Resources

Results of this assessment indicate that during a full pumping or generating event of the Project, the variation in the water level of Seminoe Reservoir at its normal maximum operating surface elevation of 6,357 feet, will be approximately 6.4 inches. The current typical daily fluctuation of Seminoe Reservoir is for most of the year (e.g., in June and July Seminoe Reservoir experiences daily fluctuations up to 0.3 foot, almost 4 inches). Therefore, operation of the Project will involve water level fluctuations generally within the range of levels currently experienced within Seminoe Reservoir at its normal maximum operating surface elevation.

Operation of the Project is not anticipated to affect erosion processes or bank stability within Seminoe Reservoir in the vicinity of the Project. Large areas of bedrock outcrops, large boulders, and cobble line the shorelines of Seminoe Reservoir in areas, especially near the proposed intake structure, and serve to protect these areas from erosion. As a result, Project operations are not expected to have adverse impacts on geological or soil resources within Seminoe Reservoir.

Underground Project Features

Underground features include the powerhouse; a power tunnel between the upper reservoir and the powerhouse; a tailrace tunnel between the powerhouse and a new intake in the existing Seminoe Reservoir; powerhouse access tunnel; and a high-voltage transmission tunnel. Extending away from their respective portals and for many hundreds of feet, the tailrace and powerhouse access tunnels will likely encounter steeply dipping sedimentary rocks of Pennsylvanian, Mississippian, and Cambrian age. The last 2,000 feet or so of tunnel toward the proposed powerhouse will likely be in Precambrian granite. The powerhouse and tailrace tunnels will be built entirely in granite (WSGS 2020, Bennett and Aalto 1982). Locating the powerhouse in granite precludes the possibility of slippage along bedding planes, which may be a problem for the structure if it was located in sedimentary units (Bennett and Aalto 1982).

Based on estimates, and information obtained from previous drilling investigations in the area of the powerhouse and access tunnels, the rock that will likely be encountered in the area will be moderately to intensely fractured. As such, support systems may be warranted (Bennett and Aalto 1982).

3.3.3 Cumulative Environmental Effects Related to Geologic and Soil Resources

As noted in Table 3.2-1, the geographic scope for geology and soils is construction workspaces since impacts on soils and surficial geology would be highly localized. The geographic scope includes the aboveground limits of the Project and extends belowground to the depth of proposed construction for the proposed Project facilities. As detailed in Table 3.2-2, there are two projects that occur within the geographic scope for geologic and soil resources: 1) Gateway West Transmission Line Project, and 2) Gateway South Transmission Line Project. These projects tie into the existing Aeolus Substation, which represents less than 7 percent (approximately 462 acres) of the Project’s Footprint of

Potential Disturbance and the only physical proximity between the Project and others within the geographic scope for potential effects on geologic and soil resources.

Issues Identified for Analysis

Potential cumulative effects on geological hazards include landslide susceptibility resulting from the loss of vegetation or ground-disturbing activities related to the construction phase of multiple projects in a localized area.

Cumulative effects on soil resources could result from either: 1) new alterations to the natural environmental and land surface that could increase the rate of soil erosion by water or wind; or 2) the permanent conversion of vegetated rangeland soils to other uses that could collectively result in limited loss of productivity of soils within the geographic scope for analysis. The implementation of appropriate mitigation measures would minimize shortterm impacts, such as new disturbance of surface soils and other new alterations to the natural environment from construction of the Project and other past, present, and future projects. Mitigation measures would allow for the local soil resources to be stabilized or returned to a state close to their preconstruction state.

Results

As discussed in Section 3.3.1.10, there are three faults identified in the area of Seminoe Dam and the proposed upper reservoir and hydraulic conveyance structures (Black Seam Fault, Red Seam Fault, and Reservoir Fault). These faults were studied as part of an Assessment of Potential for Surface Faulting for Seminoe and Kortes Dams (Geomatrix Consultants 1988), which concluded that there has been no late Quaternary displacement of these faults, and, thus, the potential for future surface faulting is very low. Furthermore, neither the Gateway West Transmission Line Project or the Gateway South Transmission Line Project cross any of these fault lines. Therefore, there are not foreseeable future actions that have the potential to affect geologic stability or sensitive geologic formations. Cumulative effects of past and present actions are not expected to affect faults or flooding.

Aeolus Substation, located at the eastern terminus of the proposed Project transmission lines, is the only area of overlap between the Project and other reasonably foreseeable projects. Aeolus substation is sited in regionally (moderate hazard) susceptible expansive soils (Figure 3.3-6). This area, which contains soils susceptible to wind or water erosion, may represent some potential for cumulative effects on geologic and soil resources, since the Project overlaps with the two aforementioned reasonably foreseeable future actions. However, soils in the vicinity of the Aeolus substation are disturbed and largely unvegetated as part of baseline conditions, because it is already constructed and in use as a regional transmission hub. The remaining portion of the geographic scope of the Project would experience the least cumulative effects since most of the soils are not susceptible to erosion or other hazards (Figure 3.3-6).

Past and present surface disturbance of soils occurs from existing and reasonably foreseeable grazing that occurs on many of the BLM-managed lands near the Project and in Wyoming. Project construction will remove certain lands from grazing rotations. Soil