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

reservoir averages in figure 15 are more interesting. The
river at the inlet curve does follow a classic winter highl
summer low seasonal trend, while the sketch for the
reservoir averages does not consistently follow such a
trend. The data in figure 15 further indicate that the
dissolved manganese at the inlet is consistently higher
than at the outlet, and the pool average (at least at the
peaks) is higher than either. The behavior of the man-
ganese in the reservoir pool can be readily explained
in terms of exchange of manganese with the sediments.
As mentioned previously in this report, there were
sharp depth profiles observed at times (particularly at
sites A and B) for temperature, D.O., zinc, iron, man-
ganese, and others. This particular trend was observed
for manganese more consistently than most other para-
meters. The extreme values seem to account for the
high averages obtained for the reservoir at unexpected
times. The suspended material does not seem to con-
tain enough soluble manganese to account for some
of the extreme values observed. Therefore, at times
the manganese must be released by the sediments into
the water. This mechanism appears to be working both
ways. Figures 19 and 20 show the surface trends ob-
served for several parameters, including manganese for
the November 1974 sampling. All the parameters show
slight to very marked decreases, moving from the out-
let to the inlet through the reservoir pool, except man-
ganese, which shows a very marked increase, This indi-
cates that during this period the dissolved manganese
is being precipitated in the reservoir and is enriching
the sediments. The sediment data (table 4) indicate
that this mechanism may be very important at the up-
per end of the reservoir. The composition of the sedi-
ments at sites E and F appear to be significantly en-
riched in manganese relative to the rest of the reservoir
and the pre-impoundment flood plain sediments (sites
19 and 24 excepted).

Iron. - Hem [3] discusses the solubility controls on
iron, Iron can be dissolved to a considerable extent in
the form of the ferrous ion (Fe+2), which is observed
in ground and other unoxygenated water. However, in
flowing streams and other oxygenated water, iron will
be oxidized to the ferric ion (Fe+3) and, at pH's en-
countered higher than three, vyill be precipitated as
F~(OH)3 or FeC03' However, there are other mechan-
isms by which iron can exist as part of the dissolved
fraction. One is in the form as colloidal ferric hydrox-
ide (Fe(OH)3), another of complexed iron if organic
material is present, and still another is the formation of
ion pairs such as FeS04'

Kopp and Kroner [9] reported a mean of 0.052 mg/l
dissolved in surface waters of the United States and 3.0
mg/l in the suspended fraction. The reservoir pool in
this study averaged values of 0,006 :t 0.004 mg/l and

0.005 :t 0.003 mg/l dissolve iron, respectively, in the
2-year study. Correspond in average concentrations at
the inlet were 0.004 :t 0,0 mg/l and 0.005 :t 0.006
mg/l. In the pre-impound me t study an average concen-
tration of 0.030 :t 0.016 m II was obtained. It should
be noted, however, that a hange in analytical proce-
dure was made going from t e pre-impoundment to the
post-impoundment study. D ring the pre-impoundment
study, iron was analyzed by irect aspiration AA, Most
of concentrations observed re very close to the detec-
tion limits, In the post-imp undment phase, the more
sensitive extraction techniqu was used.

In the suspended fraction, orresponding averages for
the pool were 0.53 :t 0,87 g/l and 0,31 :t 0,31 mg/l
and for the inlet 1.80:t 3.3 g/l and 0.30 :t 0.42 mg/l.
The standard deviations obtained indicate a high degree
of scattering.

The sediment data are also i teresting. An average com.
position of about 1.6 perce t Fe was obtained by AA
for pre-impoundment flood- lain samples, and nowhere
did iron exceed 2.0 percent in composition. However,
the sediments at sites E and were running 2.6 and 2.7
percent iron, respectively, a ter 1 year, indicating pos-
sible accumulation of iron i the sediments, The X-ray
analysis gave lower values, ut the same trends were
observed.

The trends observed with di solved iron are quite inter-
esting as was the case with zinc and manganese. The
seasonal trends as plotted in igure 16 for the inlet, out-
let, and pOol average for t e dissolved fraction com-
pared to the 1972-74 Arka sas River (fig, 24) appear
to correlate fairly closely ith peaks occurring from
early spring through the su mer and into the fall with
lowest values during midwi ter, almost an inverse sea-
sonal effect. At first glance it might appear that, like
zinc, the maxima for dissolv d iron correlates well with
the suspended matter load f the river and reservoir,
However, close comparison ith figure 22 reveals that
iron correlates with streamf ow rather than suspended
matter.

Figure 21 does not indicate pronounced surface trend
for iron during November 1974. Plots showing spatial
trends produced, in most c. ses, scattered data points.
One exception was March 1 75, in which the iron con-
centration was observed to b increasing as it moved to-
ward the inlet. Depth profil s for iron were commonly
observed at the same time and at the same sites as
were observed for manganes and zinc.

Further examination of the ron data may prove useful
in elucidating depositional m chanisms.

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