting as a partial barrier to groundwater flow (Western Water Consultants, Inc., 1983c).
Detailed mapping of the potentiometric surface of the Tensleep aquifer was conducted in the southeastern BHB by Jarvis (1986, Plate 4) and southwestern BHB by Spencer (1986a, Plate 4). Cooley (1986a) mapped the potentiometric surface of the aquifer in the Tensleep area [a small part of the same area mapped by Jarvis (1986a)]. The potentiometric-surface maps of Jarvis (1986, Plate 4) and Spencer (1986a, Plate 4) were meant to complement one another, and both maps have been combined into a single potentiometric-surface map for this study on Plate XIII. The investigators accounted for the many effects of folding and faulting on interpretation of the potentiometric surface, incorporating and expanding upon the improved understanding of hydraulic interconnection between recharge areas along Wyoming structural basin margins and deep central basins provided by Huntoon (1985a). The potentiometric-surface maps of the Tensleep aquifer constructed by Jarvis and Spencer are the most detailed available for any aquifer in the BHB, and the effects of these numerous structures on the potentiometric surface of the Tensleep aquifer can be seen on Plate XIII.
Groundwater-quality data are presented and described for only one lithostratigraphic unit (Tensleep Sandstone) included in the Tensleep aquifer. Groundwater-quality data for the underlying Ranchester Limestone Member of the Amsden Formation are included in the description of the Amsden aquifer because groundwater samples from the Amsden aquifer generally were only associated with the Amsden Formation, not the three members composing it.
The chemical composition of groundwater in the Tensleep aquifer in the BHB was characterized and the quality evaluated on the basis of environmental water samples from 38 wells and six springs. Major-ion composition in relation to TDS is shown on a trilinear diagram (Appendix G2, diagram K). Summary statistics calculated for available constituents are listed in Appendix E2. TDS concentrations were variable and indicated that most waters were fresh (82 percent of samples) and the remaining waters were slightly to moderately saline (Appendix E2; Appendix G2, diagram K; supplementary data tables). TDS concentrations ranged from 156 to 3,750 mg/L, with a median of 259 mg/L.
Concentrations of some properties and constituents in water from the Tensleep aquifer in the BHB approached or exceeded applicable USEPA or State of Wyoming water-quality standards and could limit suitability for some uses. Most environmental waters were suitable for domestic use, but concentrations of one constituent infrequently exceeded health-based standards (USEPA MCLs and HALs): fluoride (2 percent of samples analyzed for the constituent). Concentrations of several properties and constituents exceeded aesthetic standards (USEPA SMCLs) for domestic use: aluminum (100 percent exceeded lower limit), TDS (27 percent), sulfate (23 percent), filtered iron (21 percent), fluoride (15 percent), pH (3 percent below lower limit), and chloride (2 percent).
The chemical composition of groundwater in the Tensleep aquifer also was characterized and the quality evaluated on the basis of 504 produced-water samples from wells. Majorion composition in relation to TDS is shown on a trilinear diagram (Appendix H2, diagram G). Summary statistics calculated for available constituents are listed in Appendix F2. TDS concentrations from produced-water samples were highly variable and ranged from fresh to very saline; however, the concentrations indicated that most of the waters were moderately saline (61 percent) (Appendix F2; Appendix H2, diagram G; supplementary data tables). TDS concentrations ranged from 324 to 33,700 mg/L, with a median of 3,650 mg/L.
Most available water-quality analyses were from producedwater samples, for which chemical analyses of few properties and constituents were available; thus, comparisons between concentrations in produced-water samples and health-based, aesthetic, or State of Wyoming agricultural and livestockuse standards were limited. The produced-water samples had concentrations of one constituent that exceeded health-based standards (USEPA MCLs and HALs): strontium (50 percent). The produced-water samples generally had concentrations of several properties and constituents that exceeded aesthetic standards (USEPA SMCLs) for domestic use: filtered iron (100 percent), manganese (100 percent), TDS (99 percent), sulfate (96 percent), chloride (38 percent), and pH (1 percent below lower limit and 4 percent above upper limit). TDS concentrations in 6 percent of produced-water samples from the Tensleep aquifer exceeded State of Wyoming Class IV standards.
Concentrations of some properties and constituents exceeded State of Wyoming standards for agricultural and livestock use in the BHB. Properties and constituents in environmental water samples that had concentrations greater than agricultural-use standards were sulfate (26 percent), TDS (14 percent), chloride (7 percent), boron (5 percent), and filtered iron (5 percent). One property (pH) had values greater than a livestock-use standard (3 percent below lower limit). The produced-water samples generally had concentrations of several properties and constituents that exceeded agricultural-use standards: sulfate (97 percent), TDS (89 percent), chloride (60 percent), filtered iron (50 percent), and pH (less than 1 percent below lower limit). The produced-water samples generally had concentrations of several properties and constituents that exceeded livestockuse standards: TDS (28 percent), sulfate (21 percent), pH (1 percent below lower limit and 4 percent above upper limit), and chloride (2 percent).
Jarvis (1986) noted two types of water in the Tensleep aquifer. Calcium-bicarbonate-type waters were predominant near the basin margin, whereas calcium-sulfate-type waters were predominant in the basin interior. Jarvis also attributed large concentrations of sodium, calcium, sulfate, and chloride in water from the Tensleep aquifer to anhydrite dissolution. Evaluation of a much larger number of groundwater samples compiled for this study indicates that ionic composition (water type) clearly changes with increasing TDS concentration (Appendix G2, diagram K; Appendix H2, diagram G). Groundwaters from the Tensleep aquifer classified as fresh or slightly saline generally were calcium-bicarbonate and calcium-magnesiumbicarbonate type, whereas waters classified as moderately to very saline generally were classified as (and evolve toward) sodium-sulfate type.
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