Shallow groundwater and surface drainages in Las Vegas Wash are known to have elevated level of selenium which mainly comes from the naturally occurring geological hotspots on the southeast side of the Las Vegas Valley. Selenium fate and transport after it enters into Lake Mead from the Las Vegas Wash are not clearly understood. An open sourced three-dimensional Environmental Fluid Dynamic Code model (EFDC), developed by the United States Environmental Protection Agency, was used to model movement of selenium in the Boulder Basin, Lake Mead. The model was calibrated by observed data from 2006 to 2007. The concept of Lagrangian particle tracking was used to understand the specific motion trail of selenium in the Boulder Basin. The model results showed that under the present condition, it takes approximately 260 days for selenium particles to be transported from the Las Vegas Wash to the Hoover Dam. The highest depth-averaged (mean of 10 vertical layers) selenium concentration near the Hoover Dam was 0.089 µg L-1 based on 3.5 µg L-1 average inflow from the Las Vegas Wash. The results provide that the transport of selenium in the Lake is strongly influenced by hydrodynamic conditions induced by various water levels and wind intensity and directions. With increases in water level drops, selenium particles will likely move out faster from the Hoover Dam and selenium concentration throughout the Lake will decrease. The wind conditions in Lake Mead are highly variable. Southeasterly and northeasterly are the prevailing wind directions in summer and other seasons, respectively. The modelu27s results imply that the dominant wind condition combined with intensity wind do have a huge impact on selenium dispersal in both the horizontal and vertical directions in the Boulder Basin. Even under a hypothetical elevated inflow concentration, selenium value within the Lake was still relatively low. However, since selenium can bioaccumulate rapidly in aquatic organisms, continued environmental monitoring of the Lake system should be implemented to prevent potential future impact on the Lake biota. This study provides useful information for understanding complex hydrodynamic processes and selenium fate and transport in the Boulder Basin, which is important for managing this large evolving body of water.
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