SOURCE ELEVATION FOR WATER IN CAVE POOLS AT TIMPANOGOS CAVE NATIONAL MONUMENT, UTAH AS DETERMINED BY STABLE ISOTOPES OF WATER

This abstract focuses upon δ¹⁸O and δ²H of water samples collected in 2012 from cave pools and the American Fork River within Timpanogos Cave National Monument (TICA) in Utah. Samples from nearby Cascade Springs, a karst spring mantled with glacial till, and two thermal springs near Midway provide comparison with active groundwater flow systems. In the cave pools of TICA, there is clear clustering before June with a higher average D_ex. After June, the isotopic composition of the samples trends toward more negative values of δ²H and progresses below the Global Meteoric Water Line. It is possible that these trends represent a change from reservoirs dominated by melt waters to reservoirs progressively influenced by waters with significantly lower D_ex, perhaps by evaporative fractionation.

The American Fork River has the most negative δ¹⁸O, consistent with water derived from snowmelt at elevations above 2,900m. Samples at the same elevation from cave pools in TICA cluster at different locations on a LWL, suggesting source waters at differing elevations. The average δ¹⁸O in Cavern of Sleep is the least negative, suggesting a source at the elevation of the cave—2,025m. The average δ¹⁸O in Hidden Lake and Middle Lake are more negative than Cavern of Sleep, perhaps from water infiltrating the epikarst above the cave—2,195m and 2,121m, respectively. Hansen Lake has the lowest average δ¹⁸O in TICA, suggesting the canyon rim as the source—2,440m. Isotope data from Soda Pop Pit shift from a composition similar to Hansen Lake before June toward values similar to Middle Lake after June. This implies a change in source-water from the canyon rim to the overlying epikarst. A regression between δ¹⁸O and proposed source elevations gives an altitude lapse rate of 0.4‰ 100m^-1 along the Wasatch Front. Using this lapse rate, the δ¹⁸O data from Cascade Springs and the thermal springs in Midway suggest source elevations between 2,800 and 3,100 m or ¹⁸O depletion from orographic rainout.