Bioassessment Study

The Las Vegas Wash is the sole drainage from the Las Vegas Valley watershed to Lake Mead.  The four flow components of flows in the Las Vegas Wash are tertiary treated municipal wastewater, urban runoff, shallow ground water, and storm water.  Increased urbanization in the valley over the past two decades has resulted in increased flows through the Wash, which has caused significant erosion and wetland destruction. 

Since 1998, the Las Vegas Wash Coordination Committee has implemented long-term management strategies for the Las Vegas Wash.  A series of projects was undertaken to control erosion, improve water quality, and enhance the ecosystem of the Wash.  These projects include construction of several erosion control structures (Zhou et al. 2004) and a wetland park.  While these projects have provided benefits in terms of water quality improvement and ecosystem enhancement, their potential to change the flow regime of the wash by creating ponds and slowing the flow of the Wash to Lake Mead has created concern regarding the potential for effects on accumulation of contaminants in the Wash.  The pools behind the erosion control structures provide habitat for a variety of fish and wildlife, particularly migratory birds.  Wetlands have the potential to be contaminant “sinks” or “hot spots” for exposure of fish and wildlife, both resident and migratory, to toxic contaminants including pesticides (Beyer et al. 1996).

Other factors might also alter the flow of water in the Las Vegas Wash and affect water quality conditions.  Changing lake levels, erosion and formation of deltas, or increasing assementflows of municipal wastewater treatment plant effluent or diversion of these effluents might result in changes in water quality parameters that affect the cycling, degradation, accumulation, and toxicity of contaminants.  For example, fish in newly flooded reservoirs often have elevated concentrations of toxic methylmercury in their tissues.  When terrestrial zones are flooded during reservoir filling, enhanced microbial methylation of inorganic mercury in the terrestrial zone occurs, which causes a rapid increase in bioaccumulation of methylmercury in fish (Beyer et al. 1996).  Any factor that causes the level that inundated surrounding terrestrial areas could cause an increase in methylmercury bioaccumulation in the aquatic food web.

The bioassessment 2003 final report is an assessment of the concentrations of a selected suite of contaminants of potential concern (COPC) in water, sediment, whole fish, and bird eggs collected as part of the 2003 Las Vegas Wash Monitoring and Characterization Study.  Contaminant concentrations were assessed to determine whether specific contaminants are cause for concern related to effects on fish and birds of the Las Vegas Wash.