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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In May 1998, the USGS, in cooperation with UNLV, investigated sediment-deposition rates and sediment chemistry at four sites in Lake Mead. Two of the sites (one shallow and one deep) were in Las Vegas Bay, one was in the Overton Arm, and one was near the historic confluence of the Colorado and Virgin Rivers. All sediment cores collected were determined to have penetrated pre-impoundment sediments, thus providing a complete record of sediment deposition after impoundment. Total thicknesses of the post-impoundment sediments accumulated at sites 1, 2, 3, and 4 measured about 116, 118, 75, and 85 cm, respectively.
Sediment-deposition rates after impoundment of the Colorado River by Hoover Dam were calculated by measuring the accumulation of mass during three different periods: (1) from the approximate impoundment date for each site (1935-37) to the initial occurrence of 137Cs in the atmosphere (1952); (2) from 1952 to the maximum concentration of 137Cs in the atmosphere (about 1964); and (3) from 1964 to the collection date of the sample (1998). Sediment-deposition rates for the entire post-impoundment period (1935-98) averaged 1.45 (g/cm2)/yr at the Las Vegas Bay shallow site, 1.25 (g/cm2)/yr at the Las Vegas Bay deep site, 0.80 (g/cm2)/yr at the Overton Arm site, and 0.65 (g/cm2)/yr at the Colorado and Virgin Rivers confluence site. Sediment-deposition rates from the pre-impoundment interface to 1952 ranged from 0.25 (g/cm2)/yr at the Overton Arm site to 1.11 (g/cm2)/yr at the Las Vegas Bay deep site. From 1952 to 1964, deposition rates ranged from 1.27 (g/cm2)/yr at the Colorado and Virgin Rivers confluence site to 1.68 (g/cm2)/yr at the Las Vegas Bay shallow site. From 1964 to 1998, deposition rates ranged from 0.26 (g/cm2)/yr at the Colorado and Virgin Rivers confluence site to 1.81 (g/cm2)/yr at the Las Vegas Bay shallow site. Sediment-deposition rates from 1952 to 1964 and from 1964 to 1998 were not calculated for samples collected at the Overton Arm because the 1964 137Cs peak could not be determined with any certainty.
Estimated values were not used in reporting minimum constituent values. OCs and SVOCs were detected in the Lake Mead bottom-sediment core samples. The numbers of organic compounds detected were 48 at the Las Vegas Bay shallow site, 57 at the Las Vegas Bay deep site, 26 at the Overton Arm site, and 31 at the Colorado and Virgin Rivers confluence site. The most commonly detected organochlorine pesticides/polychlorinated biphenyl compounds were DDD, which ranged from 0.5 µg/kg at the Las Vegas Bay shallow site to 38 µg/kg at the Las Vegas Bay deep site, and DDE, which ranged from 0.51 µg/kg at the Overton Arm site to 57 µg/kg at the Las Vegas Bay deep site.
The most frequently detected dioxin compounds were TCDD, which ranged from <0.52 pg/g at the Colorado and Virgin Rivers confluence site to 39 pg/g at the Las Vegas Bay shallow site, and OCDD, which ranged from <2.5 pg/g at the Las Vegas deep site to 93 pg/g at the Las Vegas Bay shallow site.
The most frequently detected furan compounds included TCDF, which ranged from <0.55 pg/g at the Colorado and Virgin Rivers confluence site to 320 pg/g at the Las Vegas Bay deep site, and OCDF, which ranged from <0.74 pg/g at the Colorado and Virgin Rivers confluence site to 280 pg/g at the Las Vegas Bay deep site. Dioxin and furan analyses were not available for samples collected from the Overton Arm site.
The most frequently detected polycyclic aromatic hydrocarbons included: perylene, which ranged from 5.2 µg/kg at the Las Vegas Bay deep site to 259 µg/kg at the Colorado and Virgin Rivers confluence site; benzo[g,h,i]perylene, which ranged from 6.2 µg/kg at the Las Vegas-shallow site to 17.4 µg/kg at the Las Vegas Bay deep site; 2,6-dimethylnaphthalene, which ranged from 5.5 µg/kg at the Las Vegas-deep site to 123 µg/kg at the Las Vegas Bay deep site; and indeno[1,2,3-c,d]pyrene, which ranged from 6.5 µg/kg at the Las Vegas shallow site to 21.2 µg/kg at the Las Vegas Bay deep site.
The most frequently detected phenol compound was phenol, which ranged from 5.1 µg/kg at the Las Vegas-shallow site to 33 µg/kg at the Colorado and Virgin Rivers confluence site.
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