AWWA SOURCES59201

The Las Vegas Wash (Wash) is the sole drainage from the Las Vegas Valley watershed (~1,600 square miles) that discharges to Lake Mead. Flows to the Wash include highly-treated wastewater from three wastewater treatment facilities in the valley, urban runoff, shallow groundwater, and stormwater. Increased population and urbanization in the valley over the last three decades has resulted in increased flows to the Wash that caused significant erosion and wetland loss. Since 1998, a Comprehensive Adaptive Management Plan (CAMP) has been developed and implemented to control erosion, to improve water quality, and to enhance the ecosystem in the Wash. Constructing erosion control structures along the Wash has created more wetland acreage in the Wash, which offers many positive environmental benefits. However, increased areas of wetlands have also caused some concern over the bioaccumulation of several trace metals and metalloids. Due to its tendency to bioaccumulate in aquatic ecosystems, selenium (Se) has regularly been monitored in the water of the Las Vegas Wash and the tributaries and seeps into the Wash. This monitoring program has established a baseline dataset of Se concentrations in the Wash and its tributaries. Water samples have been collected monthly from eight locations in the mainstream Wash channel and quarterly from eight locations in the tributaries. All samples were collected and preserved using ultra-clean sampling and preservation techniques. To ensure proper QA/QC and because of the low detection limits, water samples were analyzed by three different laboratories for verification. Selenium data continues to be fairly consistent for each sample location and among the different laboratories. In general, the tributaries with shallow groundwater inputs or long flow paths have elevated total selenium concentrations, ranging from 10 to 25 ppb, whereas samples from the mainstream Wash sites are relatively low in total selenium (< 5 ppb) due to the dilution provided by inflows of wastewater treatment plant effluent. Additional data collection has allowed for the identification of zones of elevated Se concentrations in the tributaries and for the Se mass balance calculations within the system. Includes 13 references, tables, figures.