A STABLE ISOTOPE STUDY OF UTAH LAKE (UTAH, USA)

Utah Lake is the largest natural freshwater lake in the western US. The lake represents a crucial water source for Utah Valley and is also important for recreational activities and as a biological habitat. The major inlets to Utah Lake are the American Fork River, Provo River, Hobble Creek, and Spanish Fork River. Utah Lake has only one outlet, the Jordan River, which drains the lake north to the Great Salt Lake. To investigate the water budget of the lake, we analyzed the hydrogen (δD) and oxygen (δ¹⁸O) stable isotope composition of water samples collected biweekly during the period March 2015-March 2016 from the lake, the inlets, a shallow groundwater well, and from the Jordan River. To investigate the vertical and horizontal variability in the δD and δ¹⁸O values of lake water we also seasonally collected and analyzed shallow water samples from different locations in the lake and samples along a vertical profile at the deepest lake site.

With respect to the inlets, the average (±1 sd) values (vs. V-SMOW) of the Provo River (δD=-118.8±0.9‰; δ¹⁸O=-16.07±0.32‰), Spanish Fork River (δD=-112.3±2.9‰; δ¹⁸O=-14.69±0.54‰), American Fork River (δD=-119.9±1.4‰; δ¹⁸O=-16.54±0.42‰) and Hobble Creek (δD=-114.4±8.4‰; δ¹⁸O=-15.17±1.58‰) are similar to the average values of the shallow groundwater (δD=-126.2±0.7‰; δ¹⁸O=-17.08±0.23‰), which in turn has a composition that is similar to winter precipitation. This suggests that snowmelt-fed groundwater is the main source of Utah Valley surface waters. In addition, river water samples plot on or close to the local meteoric water line (LMWL), suggesting that no significant evaporation is occurring in the inlets. With respect to the lake (δD=-76.0±10.2‰; δ¹⁸O=-8.32±1.87‰) and the Jordan River (δD=-90.4±14.0‰; δ¹⁸O=-10.75±2.66‰), their average values are higher than those of the inlets and plot below the LMWL, suggesting that significant evaporation is occurring in these water bodies. Seasonal samples indicate that the lake is isotopically poorly mixed horizontally but well mixed vertically. Results from this study suggest that global climate change, by changing precipitation seasonality and the proportion of precipitation that falls as rain vs. snow may affect the availability of surface water in Utah Valley.