Open-File Report 01-282
Sediment-Deposition Rates and Organic Compounds in Bottom Sediment at Four Sites in Lake Mead, Nevada, May 1998
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Lake Mead, on the Colorado River in Nevada and Arizona (fig. 1), has been impounded by Hoover Dam since 1935. The lake is the largest reservoir by volume in the United States and is an important source of water for more than 22 million residents of southern Nevada, Arizona, and southern California. Sources of inflow to Lake Mead include the Colorado River, Las Vegas Wash, and the combined flow of the Virgin and Muddy Rivers.
The Las Vegas area, Nevada, is one of the Nation's fastest-growing urban areas. Urban runoff, industrial drainage, and treated municipal wastewater from the Las Vegas area are transported by Las Vegas Wash to Las Vegas Bay on Lake Mead. Bevans and others (1996) indicated that the discharge from the Las Vegas area probably is a source of organochlorine compounds (OCs) and semivolatile organic compounds (SVOCs) to Las Vegas Wash and Las Vegas Bay on Lake Mead. OCs include organochlorine pesticides and degradation products, polychlorinated biphenyls (PCBs), dioxins, and furans. SVOCs include polycyclic aromatic hydrocarbons (PAHs), phthalates, and phenols. Concentrations of some OCs and SVOCs in the water column, in bottom sediments, or in fish-tissue samples were nearly an order of magnitude larger in Las Vegas Wash and Las Vegas Bay than in an upstream area on Lake Mead (Bevans and others, 1996).
The U.S. Geological Survey's (USGS) National Water-Quality Assessment (NAWQA) Program and the Reconstructed Trends (RT) Study, in cooperation with the Department of Health Physics, University of Nevada, Las Vegas (UNLV), collected bottom-sediment core samples to investigate sediment-deposition rates and historic concentrations of OCs and SVOCs near the major inflows to Lake Mead.
This report presents data on sediment-deposition rates and chemical analyses of samples from bottom-sediment cores collected in May 1998 at four sites in Lake Mead. These sites were selected to provide information on the spatial and temporal characteristics of sedimentation and selected chemical constituents of the lake bottom adjacent to the major contributory inflows. Bottom-sediment cores at site 1 were collected in the delta of Las Vegas Wash where it enters Las Vegas Bay. Cores at site 2 were collected farther out in Las Vegas Bay. Cores at site 3 were collected in the Overton Arm of Lake Mead across from Stewart Point. Cores at site 4 were collected near the pre-impoundment confluence of the Colorado and Virgin Rivers (fig. 1).
Sediment-deposition rates were calculated using the pre-impoundment interface and cesium-137 (137Cs) activities in bottom-sediment core samples at the four sites. The pre-impoundment interface at each of the four sites occurred between 1935 and 1937. A by-product of nuclear weapons testing, 137Cs first occurred in the atmosphere in about 1952 and peaked in concentration in about 1964 (Van Metre and others, 1996, Van Metre and others, 1997a and 1997b, and Hoffman and Taylor, 1998). Separate sediment-deposition rates are reported for three periods -- from the pre-impoundment interface to 1952, from 1952 to 1964, and from 1964 to 1998.
Chemical analyses of bottom-sediment samples collected from Lake Mead during the study included organochlorine pesticides and degradation products, PCBs, dioxins, furans, PAHs, and phenols.
Lake Mead is the largest reservoir in the United States by volume and is second largest in terms of surface area (Lara and Sanders, 1970). Its maximum depth is about 180 m and the surface area is about 660 km2 (Paulson and Baker, 1980). At a maximum lake-surface elevation of 374 m above sea level, the lake extends about 106 km from Hoover Dam up the Colorado River. The maximum width of Lake Mead is about 15 km and its irregular shoreline is about 885 km in length (LaBounty and Horn, 1997). Lake Mead has four major subbasins; Boulder, Virgin, Temple, and Gregg; and three narrow canyons: Boulder, Virgin, and Iceberg (fig. 1).
Depending on release and inflow patterns, the retention time of water in Lake Mead averages 3.9 years (LaBounty and Horn, 1997). The Colorado River contributes about 98 percent of the annual inflow; the remainder is contributed by Las Vegas Wash and by the combined flow of the Virgin and Muddy Rivers (fig. 1). Las Vegas Wash contributes the second highest percentage of inflow to the lake. From 1992 to 1998, the mean daily discharge for Las Vegas Wash was about 5.9 m3/s (Preissler and others, 1999). Flow in Las Vegas Wash is perennial because of discharge from municipal wastewater treatment plants (Covay and others, 1996) and urban runoff. Discharge of treated wastewater effluent to lower Las Vegas Wash has steadily increased since the 1940s. In 1993, treated wastewater effluent constituted about 96 percent of the annual discharge into Las Vegas Wash (Bevans and others, 1996), which is the likely source of some of the synthetic organic compounds being discharged into Las Vegas Bay (Covay and Leiker, 1998; Bevans and others, 1996). A complete description of the environmental and hydrologic setting of the Las Vegas area is available in a report by Covay and others (1996).
Lake Mead is classified as mildly mesotrophic (Vollenweider, 1970; Carlson, 1977), based on its moderate nutrient concentrations and biological productivity. The hydrodynamics of Lake Mead are very complex and not well understood. Each basin in Lake Mead responds differently to the inflow-outflow regime and each basin is ecologically unique (La Bounty and Horn, 1997). Annual mean discharge for the Colorado River below Hoover Dam from 1935 to September 1998 was about 394 m3/s (Preissler and others, 1999). During bottom-sediment core sampling in May 1998, the water-surface elevation of Lake Mead was about 369 m above sea level (U.S. Bureau of Reclamation, 1998).
The authors acknowledge the following individuals and agencies who assisted this investigation. Dr. Mark J. Rudin, Chair, Department of Health Physics, UNLV, provided cooperative, technical, and field support. William J. Burke and Bryan C. Moore, Resource Specialists, Lake Mead National Recreation Area, provided logistical support and assisted in data-collection activities. F. Kent Turner, Chief, Resource Management Division, Lake Mead National Recreation Area, provided managerial support and consultation.
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