FLOOD10381

• County: Pueblo
• Stream Name: Arkansas River
• Basin: Arkansas
• Title: Effects of Impoundment on Water and Sediment in the Arkansas River at Pueblo Reservoir
• Date: 5/1/1977
• Floodplain - Doc Type: Educational/Technical/Reference Information

The average dissolved zinc for the pre-impoundment
study was 0.014:t 0.015 mg/I, while the average value
obtained at the reservoir inlet were somewhat lower at
0.008 :t 0.009 mg/I and 0.007 :t 0.004 mg/l, respec-
tively, for the 1974-75 and 1975-76 sampling years.
These values compare to the range obtained by Durum,
et al. [10] of 0.010 to 0.050 mg/I for most samples and
the mean of Kopp and Kroner [9] of 0.064 mg/I ob-
tained from the 75.5 percent of the samples in which
zinc was detected.

Plots made to show seasonal trends for the pre-impound-
ment river (fig. 24) produced highly scattered data
points. A similar plot for dissolved zinc (fig. 14) in the
present study shows that the inlet and outlet zinc rises
and falls with the reservoir pool (averages), but the pool
maxima are always higher. The concentration in the
suspended fraction was 0.021 :t 0.030 mg/I and 0.010
:t 0.010 mg/I, respectively, for the two years with a low
average of 0.003 :t 0.005 (March 1976). Higher levels
of suspended zinc occurred generally during the first
sampling year. These values compared to Kopp and
Kroner [9] of 0.062 mg/I mean obtained for the 62
percent of the samples in which zinc was detected. Our
data (even taking into account the differences in meth-
ods of obtaining the means) indicate that the zinc loads
in the Arkansas River and Pueblo Reservoir are lower
than average.

Zinc values tended to be highly variable in both the dis-
solved and suspended fractions. There does not seem to
be a definite seasonal trend (fig. 14) as observed for
most of the gross Iimnological parameters, but rather a
trend that could better correlate with the concentra-
tion of suspended matter carried into the reservoir. Ex-
cellent evidence for this mechanism can be seen by
comparing figure 14 with figure 22 giving the concen-
tration of suspended matter carried into the reservoir.
An excellent correlation is observed and is consistent
with the prediction of Hem [3] that dissolved zinc
concentration is a function of the zinc available in rock
and soil. Another possible important source of zinc is
from exchange with the sediments. As already noted
there were occasions when there was an increase in zinc
concentration with depth, with extreme concentrations
observed at the bottom, A review of the trace zinc con-
tent in the sediments of the reservoir indicate that there
is abundant zinc available for exchange. Furthermore,
it should be noted that the average zinc content in the
reservoir sediments is 4 to 5 times greater than the
trace zinc content of the pre-impoundment flood plain
sediments, which indicates zinc loading into the sedi-
ments. The greatest loading appears to be occurring
around site F, which is near the inlet of the reservoir at
the conservation pool level.

Zinc appears to be a key element in relating sedimenta-
tion of the reservoir to water quality with regard to
trace metals. A more thorough statistical analysis of
the data with regard to zinc would be helpful.

Manganese. - Manganese was perhaps the most inter-
esting element studied because of its unpredictability.
When thermal stratification occurs in lakes and rivers,
it is observed that manganese deposited there under
oxidizing conditions may be dissolved. At pH's near
neutrality the predominant dissolved species would be
the Mn+2, and concentrations as high as 1.0 and 10
mg/I would be stable in aerated water. Small increases
in pH, however, will shift the manganese equilibria to-
ward th.e formation of crystalline oxides, principally
Mn02(s)' Manganese can also exist in dissolved form as
complex ions with organic material and also forms the
MnHC03 complex ion and the MnS04 ion pair, both
of which can be important species in natural water [3].

In this study, manganese was detected at highly vari-
able levels. The average dissolved value obtained for
the reservoir pool was 0.022 :t 0.090 mg/I and 0.016:t
0.016 mg/I for the two sampling years. Those can be
compared to inlet averages of 0.016 :t 0.015 mg/I and
0.020 :t 0.012 mg/I and pre-impoundment river levels
averaging 0.025 :t 0.017 mg/1. A single high value of
0.63 mg/I is reported for August 1975, at the bottom
of the reservoir at site B.

The highest monthly average was October 1974, at
0.044 :t 0.035 mg/I, and the lowest was July 1975,
at 0.007 :t 0.009 mg/1. In the suspended fraction, aver-
ages of 0.022 :t 0.014 mg/I and 0.056 :t 0.058 mg/I
were obtained. June 1975 was the highest monthly
average at 0.20 :t 0.11 mg/I, and the lowest monthly
average was 0.006 :t 0.001 mg/I in December 1975.
These values compare to Kopp and Kroner [9] means
of 0.074 mg/I in the 31 percent of the samples in which
dissolved manganese was detected and 0.105 mg/I in
the 93 percent of the samples in which suspended
manganese was detected.

The data in table 4 indicate that there was abundant
manganese in the flood plain sediments, and sedimenta-
tion during the first year of the reservoir enriched the
sediments further with manganese, particularly at site
F near the inlet.

Figures 15 and 24 plot the seasonal trends observed in
the river and reservoir (1974-75) and the pre,impound-
ment river (1972-74). respectively. The pre-impound-
ment river (fig. 24) does not indicate the classic winter
high/summer low trend, but rather, exhibits widely
scattered points. The plots for the inlet, outlet, and

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