USE OF PASSIVE SAMPLERS FOR DETECTING VERTICAL GRADIENTS AND POTENTIAL SOURCES OF ORGANIC CONTAMINANTS TO LAKE MEAD, NEVADA, U.S.A

Synthetic organic compounds (SOCs), both hydrophobic and hydrophilic, pose a potential chronic or acute threat to the health of living organisms. Lake Mead, in southern Nevada, is the major source of drinking water for about 1.6 million residents of Las Vegas Valley, and millions of annual tourists, as well as downstream users of the Colorado River. The Las Vegas metropolitan area is one of the fastest growing areas in the US, with a net gain of about 6,000 new residents monthly. Wastewater from the metropolitan area, about 170 million gallons per day, flows into Lake Mead through Las Vegas Wash (LVW), which enters into Las Vegas Bay (LVB). SOCs discharging into LVB can originate from many sources including irrigated-urban runoff, storm-water runoff, subsurface inflow, accidental spills, and treated sewage effluent. Many SOCs have been found previously in bottom sediment, fish tissue and plasma, and the water column within LVB. However, it is not known whether the lake sediment acts as a source and or sink of SOCs. In order to assess whether hydrophobic compounds are released from sediment, semipermeable membrane devices (SPMDs) were deployed at a location in LVB with 18 meters of water at depths of 4, 8, 12 and 18 meters for one month in March 2006 to determine if there was a vertical gradient of SOCs with higher concentrations near the bottom than in other parts of the water column. Continuous temperature measurements were taken concurrently at each depth and performance compounds were used to adjust for any differences in uptake of SOCs for each SPMD. Concentrations of hydrophobic SOCs are 2 to 4 times higher in the 18 meter SPMD than in the shallower SPMDs. Personal care products such as musks and fragrances are most prevalent, but PCBs are among the other compounds found at the deepest site. The total SOC concentrations are relatively similar in the upper three SPMDs indicating that high concentrations are confined to the lower 6 meters of the water column. The high SOC concentrations at depth may be explained by, 1) a high density plume with high SOC concentration of LVW water sinks to the bottom of LVB, or 2) release of SOCs from sediment due to biological activity, chemical fluxes or resuspension. Determining the process of how SOCs become concentrated near the bottom of LVB may be important for management of contaminants in Lake Mead.