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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Sediment-chemistry data indicate that synthetic organic compounds (OCs and SVOCs) were present in Lake Mead sediment samples. OCs include chlorinated pesticides and degradation products, PCBs, dioxins, and furans. PCBs have been used as plasticizers and hydraulic lubricants, in heat-transfer systems, and in electrical capacitors and transformers (Smith and others, 1998). Dioxins and furans were first produced in the manufacture of herbicides and PCBs. They are formed during municipal waste combustion (Sijm and Opperhuizen, 1996) and commonly are discharged into surface waters by chemical-manufacturing and sewage-treatment plants. SVOCs include PAHs, which originate from anthropogenic and natural sources, and phenols. PAHs mainly are produced by high-temperature reactions such as incineration or fires; however, some are produced commercially for use in pesticides, resins, dyes, cutting fluids, solvents, and lubricants (Smith and others, 1998). Phenols are used to manufacture phenolic resins, herbicides, pharmaceuticals, dyes, plastics, and explosives (Smith and others, 1998).
Concentrations of synthetic organic compounds and their degradation products varied with depth within the sediment cores. Selected synthetic organic compounds, with maximum concentration and depth, for the four sites is listed in table 6.
Dichlorodiphenyldichloroethane (DDD) and dichlorodiphenyldichloroethylene (DDE) are degradation products of dichlorodiphenyltrichloroethane (DDT), an organochlorine pesticide. DDT was first used as a pesticide in 1939 and usage peaked in the United States in the early 1960's (Van Metre and others, 1997b). It was widely used until about 1970 and was banned in 1972. These compounds are considered toxic, relatively stable in the environment, and very resistant to biodegradation. In addition, these compounds have relatively low solubilities in water and a strong tendency to sorb to particulate matter in water, bed sediment, and soil (Van Metre and Callender, 1997). Concentrations of DDD and DDE in the Lake Mead samples are plotted against depth in figure 3. Concentrations of other pesticides sampled from the four sites either were not detected or were detected at estimated concentrations lower than laboratory reporting limits.
DDD concentrations varied with depth at all four sites. These concentrations were greater at sites 1 and 2 than at sites 3 and 4 (table 7). The maximum DDD concentration at site 1 was 11 µg/kg at a depth range of 30-45 cm; at site 2, 38 µg/kg at a depth range of 20-25 cm; at site 3, <1.5 µg/kg at a depth range of 0-5 cm; and at site 4, 2.0 µg/kg at a depth range of 24-35 cm. The maximum DDD concentrations at sites 1 and 2 occurred in sediment dating between 1964 and 1998, whereas the maximum DDD concentration at site 4 was between 1952 and 1964.
DDE concentrations also varied with depth at all four sites. Concentrations in samples from sites 1 and 2 generally were greater than were concentrations in samples from sites 3 and 4 (table 7). The maximum concentration at site 1 was 25 µg/kg at a depth range of 30-35 cm; at site 2, 57 µg/kg at a depth range of 20-25 cm; at site 3, 1.4 µg/kg at a depth range of 30-35 cm; and at site 4, 2.3 µg/kg at a depth range of 18-21 cm. The maximum concentrations at sites 1 and 2 occurred in sediment dating between 1964 and 1998 and the maximum concentration at site 4 was near the 1964 interface.
Although analyses of total PCBs were available for all four sites, most concentrations either were not detected or were reported as estimated concentrations lower than the laboratory reporting limit. The maximum concentration at site 1 was <30 µg/kg at a depth range of 10-15 cm; at site 2, 18.2 µg/kg at a depth range of 20-25 cm; at site 3, <45 µg/kg at a depth range of 0-5 cm; and at site 4, <45 µg/kg at a depth range of 40-45 cm. Analytical results of chlorinated pesticides and PCBs in cores from the four sites are listed in table 7.
Analyses for dioxins were available only for sites 1, 2, and 4. Concentrations of total tetrachlorodibenzo-p-dioxin (TCDD) varied with depth at all three sites, with greater concentrations occurring near the top of the cores (table 8). Concentrations in samples from sites 1 and 2 were significantly greater than in samples from site 4, in which all concentrations were not-detected values. The maximum concentration at site 1 was 39 pg/g at a depth range of 5-10 cm; at site 2, 22 pg/g at a depth range of 15-20 cm; and at site 4, <0.76 pg/g at a depth range of 3-6 cm. TCDD concentrations were relatively constant at sites 1 and 2 before 1964; however, sometime after 1964, TCDD concentrations began to increase. TCDD concentrations at site 1 continued to increase up to the top of the core. TCDD concentrations at site 2 increased for about half the distance of sediment dating between 1964 and 1998, stabilized, then showed a small decrease at the top of the core. Concentrations of TCDD for the three sites are plotted against depth in figure 4.
Concentrations of octochlorodibenzo-p-dioxin (OCDD) varied with depth at all three sites (table 8). OCDD concentrations in samples from sites 1 and 2 generally were greater than in samples from site 4, in which all concentrations were not-detected values. The maximum concentration at site 1 was 93 pg/g at a depth range of 95-100 cm; at site 2, 44 pg/g at a depth range of 5-10 cm; and at site 4, <11 pg/g at a depth range of 3-6 cm. The maximum concentration at site 1 occurred in sediment dating between 1952 and 1964, whereas the maximum concentrations at site 2 were at the top of the core. Concentrations of OCDD for sites 1, 2, and 4 are plotted against depth in figure 4.
Analyses for furan compounds were available only for sites 1, 2, and 4. Concentrations of total tetrachlorodibenzofuran (TCDF) generally were constant at sites 1 and 4, but significantly varied with depth at site 2 (table 9). TCDF concentrations generally were greater in samples from sites 1 and 2 than in samples from site 4, in which all concentrations were not-detected values. The maximum TCDF concentration at site 1 was 150 pg/g at a depth range of 35-40 cm; at site 2, 320 pg/g at a depth range of 15-20 cm; and at site 4, <0.89 pg/g at a depth range of 15-18 cm. The maximum TCDF concentrations at sites 1 and 2 occurred in sediment dating between 1964 and 1998. TCDF concentrations for the three sites are plotted against depth in figure 5.
Octochlorodibenzofuran (OCDF) concentrations were fairly constant at sites 1 and 4, but varied near the upper end of the core at site 2 (table 9). OCDF concentrations generally were greater in samples from sites 1 and 2 than in samples from site 4, in which all concentrations were not-detected values. The maximum OCDF concentration at site 1 was 110 pg/g at a depth range of 35-40 cm; at site 2, 280 pg/g at a depth range of 25-30 cm; and at site 4, <4.8 pg/g at a depth range of 3-6 cm. Maximum OCDF concentrations at sites 1 and 2 occurred in sediment dating between 1964 and 1998. OCDF concentrations for the three sites are plotted against depth in figure 5.
Analyses for PAHs were available for all four sites. Concentrations of benzo[g,h,i]perylene were fairly constant at sites 1, 2, and 3, but varied with depth in samples from site 4 (table 10). Benzo[g,h,i]perylene concentrations in samples from sites 1 and 2 generally were greater than in samples from sites 3 and 4, in which all concentrations were non-detected values or were estimated values lower than laboratory reporting limits. The maximum concentration at site 1 was 16.5 µg/kg at a depth range of 30-35 cm; at site 2, 17.4 µg/kg at a depth range of 20-25 cm; at site 3, estimated as 3.4 µg/kg at a depth range of 0-5 cm; and at site 4, estimated as 7.6 µg/kg at a depth range of 40-45 cm. Maximum concentrations at sites 1, 2, and 3 occurred in sediment dating between 1964 and 1998, while the maximum concentration at site 4 was between 1952 and 1964. Concentrations of benzo[g,h,i]perylene for the four sites are plotted against depth in figure 6.
Concentrations of 2,6-dimethylnaphthalene varied with depth at all four sites (table 10). The maximum concentration at site 1 was 81.9 µg/kg at a depth range of 60-65 cm; at site 2, 123 µg/kg at a depth range of 60-65 cm; at site 3, 70.7 µg/kg at a depth range of 30-35 cm; and at site 4, 77.8 µg/kg at a depth range of 18-21 cm. The maximum concentration at site 1 occurred in sediment dating between 1964 and 1998, and at sites 2 and 4 near the 1964 interface. Concentrations of 2,6-dimethylnaphthalene for the four sites are plotted against depth in figure 6.
Concentrations of indeno[1,2,3-c,d]pyrene at all four sites were fairly constant with depth (table 10). Concentrations at sites 1 and 2 were greater than those at sites 3 and 4, at which all concentrations were either non-detected values or were estimated values lower than laboratory reporting limits. The maximum concentration at site 1 was 15.3 µg/kg at a depth range of 30-35 cm; at site 2, 21.2 µg/kg at a depth range of 80-85 cm; at site 3, estimated as 3.5 µg/kg at a depth range of 0-5 cm; and at site 4, estimated as 5.8 µg/kg at a depth range of 40-45 cm. Maximum concentrations at sites 2 and 4 occurred in sediment dating between 1952 and 1964, whereas the maximum concentration at site 1 was between 1964 and 1998. Concentrations of indeno[1,2,3-c,d]pyrene for the four sites are plotted against depth in figure 7.
Perylene concentrations in samples from sites 2, 3, and 4 varied with depth, whereas perylene concentrations in samples from site 1 were fairly constant with depth (table 10). Perylene concentrations in samples from sites 3 and 4 were significantly greater than in samples from sites 1 and 2, in which most concentrations were estimated values lower than laboratory reporting limits. The maximum concentration at site 1 was estimated as 7.6 µg/kg at a depth range of 10-15 cm; at site 2, 11.3 µg/kg at a depth range of 80-85 cm; at site 3, 197 µg/kg at a depth range of 30-35 cm; and at site 4, 259 µg/kg at a depth range of 75-80 cm. The maximum concentration at site 1 occurred in sediment dating between 1964 and 1998; at site 2, between 1952 and 1964; and at site 4, between the pre-impoundment interface and 1952.
Pyrene concentrations were fairly constant with depth in samples from all four sites. Pyrene concentrations in samples from sites 1 and 2 generally were greater than in samples from sites 3 and 4 (table 10). All concentrations in samples from sites 3 and 4 were estimated values lower than laboratory reporting limits. The maximum concentration at site 1 was 10.7 µg/kg at a depth range of 10-15 cm; at site 2, 8.8 µg/kg at a depth range of 80-85 cm; at site 3, estimated as 3.6 µg/kg at a depth range of 10-15 cm; and at site 4, estimated as 5.8 µg/kg at a depth range of 40-45 cm. Maximum concentrations at sites 2 and 4 occurred in sediment dating between 1952 and 1964, whereas the maximum concentration at site 1 was between 1964 and 1998. Pyrene concentrations in samples from the four sites are plotted against depth in figure 8.
Analyses for phenols were available for all four sites. Phenol concentrations were fairly constant with depth in samples from sites 1, 2, and 3, but varied with depth in samples from site 4 (table 10). All phenol concentrations in samples from site 3 and most concentrations in samples from sites 1 and 4 were estimated values lower than laboratory reporting limits. The maximum concentration at site 1 was estimated as 9.8 µg/kg at a depth range of 20-25 cm; at site 2, 18.3 µg/kg at a depth range of 80-85 cm; at site 3, estimated as 8.2 µg/kg at a depth range of 0-5 cm; and at site 4, 33 µg/kg at a depth range of 50-55 cm. The maximum concentration at site 1 occurred in sediment dating between 1964 and 1998, at site 2 between 1952 and 1964, and at site 4 near the 1952 interface. Phenol concentrations in samples from the four sites are plotted against depth in figure 8. Analytical results for PAHs and phenols in cores from the four sites are listed in table 10.
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