A MASS BALANCE FOR SELENIUM IN THE UPPER POND OF THE CLARK COUNTY WETLANDS PARK, NATURE PRESERVE, LAS VEGAS, NEVADA

Selenium (Se), a necessary nutrient to fish and wildlife, can bioaccumulate in aquatic food webs to toxic levels.  The alkaline soils and arid climate of the Western United States may provide an environment in which Se can mobilize and become available for biological uptake.  Wetlands, in particular, have been shown to bioaccumulate Se.  

Past studies at the Nature Preserve, in the Wetlands Park, Las Vegas, NV, documented surface water Se concentrations from 12 to 316 μg/L.  In order to determine if the Nature Preserve is accumulating Se, we constructed a mass balance in its upper pond.  The mass balance will assist in developing management practices to minimize bioaccumulation, serve as a road map for other potential mass balance studies along the Las Vegas Wash, and contribute a greater understanding of Se mobility in the wetland environment.  

A water budget was constructed from surface water inflow and outflow, precipitation, and evapotranspiration. Groundwater recharge/discharge was calculated as the budget error term. The dominant components in the water budget are the surface inflow and outflows, which average 129 L/s and 98 L/s, respectively. Once the water budget was known, a Se budget was constructed using Se concentrations in surface water, groundwater, atmospheric deposition, volatilization, plant uptake, and sediment adsorption/precipitation.   

Selenium surface water and groundwater concentrations have increased with time.  In August, 2004, the average surface water concentration was 9.38 μg/L, which was greater than the average groundwater concentration of 7.13 μg/L.  At the end of November, the average surface water concentration rose to 20.4 μg/L, while the average groundwater concentration rose to 116 μg/L.  

Selenium influx has increased from August, 2004, to December, 2004, from 63 to 201 g Se/day.  The Se outflux has also increased during this time period, from 65 to 164 g Se/day.   On average, the wetland appears to be gaining 42 g Se/day.   

At this point we hypothesize that residual Se is accumulating in the plants and sediment of the pond.  The groundwater concentrations may be increasing due to flushing from repeated storm events, and from plant senescence and dormancy during the fall and winter.  We will monitor the Se concentrations and flux through October, 2005, to test our hypotheses and revise our interpretations.