The Influence of the wastewater drainage from the Las Vegas Valley on the Limnology of Boulder Basin, Lake Mead, Nevada-Arizona

摘要

Lake Mead, Colorado River, Arizona-Nevada, is one of the most heavily used reservoirs in the western United States, providing abundant recreational opportunities as well as downstream domestic and agricultural water for over 22 million users. Based on average nutrient levels and productivity, Lake Mead is classified as mildly mesotrophic. The interflow of the Colorado River dominates the limnology of much of the 106 km-long reservoir, and may still be identified at Hoover Dam under certain conditions. The lower basin of Lake Mead ending at Hoover Dam is known as Boulder Basin and is near the Las Vegas metropolitan area. Las Vegas Bay, which comprises the northwestern portion of Boulder Basin,receives all runoff including secondary and tertiary treated municipal sewage effluent from the Las Vegas Valley via Las Vegas Wash. The rapidly increasing population size of the Las Vegas Valley, and subsequent increases in inputs of point and non-point sources to Las Vegas Wash, has resulted in an increasing rate of eutrophication in Las Vegas Bay. Due to abundant nutrients, chlorophyll a concentrations during the months of June and July often exceed 100 mg-m-3, while secchi depth decreases to less than 0.5 m. Within the first 4 km of Las Vegas Bay, extending away from the wash inflow, recovery from nutrient enrichment is dramatic. Secchi readings increase by over 5 m and chlorophyll a concentrations decrease by more than 90 percent. However, the influence of the density current plume from Las Vegas Wash, which is easily identified by its relatively high specific conductance and turbidity, can be observed to extend into Boulder Basin, and at times to Hoover Dam. The thickness, the distance to which it extends into the reservoir, and depth of the plume depend on the season of the year and corresponds to the degree of thermal stratification within the reservoir. Although not directly measured, limnological data suggest the potential for this plume to be entrained by municipal water intakes located at Saddle Island near the mouth of Las Vegas Bay. Outbreaks of cryptosporidiosis in Las Vegas have been shown to be associated with time periods when the plume was observed near the same depth as the intakes. Additionally, concentrations of bacteria and organic compounds are higher in the plume relative to the surrounding water.