WSP03775

• File Number: 8507
• Description: Rio Grande Project
• State: CO
• Basin: Rio Grande
• Date: 6/1/1996
• Title: Assessment of Metal Transport into and out of Terrace Rervoir/ Conejos County/ Colorado/ April 994 through March 1995
• Water Supply Pro - Doc Type: Publication

Throughout the study, aluminum was
transported into the reservoir predominantly in
the suspended fraction (fig. 10). By comparison,
most of the aluminum transported out of the reservoir
was predominantly in the dissolved fraction, except
during the pre-peak and peak snowmelt periods
(fig. 10).
About 81 percent of the 363 tons of total
aluminum that entered the reservoir was not
transported out of the reservoir, indicating that the
reservoir was a sink for an estimated 294 tons of
aluminum (table 5; fig. 11). Although 81 percent
of the aluminum was not transported out of the
reservoir during the study, about 68.5 tons was
transported downstream to the Alamosa River, and
most ofthis load (about 63 percent) was transported
out of the reservoir during the peak snowmelt period
from May 15 through June 8, 1994 (table 5; fig. 11).
Although most of the total-aluminum load was not
transported out of the reservoir, about 75 percent of
the dissolved-aluminum load that entered the reservoir
was transported out of the reservoir during the study
(table 5).

Iron

Large variations in iron loads occurred
during the study. The largest daily total-iron loads
were transported into and out of Terrace Reservoir
during the peak snowmelt period (fig. 12). During
the peak snowmelt period, the maximum daily total-
iron load that entered the reservoir was about 25 tons
(tsble 7). Large daily total-iron loads also were
transported into the reservoir during rainstorms
during the summer period, and like total-aluminum
loads, almost all the total-iron loads that entered
the reservoir during rainstorm runoff remained in
the reservoir. The smallest daily iron loads occurred
during the base-flow period between November 1994
and February 1995 (fig. 12).
During the pre-peak and peak snowmelt
periods, more than 90 percent of the iron was
transported into and out of the reservoir in the
suspended fraction (fig. 10). However, most of
the iron transported into and out of the reservoir
during the post-peak snowmelt and summer-flow
periods was transported in the dissolved fraction.
During these periods, about 60 percent of the iron
entering the reservoir generally was in the dissolved
fraction; and generally more than 70 percent of the
iron being discharged from the reservoir was in the
dissolved fraction (fig. 10).

An estimated 75 percent of the 790 tons of
total iron that entered the reservoir remained in the
reservoir, indicating that the reservoir was a sink
for an estimated 596 tons of iron (table 5; fig. 12).
Although 75 percent of the iron was not transported
through the reservoir, about 194 tons was transported
downstream to the Alamosa River, and most of this
load (about 73 percent) was transported out of the
reservoir during the peak and post-peak snowmelt
periods from May 15 through July 14, 1994 (table 5;
fig. 12).
Between April 1 and June 8, 1994, most
of the dissolved iron that entered the reservoir
probably was in colloidal form (that is, less than
0.45 J.1m), which aggregated, then settled from the
reservoir water column under the low gradient
conditions of the reservoir, resulting in almost a
75-percent loss in dissolved iron at site AR31.0
(downstream from Terrace Reservoir). Between
June 9 and September 30, 1994, about 50 percent
of the dissolved iron that entered the reservoir was
transported out of the reservoir (table 5).

Copper

The maximum daily total-copper load
of about 3,770 Ibs (about 1.9 tons) entered the
reservoir on June 8, about a week later than the
maximum daily total-aluminum and total-iron loads
(table 7). This was a substantially smaller load than
the daily maximum total-aluminum and iron loads
that entered the reservoir. Although the maximum
daily load occurred at the end of the peak snowmelt
period, the largest copper loads were transported
into and out of the reservoir during the post-peak
snowmelt period (fig. 13). Almost 45 percent of
the annual total-copper load was transported into
and out of Terrace Reservoir during the post-peak
snowmelt period (table 5), which corresponded
to a period when streamflow had substantially
diminished. The smallest daily total-copper loads
occurred during the base-flow period between
November 1994 and February 1995 (fig. 13).
During the pre-peak and peak snowmelt
periods, most of the copper transported into and
out of the reservoir was in the suspended fraction.
During these periods, about two-thirds of the
approximately 18.5 tons of total copper that entered
the reservoir (table 5) remained in the reservoir and
was not discharged downstream to the Alamosa River.

30 AsHssment 01 Me..1 Transport Into snd Out 01 Terrsce ReservoIr, Conelos County, Coloreclo,
April 1994 Through March 1995

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