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

conditions just prior to permanent pool formation was
acquired and is hereafter called pre-impoundment data.

In July 1974, the team of Mahan, Herrmann, and stu-
dent assistants were awarded the current contract to
monitor gross and trace inorganic constituents of the
water and sediment of Pueblo Reservoir, its inlet, and
outlet. In June 1974, the impoundment phase of the
project commenced. This publication, therefore, is the
culmination of the past four years' effort in the study
of pre-impoundment and impoundment conditions in
the reach of the Arkansas River now known as Pueblo
Reservoir.

METHODS AND MATERIALS

Water Sampling

Nine sampling sites were chosen for the duration of the
project. Six of the sites were distributed over the reser-
voir surface (fig, 1, sites A-F). In addition, there were
two outlet sites (H and I, fig. 1) selected. The primary
site (H) was 2 miles downstream from the reservoir
near the headgate of the intake of the flume of the
northside Pueblo waterworks, and one (I) was immedi-
ately below the stilling basin at the dam. The location
of the inlet sampling site (G) moved up or down the
river channel, depending on the reservoir level. Sites A
and B were situated so that water conditions on either
side of the old diversion dam could be monitored. The
others were chosen so that fairly representative samples
could be collected over the total area of the reservoir.

Samples from these sites were normally collected on a
monthly basis beginning in June 1974 and continuing
until March 1976. Access problems, usually ice cover,
prevented the collection of samples at some of the sites
from time to time, Reservoir samples were collected at
the surface at 3- by 5-metre depth intervals using a non-
metallic Van Dorn sampler. Samples at the inlet and
outlet sites were collected at the surface only. Temper-
ature and dissolved oxygen were determined as samples
were collected. The water samples were then removed to
the laboratory for immediate processing and analysis.

Sediment Sampling

During the fall of 1973, sediment cores 50-mm in
length and approximately 10 mm in diameter were col-
lected from the Arkansas River flood plain at 31 differ-
ent sites (fig, 2) in areas within the basin to be flooded
by the reservoir. With the aid of the Bureau of Recla.
mation diving team from Denver, three 100- by 10-mm

sediment cores were collecte from each of the three
sites (B, D, and E) in June 974. The samples were
collected by divers from the servoir floor using PVC
(polyvinyl chloride) coring de ices. The same PVC de-
vices were also used to collec sediments from sites C,
F, and G,

Water Sample Processing

&

After being removed to the laboratory immediately
after collection, the samples ere filtered through 0.45
J-lm membrane filters, The fH rate was defined as the
"dissolved fraction" of the sa pie, while the portion of
the sample retained by the ilter was defined as the
"suspended fraction" of the sa pie [1] 1. Any colloidal
or other material in the sam. les less than 0.45 J-lm in
diameter passed through the ,ilter and was counted as
part of the dissolved fraction. The first 200 ml of each
sample was filtered through tared filter which was
then dried at a temperature f 35 to 450C, weighed
and then stored for later diss lution and analysis, The
dissolution was effected usin a HCI-HN03 digestion
procedure [1,2].

Sediment Sample Processing

After drying, 10- to 20-mg s mples were dissolved by
the HCI-HN03 procedure alrea y described. In this case
no Millipore filter was disso ved, only the sediment.
Samples prepared in this m nner were analyzed by
atomic absorption (AA), Ten- g portions of other dried
samples were mixed with gra hite and borax and com-
pressed into a pellet for anal sis by X-ray flourescence
spectro metry,

Analysis

Tables 1 and 2 summarize t e methods used for the
chemical and spectrometric a alysis performed on the
solubilized samples collected uring the course of this
project. The tables give the m thod used for each para-
meter, detection limit, unce ainty, and sample frac-
tion analyzed. The detection I mit was defined as twice
the peak-to-peak noise for te spectrophotometric or
emission measurements as obs rved in the laboratory,

Analysis of the dissolved frac ions were performed as
soon as possible after collect on, To minimize losses,
mercury analyses were alway done the same day the
samples were collected or ot at all. Analyses for
other metals were performed the same day or within

IL

1 Numbers in brackets refer to items in the bibliography.

2