3.4.1.6 Groundwater

a redistribution of the already existing water column heat due to the additional mixing during power generation; slightly warmer water is then routed through the penstocks.

The greatest temperature increase corresponds to the modeled year 1977. Based on meteorological conditions, the year 1977 represents an unusual year with extreme lowflow conditions. Under existing conditions, 1977 reflects unusually high thermally stratified conditions due to extreme low river flows and, therefore, it is more sensitive to impacts from Project operations. It also reflects the coldest penstock temperature from all three simulation years and, therefore, despite the temperature increase due to Project operations, the penstock water temperature is still significantly below the other two simulation years. It is important to note that Project operations do not add a thermal (heat) load to Seminoe Reservoir but rather slightly redistribute the naturally occurring water heat content.

3.4.1.6 Groundwater

Estimates of water used in Carbon County indicate that groundwater only accounts for approximately 2 percent of the overall water used; however, in many parts of the county it is the only supply of water available. Groundwater in the county occurs under both watertable (unconfined) and artesian (confined) conditions (Bartos et al. 2006). Recharge to aquifers in Carbon County occurs by infiltration of precipitation on outcrop areas, infiltration of snowmelt runoff from the mountains, and leakage of streamflow. Groundwater discharge in Carbon County occurs mainly as seepage to streams, discharge to springs and seeps, pumpage from wells, evapotranspiration, and underflow along streams and in aquifers that extend out of the area (Bartos et al. 2006).

The hydrogeologic units in the Platte River Basin consist of saturated strata and fractured crystalline rocks that store and convey groundwater. These rock units range in geologic age from Quaternary to Precambrian and are variably permeable. Virtually all the geologic units in the Platte River Basin, including confining units, are capable of yielding at least small quantities of groundwater to wells. Estimated average annual recharge in the Platte River Basin ranges from less than 1 inch per year in interior areas of the drainage subbasins to 28 inches per year in the surrounding mountains (Taucher et al. 2013).

The Groundwater Atlas of Wyoming (Stafford et al. 2017) was developed to enable research and comparison of groundwater wells, groundwater quality, aquifers, and recharge across the state. According to the Groundwater Atlas of Wyoming, several groundwater wells are located in the Project vicinity, some associated with the North Red Hills Campground and others on BLM land (Stafford et al. 2017).

Feasibility drilling as part of the North Platte River Hydroelectric Study (Seminoe Site) (Bennett and Aalto 1982) in the area of the proposed upper reservoir (i.e., drill hole DD-7) did not encounter any groundwater11. In wet years, Dry Lake (located within the proposed

11 Drill hole DD-7, located in the center of Dry Lake, went through 38.7 feet of material described as granitic saprolite and had a total drill hole depth of 199.9 feet (Bennett and Aalto 1982).