9343 - Laserfiche WebLink

Home Browse Search

Table 6. Percentage of copper, iron, manganese, molybdenum, and zinc loads in the dissolved phase entering and leaving Dillon Reservoir, water year 1997 [All values in percent; Dissolved molybdenum concentrations in some cases were greater than the total molybdenum concentrations. In this case. the dissolved percentage was listed as 100 percent. All concentrations were deemed accurate within laboratory method precision] Element Tenmile Creek Blue River Snake River Total incoming Blue River outgoing Copper 62 60 24 32 88 Iron 2.9 1.8 1.7 2.2 14.4 Manganese 58 28 68 62 55 Molybdenum 100 48 50 100 100 Zinc 56 76 76 73 81 larger grain-size materials settle out quickly as streams enter the reservoir; as a result, all of these samples were predominantly silt and clay. Samples from the three upper arm sites ranged from 77 to 92 percent silt and clay; lower arm sites ranged from 93 to 97 percent; and the main lake site was 98 percent silt and clay. Although the wet sieving and the sorting in the reservoir help to minimize the effects of grain size on trace-element concentrations, concentrations were normalized against the percent content of clay-sized particles prior to comparisons among sites to further reduce possible variability caused by differences in particle-size distributions. Clay-normalized concentrations of trace elements in streambed sediments and surficial reser- voir sediments are shown in figure 6. Differences between each stream and arm of the reservoir and longitudinal differences in the downstream direction are evident on these plots. The heavy metal concentra- tions that were higher in the Snake River streambed- sediment sample (barium, copper, lead, nickel, and zinc) also were higher in sediment samples from the Snake River arm of Dillon Reservoir than in sedi- ment samples from the other arms. In most cases, concentrations in the Blue River and Tenmile Creek were less than those from the Snake River streambed sediment. Among the seven elements shown in figure 6, a common longitudinal pattern is found. Concentra- tions of barium, cadmium, copper, iron, lead, nickel, and zinc display a longitudinal decrease in normalized concentration. The largest decrease in clay-normalized concentration occurs between the streambed sediment and the upper arm of the reservoir. Little change in sediment-core concentration is seen from the upper arm to the dam for the arm with the lowest concentra- tion, typically the Blue River or Tenmile Creek. If there were no additional sources of sediment between the streambed-sampling sites and near the dam in the lower part of the reservoir, and if there were no geochemical alterations of the sediment chemistry within the reservoir, normalized concentrations near the dam would be expected to be the mass-weighted average of concentrations in sediments from the arms. In this situation, the normalized concentration near the dam would be expected to be between those levels observed in the arms. The elements in figure 6 exhibit slightly lower concentrations at the site near the dam than would be expected if the above scenario were the only influence on the sedimentation of trace elements. The decrease in many of the metals in the downstream direction suggests the influence of dilution of the stream sediments by sediment with trace-element concentrations similar to or lower than that of the Blue River, the stream with typically the lowest trace- element sediment concentrations. One logical source of sediment that would act to dilute concentrations of some metals is erosion of bank material in the reser- voir. When a reservoir is built, a new shoreline is formed. It can take years or even decades for this new shoreline to stabilize, and erosion can be a major source of sediment to the reservoir. The shoreline has yet to stabilize around parts of Dillon Reservoir (fig. 7) and, because sedimentation rates in the reser- voir are relatively low (about 0.5 cm/yr at site DLN), bank erosion could be an important contributor of sediment. 18 Identification of Water-Quality Trends Using Sediment Cores from Dillon Reservoir, Summit County, Colorado