TDS and selenium projections for the Las Vegas Wash, post completion of the Systems Conveyance and Operations Program (SCOP)

Abstract

The Las Vegas Wash (Wash) is the primary drainage channel for the Las Vegas Valley. Flows from the tributaries are a major source of contaminants of particular concern to the Wash, mainly total dissolved solids (TDS) and selenium (Se). Treated wastewater effluent discharged to the Wash currently provides enough dilution to lower the TDS and to maintain selenium concentrations below the current 5μg/L aquatic life criteria recommended by the Unites States Environmental Protection Agency (EPA; USEPA 1987); however, the Systems Conveyance Operations Program (SCOP) was intended to take most of this treated effluent through a pipeline directly into the Boulder Basin of Lake Mead. While construction of the pipeline may have had many water quality benefits for the Las Vegas Valley and Lake Mead, the dilution benefit to the Wash will be significantly reduced, primarily affecting TDS and Se concentrations in the Wash. The Wash enters Lake Mead at Las Vegas Bay, home to the endangered razorback sucker. The concern is the increased concentration of TDS and Se to the Las Vegas Bay, which potentially would impact the razorback sucker. Using data collected by the Southern Nevada Water Authority's (SNWA) environmental monitoring and management water quality team, projections have been made for expected TDS and Se concentrations post completion of the SCOP. These projections show 2 options for managing TDS and Se concentrations on the Wash: dilution and/or treatment of the tributaries. Discharging 2.08 × 105 m3/d to the Wash provides the necessary dilution to lower Se to the target concentration of 5 μg/L but will still leave TDS levels higher than desired. Treating one or more of the major tributaries will bring both Se and TDS concentrations within targeted levels.