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<br />Variations In Metal Concentrations <br /> <br />Large spatial and temporal variations in <br />concentrations of the metsls of concern occurred <br />during the study. A statistical summary of metal <br />concentrations upstream (at site AR34.5) and <br />downstream (at site AR31.0) from Terrace Reservoir <br />is listed in table 2. The statistics in this summary can <br />be used to compare differences between dissolved <br />and total metals at the same site and differences in <br />metal concentrations upstream and downstream from <br />Terrace Reservoir. The median and maximum concen- <br />trations of the metals of concern were larger upstream <br />from the reservoir than downstream from the <br />reservoir. Upstream and downstream from Terrace <br />Reservoir, the largest concentrations of dissolved <br />aluminum, iron, copper, cadmium, manganese, and <br />zinc generally occurred between mid-June and <br />November (figs. 4-9). Upstream from the reservoir <br />(site AR34.5), the largest concentrations of tots I <br />aluminum, iron, copper, cadmium, manganese, and <br />zinc generally occurred between July and September <br />during storm runoff. After November, dissolved- <br />and total-metal concentrations generally decreased <br />downstream from the reservoir after the closure of <br />the reservoir outlet. Small increases in total-metals <br />concentrations that occurred upstream from the <br />reservoir in February and March may have been <br />related to early spring snowmelt runoff. <br /> <br />Percentage of Metals In the <br />Dissolved and Suspended Fraction <br /> <br />The percentage of the dissolved metal was <br />computed by dividing the dissolved-metal concentra- <br />tion by the total-metal concentration and multiplying <br />by 100. The remainder is computed to be the <br />suspended portion. The percentages are used to <br />provide an indication of the dominant mode of metal <br />transport into and out of Terrace Reservoir. Temporal <br />variations were evaluated by dividing the annual <br />streamflow hydrograph into the discrete flow periods <br />as previously defined (fig. 2). The percentage of <br />dissolved metals computed for samples collected <br />during storm runoff at site AR34.5 are presented <br />separately from other water-quality samples collected <br />between July 15, 1994, through September 30, 1994, <br />because the largest concentrations oftotal aluminum, <br />iron, copper, cadmium, manganese, and zinc that <br /> <br />occurred upstream from the reservoir generally <br />occurred between July and September during <br />storm runoff. <br /> <br />Aluminum <br /> <br />Throughout the study, aluminum was <br />transported into the reservoir predominantly <br />in the particulate or suspended form (fig. 10). <br />More than 90 percent of the aluminum was in the <br />suspended fraction during the pre-peak snowmelt, <br />peak snowmelt, base-flow, and the early spring <br />snowmelt periods. During the post-peak snowmelt, <br />the summer-flow, and the storm-runoff periods, the <br />percentage of dissolved aluminum increased slightly; <br />however, the suspended-aluminum fraction still <br />predominated. <br />Downstream from the reservoir, the <br />suspended-aluminum fraction was predominant <br />only during the pre-peak snowmelt and peak <br />snowmelt periods. During the peak snowmelt <br />period, when estimated reservoir residence times <br />were between 3 and 5 days (Stogner and Edelmann, <br />1996), a large percentage of aluminum was <br />transported out of Terrace Reservoir in the suspended <br />fraction. After June 8 (the end of the peak snowmelt <br />period), most of the aluminum that was transported <br />out of Terrace Reservoir was in the dissolved <br />fraction. <br /> <br />Iron <br /> <br />The temporal variations in the percentage <br />of dissolved and suspended fraction of iron were <br />similar upstream and downstream from Terrace <br />Reservoir. During the pre-peak snowmelt and <br />peak snowmelt periods, more than 90 percent <br />of the iron was transported into and out of the <br />reservoir in the suspended fraction (fig. 10). <br />However, the percent dissolved-iron fraction <br />generally was greater than SO percent during the <br />post-peak snowmelt and the summer-flow periods. <br />The percentage of dissolved-iron fraction upstream <br />from the reservoir decreased to about 25 percent <br />during storm runoff, indicating that most of the iron <br />transported into the reservoir during storm runoff <br />was in the suspended fraction. During the base- <br />flow and early spring snowmelt periods, most of <br />the iron generally entered the reservoir in the <br />suspended fraction. <br /> <br />12 Assessmenl of Metsl Trsnsport Inlo and Out of Terrace Reservoir, Coneloa County, Colorado, <br />Aprlll994 Through March 1995 0 0 31 G 9 <br />