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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Vertical cores of bottom sediments from Lake Mead were collected using a Benthos gravity corer and a Benthos piston corer (each is 4 m long and 6.3 cm in diameter). The corers were deployed from a custom-built pontoon boat (Van Metre and others, 1997a and 1997b). An A-frame and hydraulic winch were used to raise and lower the coring devices. Four cores ranging in length from 48 to 158 cm were taken from site 1. Three cores ranging in length from 111 to 161 cm were taken from site 2. Five cores ranging in length from 70 to 90 cm were taken from site 3. Four cores ranging in length from 65 to 127 cm were taken from site 4. All cores were believed to have penetrated pre-impoundment sediments. The pre-impoundment interface in each core was ascribed to a change in the physical appearance of the sediment, its particle-size composition, or the presence of pre-impoundment soil-surface organic matter. The presence of pre-impoundment sediment ensured that a complete post-impoundment sedimentation record was represented by each core.
The replicate cores collected at each site were used for different physical and chemical analyses. At each site, one core was split lengthwise to reveal its physical characteristics. One extracted core from each site was subsampled to determine wet and dry weights needed to calculate porosity for subsampled intervals. Other cores were extruded vertically and samples were collected for chemical analyses of organochlorine pesticides and PCBs, dioxins, furans, PAHs, phenols, and 137Cs (table 1). Organochlorine pesticides and PCBs were analyzed in organic-solvent extracts using dual capillary-column gas chromatography with dual electron-capture detectors (Wershaw and others, 1987; Foreman and others, 1995). Dioxins and furans were analyzed in organic-solvent extracts using gas chromatography/high-resolution mass spectrometry (U.S. Environmental Protection Agency, 1986). PAHs and phenols were analyzed in organic-solvent extracts using gas chromatography/mass spectrometry (Furlong and others, 1996). 137Cs was analyzed using a high-purity intrinsic germanium detector gamma spectrometer (Dr. Mark J. Rudin, University of Nevada, Las Vegas, Department of Health Physics, oral commun., 1999).
Post-impoundment sediment-deposition rates at each site were calculated by measuring the accumulation of mass from (1) the approximate impoundment date at each site (ranging from 1935 to 1937) to the first occurrence of 137Cs in the atmosphere in 1952; (2) from 1952 to the maximum activity of 137Cs in the atmosphere in 1964, and (3) from 1964 to the collection date of the sample in 1998. Detailed procedures for calculating mass accumulation rates are presented in Van Metre (U.S. Geological Survey, written commun., 1997) and Van Metre and others (1996, 1997a, and 1997b).
Physical characteristics, determined in the field from one split core from each of the four sites, included water content, particle size, color, and odor (table 2). All four cores examined were water saturated near the top; however, water content decreased with depth and compaction. Particle sizes of core sediments varied from site to site. The predominant particle sizes near the top of the cores consisted of clay and silt. The sediment in the cores became more coarse with depth, consisting of fine and coarse sand and some gravel near the pre-impoundment surface. The color of the cores are described as blends of yellow, tan, gray, brown, black, and olive. At site 3, live and dead Asiatic clams (Corbicula fluminea) were observed. A moderate odor of marsh gas, possibly hydrogen sulfide, was detected at a depth from 5 to 85 cm in the core from site 4. Distinct sediment layers in the cores were readily observed except in zones where clays were disturbed by the core barrel.
Sediment cores from Lake Mead were age-dated by using core depth and observed 137Cs activities (table 3; Van Metre and others, 1997a and 1997b). 137Cs, an anthropogenic by-product of nuclear weapons testing, has a half-life of about 30.1 years. 137Cs first occurred in the atmosphere in about 1952, with peak activities occurring in 1963-64 (Hoffman and Taylor, 1998). 137Cs strongly sorbs to sediments and is a useful tool for age-dating and assessing sediment input to a reservoir or lake from its watershed. Sediment with sorbed 137Cs that enters a reservoir or lake tends to sink and accumulate on the bottom (Krishnaswami and Lal, 1978, p. 153-177; Hoffman and Taylor, 1998).
The cores from Lake Mead are assumed to represent the entire period of deposition from the time impounded water reached the sampling site to the date of collection in 1998. The top of each core (0 cm) was assigned the sampling date. The pre-impoundment interface (from 1935 to 1937 depending on site location) was determined by a change in either the physical appearance of the sediment, its particle-size composition, or the presence of pre-impoundment soil-surface organic matter. Dates corresponding to the first occurrence of 137Cs in the atmosphere (1952) and the peak activity of 137Cs in the atmosphere (about 1964) were determined by evaluating 137Cs activities in the core samples.
Activities of 137Cs (table 3) are plotted against depth in figure 2. Activities and peaks of 137Cs were variable in sediment samples from each site. The 137Cs peaks at sites 1, 2, 3 and 4 were 0.98, 0.76, 0.69, and 0.95 pCi/g, respectively. Many of the 137Cs activities at sites 1 and 4 could not be determined precisely and were reported as non-detected values less than an assigned reporting limit. Site 1 had two identical peaks, one at a depth of about 88 cm and the second at a depth of about 103 cm. The 137Cs peak at a depth of 88 cm was designated as the 1964 peak (Edward Callender and Peter C. Van Metre, U.S. Geological Survey, written commun., 1998) because it was consistent with the 137Cs peak at site 2, which occurred at a depth of about 63 cm. The 137Cs peak at a depth of 103 cm at site 1 was attributed to nuclear testing in 1958 in Nevada (Peter C. Van Metre, U.S. Geological Survey, written commun., 1998). The 137Cs peak at site 3 for 1964 could not be determined with any certainty and was not used to calculate sediment-deposition rates for the periods 1952-64 and 1964-98. The 137Cs peak at site 4 occurred at a depth of about 20 cm. The shallow depth of the 137Cs peak at site 4 could have resulted from the attenuation of peak flows discharging from the Colorado and Virgin Rivers as a result of impoundment of Lake Mead and regulation of the Colorado River above Lake Mead after 1964.
Sediment-deposition rates (table 4) at each site were calculated using mass-accumulation rates to normalize for compaction. These calculations assume that sedimentation rates probably are constant in terms of mass, and that the density of solids is 2.5 g/cm3 (Van Metre and others, 1997a and 1997b). In terms of mass, deposition rates probably became more constant after 1964, when Glen Canyon Dam (fig. 1) began to regulate Colorado River inflow to Lake Mead. Porosity values for subsampled intervals in cores from each site were calculated with the wet and dry weight of sediment samples. These values are listed in table 5.
Sediment-deposition rates calculated for sites 1, 2, 3, and 4 from the pre-impoundment interval to 1998 were 1.45, 1.25, 0.80, and 0.65 (g/cm2)/yr, respectively. The variation of sediment-deposition rates in Lake Mead could be the result of a combination of factors, such as upstream regulation of the Colorado River, rapid urbanization of Las Vegas Valley, rapid episodic erosion in Las Vegas Wash, the complex hydrology of the lake, and, to a lesser extent, urban development in the Virgin-Muddy River Basin.
Sediment-deposition rates calculated for sites 1, 2, 3, and 4 for the pre-impoundment interval to 1952, when 137Cs first occurred in the atmosphere, were 0.41, 1.11, 0.25, and 1.03 (g/cm2)/yr, respectively. The variability of these rates could be attributed, in part, to the different date that each site was initially inundated. Inundation dates determined for sites 1, 2, 3, and 4 were April 1937, June 1935, June 1936, and May 1935, respectively.
Sediment-deposition rates for sites 1, 2, and 4 were calculated from 1952 to 1964 and from 1964 to 1998. The 137Cs peak in 1964 coincided with completion of Glen Canyon Dam upstream of Lake Mead (fig. 1). This impoundment of the Colorado River regulated streamflow and reduced the sediment load to Lake Mead. Deposition rates for site 3 from 1952 to 1964 and from 1964 to 1998 were not calculated because the 137Cs peak for 1964 could not be determined with any certainty.
Sediment-deposition rates calculated from 1952 to 1964 at sites 1, 2, and 4 were 1.68, 1.44, and 1.27 (g/cm2)/yr, respectively. Deposition rates at these three sites from 1964 to 1998 were 1.81, 1.26 and 0.26 (g/cm2)/yr, respectively. The increased rate of sediment deposition at site 1 likely is the result of increased discharge of treated municipal wastewater effluent and subsequent erosion within lower Las Vegas Wash (P.A. Glancy, U.S. Geological Survey, oral commun., 1998). Sediment-deposition rates at site 2 increased after 1952, but decreased after 1964. This decrease could be the result of higher lake levels and regulation of the Colorado River above Lake Mead after 1964. Sediment-deposition rates at site 4, located near the historic thalweg of the Colorado River, significantly decreased after 1964. This decrease also could be the result of the completion of Glen Canyon Dam, which reduced the sediment load into Lake Mead.
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section: Occurrence of Organic Compounds
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