Nutrient Atmospheric Deposition on Utah Lake: A Comparison of Sampling and Analytical Methods

We describe modified sampling and analysis methods to quantify nutrient atmospheric deposition (AD) and estimate Utah Lake nutrient loading. We address criticisms of previous published collection methods, specifically collection table height, screened buckets, and assumptions of AD spatial patterns. We generally follow National Atmospheric Deposition Program (NADP) recommendations but deviate to measure lake AD, which includes deposition from both local and long-range sources. The NADP guidelines are designed to eliminate local contributions to the extent possible, while lake AD loads should include local contributions. We collected side-by-side data with tables at 1 m (previous results) and 2 m (NADP guidelines) above the ground at two separate locations. We found no statistically significant difference between data collected at the different heights. Previous published work assumed AD rates would decrease rapidly from the shore. We collected data from the lake interior and show that AD rates do not significantly decline away from the shore. This demonstrates that AD loads should be estimated by using the available data and geostatistical methods even if all data are from shoreline stations. We evaluated screening collection buckets. Standard unscreened AD samples had up to 3-fold higher nutrient concentrations than screened AD collections. It is not clear which samples best represent lake AD rates, but we recommend the use of screens and placed screens on all sample buckets for the majority of the 2020 data to exclude insects and other larger objects such as leaves. We updated AD load estimates for Utah Lake. Previous published estimates computed total AD loads of 350 and 153 tons of total phosphorous (TP) and 460 and 505 tons of dissolve inorganic nitrogen (DIN) for 2017 and 2018, respectively. Using updated collection methods, we estimated 262 and 133 tons of TP and 1052 and 482 tons of DIN for 2019 and 2020, respectively. The 2020 results used screened samplers with lower AD rates, which resulted in significantly lower totals than 2019. We present these modified methods and use data and analysis to support the updated methods and assumptions to help guide other studies of nutrient AD on lakes and reservoirs. We show that AD nutrient loads can be a significant amount of the total load and should be included in load studies.

Using Stable Isotopes to Determine the Water Balance of Utah Lake (Utah, USA)

To investigate the hydrology of Utah Lake, we analyzed the hydrogen (δ2H) and oxygen (δ18O) stable isotope composition of water samples collected from the various components of its system. The average δ2H and δ18O values of the inlets are similar to the average values of groundwater, 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 river waters. In addition, samples from the inlets plot close to the local meteoric water line, suggesting that no significant evaporation is occurring in these rivers. In contrast, the lake and its outlet have higher average δ-values than the inlets and plot along evaporation lines, suggesting the occurrence of significant evaporation. Isotope data also indicate that the lake is poorly mixed horizontally, but well mixed vertically. Calculations based on mass balance equations provide estimates for the percentage of input water lost by evaporation (~47%), for the residence time of water in the lake (~0.5 years), and for the volume of groundwater inflow (~700 million m3) during the period April to November. The short water residence time and the high percentage of total inflow coming from groundwater might suggest that the lake is more susceptible to groundwater pollution than to surface water pollution.

Assessing the Impact of ~65 years of Land Use and Land Cover Change on the Utah Lake Watershed with Remote Sensing and Spatial Modeling

<p>Utah Lake is one of the largest natural freshwater lakes in the western United States. Its watershed is 9,800 km<sup>2</sup>. Utah Lake is located in Utah County which is expected to have the highest population growth in the state through 2060. Land use and water regulation has shifted the Utah Lake shoreline since the 1900s. Monitoring the land use and land cover change (LULCC) in the watershed is critical to understanding surrounding hydrology and future sustainability. In this study, we compared the Utah Lake shoreline change from 1953-2014 and classified the land cover in the Utah Lake watershed from 1985-2018. Our results show that there was a 41.45 km<sup>2  </sup>decrease in lake surface from 1953 to 2014. The shoreline around the Provo Bay and Goshen Bay has receded lake-ward considerably in 2014 compared to the 1953 shoreline, and the lost water and wetland area was equivalent to 3,851 football fields in size. Land cover change calculations indicate that from 1985 to 2018 urbanization increased by 6%, forest by 2%, and barren by 3%, whereas water and agriculture decreased by 1% and 6%, respectively. The findings from this project could be used by Utah’s legislature to implement meaningful watershed planning and management, especially in light of the state considering House Bill 272 that promotes “comprehensive restoration of Utah Lake by building an island on it.” The bill proposes an island in Utah Lake which could dramatically alter LULCC around the lake. In addition, any significant LULCC on and around the lake will modify the lake water budget, its ecosystem, and have profound consequences on Utah Lake watershed and the surrounding regions.</p>