USE OF SEDIMENT CORES TO EVALUATE THE HISTORY OF CONTAMINANT INPUTS TO LAKE MEAD

Assessing changes in contaminant inputs (both organic and inorganic) over time is important for evaluating their potential sources and sinks. Variations in contaminant input were assessed in four sediment cores taken in 1998 from three different parts of Lake Mead (two from Las Vegas Bay (LVB) and one each from Overton Arm and the Virgin Basin). Sediments were analyzed for major and trace elements, radionuclides, and organic compounds. The Overton Arm is the sub-basin of Lake Mead least affected by anthropogenic contaminant inputs. Anthropogenic contaminant concentrations are greatest in Las Vegas Bay, reflecting inputs from the Las Vegas urban area through the Las Vegas Wash tributary. Although contaminant concentrations remain low overall, compared to Canadian sediment quality guidelines and compared to sediments in other USA lakes, there are higher median mercury concentrations in the Virgin Basin (0.06 ppm), which is the main body of the reservoir and is influenced by the hydrology of the Colorado River, compared to LVB (median = 0.04 ppm). Major- and trace-element concentrations in the Virgin Basin core show pronounced shifts (approximately 50 % declines in concentrations depending on the element) after building the Glen Canyon Dam on the Colorado River in 1960s, before gradually returning to concentrations more typical of pre-1960s by the 1980s and 1990s, after the filling of Lake Powell upstream. A longitudinal transect of 3 cores from inner to outer Las Vegas Bay taken in 2007 were analyzed for emerging contaminants. Detections of musk fragrances and polybrominated diphenyl ethers have been found at higher concentrations (up to 65 ng/g, and 38 ng/g, respectively) in the upper 10 – 15 cm of the core declining to non-detection in the lower parts of the cores, indicating that these compounds either degrade with time or have been increasing in use for the past 10 – 20 years. In addition, concentrations declined from the inner bay to the outer bay, indicating their source is from the Las Vegas Wash. These data show temporal and spatial trends in contaminant histories that can be related to human disturbance from both within and outside the basin.