Investigating Temporal and Spatial Variations of Nutrient and Trace Metal Loading in Utah Lake (Utah, USA)

Utah, the second-driest state in the nation, faces a future encompassing population growth and climate change, both of which have the potential to impact the region’s hydrologic system. Consequently, these factors may affect both the availability and quality of water necessary for beneficial human use. Utah Lake is the largest freshwater lake in Utah. The lake is 35.4 km long and 16.1 km wide, with an average depth of 2.4 m and a maximum of 4 m [1]. It occupies much of Utah Valley and is used by the region as a water source for agriculture, irrigation, and water recreation. The major inflows to Utah Lake are the American Fork River, Provo River, Hobble Creek, and Spanish Fork River. The Provo River and Spanish Fork River account for nearly 60% of inflow into Utah Lake. Utah Lake has only one outlet, the Jordan River, which drains the lake north to the Great Salt Lake [2,3]. Although Utah Lake and its associated watershed have been of importance to humans for at least several millennia, Utah Lake has long been considered severely polluted after the pioneer settlement. This is largely caused by heavy loadings of various pollutants related to anthropogenic activities on Utah Lake [4,5]. Water quality issues primarily result from excessive nutrient inputs related to agricultural and animal farming practices, urban runoff, effluent from wastewater treatment plants, and elevated concentrations of trace metals associated with mining activities since the mid-1800s. Anthropogenic activities have shown an impact on both hydrological patterns and pollutant discharge within watersheds [6]. Microbes in these nutrient sources potentially alter the biogeochemical processes and lead to N₂O emissions in waters to the atmosphere, which can, in turn, affect nitrification and denitrification processes [6,7,8]. Furthermore, increasing discharges of excessive fertilizers from farming lands, residential lawns, and effluent discharge gradually led to the eutrophication of Utah Lake. Lake eutrophication is often linked to nutrient changes over time [9,10]. Shallow lakes are far more vulnerable than deep-water ecosystems due to their limited capacity to assimilate contaminants or nutrient loads [11,12]. As a result, shallow lakes can easily transition from an “oligotrophic state” to a “eutrophic state” [13]. Consequently, the excess nutrient levels are resulting in periodic harmful algal blooms (HABs) in the lake. Severe HABs have occurred in the summer since 2016 (DEQ) [14], prompting the closure of several locations on Utah Lake. Utah lake eutrophication is a serious problem that imposes hazardous health effects on local communities in addition to its adverse impacts on the local economy and recreational activities.

Besides the eutrophication problem, Utah Lake, like other lakes in the world, often sequesters trace elements and other contaminants through several natural processes, including sediment trapping and bioaccumulation of contaminants in fish as well as precipitation of contaminants as insoluble solids in the water column [15]. Although many metals are biologically essential in trace amounts, such as magnesium (Mg), iron (Fe), copper (Cu), and zinc (Zn) [16], excessive quantities can interfere with physiological processes. Other metals, such as arsenic (As) and lead (Pb), can accumulate in the tissues of aquatic organisms and cause adverse biological impacts in aquatic organisms [17,18,19]. Wang et al. (2017) discovered that As concentration in the floc layer (the top 20 cm mixture of water and sediments at the lake bottom) along the Utah Lake-Jordan River transition area was significantly higher than background levels [20]. The floc layer is very important since it is the layer where benthivorous fish species take in nutrients and trace metals that will eventually affect the trace metal levels in their tissues. More importantly, many people consume fish from the lake, which makes it even more important to monitor trace metal levels in the lake. Furthermore, previous studies also indicated that Utah Lake is not as horizontally well mixed as previously thought [20,21]. Therefore, it is essential to capture and quantify the temporal and spatial variations of nutrients and trace metals in the inflows to the lake to set the stage for a better understanding of the current state of Utah Lake.

To address the temporospatial variations of the nutrient and trace metal concentrations in the lake, we applied Geographic Information System (GIS) geospatial analysis techniques to visualize the variations. Specifically, we utilized inverse distance weighting (IDW) interpolation, a common GIS interpolation method used to predict values for unmeasured locations within an area using existing sample data points. The underlying assumption of IDW is that proximate locations should exhibit more similar parameter values than distant locations [22]. As such, IDW can generate continuous surfaces using weighted averages of point measurements and is useful for interpolating values such as rainfall, temperature, chemical concentrations, and elevation between measurement locations [23]. In this study, we leveraged available lake and river data to conduct IDW analysis. The accuracy of IDW interpolation relies on the distribution of measured locations [22]. Notably, our lake sampling sites captured the major inlets for nutrient and trace metal loading. Therefore, these sites are well-suited for utilizing IDW geospatial analysis to generate interpolated estimates between data points within the lake system. While IDW has limitations, our measurement locations help minimize estimation errors and allow reasonable mapping of the temporal and spatial variations in trace metal and nutrient concentrations across the lake.