WSP10787

• File Number: 8170
• Description: Arkansas Basin Water Quality Issues
• State: CO
• Basin: Arkansas
• Water Division: 2
• Date: 9/1/1981
• Author: Bureau of Reclamati
• Title: Heavy Metals Pollution of the Upper Arkansas River - Colorado - and its Effects on the Distribution of the Aquatic Macrofauna
• Water Supply Pro - Doc Type: Report/Study

The order of decreasing ease of reduction of the
most important heavy metals is: manganese,
zinc. iron, cadmium. lead, and copper. However.
this order is a function of all the constituent ele-
ments and their chemical environments and is
only a general indication of mobility under reduc-
ing conditions (Mortimer. 1942 1114]). Zinc,
being highly toxic and relatively easily reduced.
would likely be the most severe heavy metal pol-
lution problem in the upper Arkansas River. Cad-
mium may also be a serious problem due to its
greater toxicity to aquatic organisms.

California Gulch and AR-3 contain visible evi-
dence of a precipitated iron layer. Hutchinson
(1957) 181] and Cole (1979) 1281 consider
this oxidized (ferric) iron layer an effective bar-
rier to the diffusion of reduced substances from
the deeper layers into the overlying water.
Reduction of this oxidized layer to ferrous com-
pounds allows other reduced metals in the sedi-
ment to move into the water. An E, value of
100 mV or below would indicate reduction of
sulfate to sulfide, and ferrous sulfide and other
heavy metal sulfides are generally insoluble and
tend to precipitate (Sartoris et aI., 1977
11331 ). This phenomenon, however, may only
occur under anaerobic conditions. Reducing en-
vironments also favor slower degradation of
organics and the formation of structurally com-
plex, large-molecular-weight molecules which
are effective in binding metals (Gambrell et al..
1976 f60/ ). These electrochemical factors and
the uncertainty of which elements are in ionic or
labile forms make precise heavy metals toxicity
predictions difficult.

Current

Water velocities determined by discharge meas-
urements are questionable in correlations with
suitable habitat for macroinvertebrate because
the velocity adjacent to the substrate is normally
much slower than the calculated average stream
velocity (Cummins. 1962 131) I. It was ob-
vious. however, that velocities were high
enough to suspend sediments and some degree
of benthic scour had occurred during the spring
runoff. Concentrations of metals generally were
higher downstream of AR-3, indicating that
sediments were suspended in the areas of the
Leadville Drain and California Gulch and carried
downstream. and higher metal loads were enter-
ing the river through increasing inflows.

Total numbers of benthic organisms were
generally lower at all stations, except EF-1, dur-
ing spring runoff. This may have been due to dif-
ficulty in sampling during high flows, or the
emergence of aquatic insects; however.
Bogatov (1978) 1121 found a sharp depletion of
benthos in riverbeds following a flood period. An
increase in Saetis abundance at AR-3 following
spring runoff indicates that metal-laden sedi-
ments were scoured from the streambed and
that the habitat improved for a time.

Substrate

The substrate sampled was relatively uniform
throughout the study area. Slightly larger stones
were present at the downstream stations. as
were deeper channels and pools. The larger sub-
strate at AR-8, particularly, was likely the reason
for a reduction in the mean d for macroinverte-
brates at this station. This substrate was not as
preferred. sampling was more difficult, or both.
Riffle-type habitat and riparian vegetation
dominated in the upper section of the river and
became less abundant in the lower reaches.

Ephemeropterans were more abundant in the
areas of shallow riffles and trichopterans were
more abundant in deeper channel sections. The
abundance of trichoptemns in the river below
the Lake Fork inflow may reflect an increase in
food supply being released from the upstream
reservoir (Muller. 1955, 1956 [116. 117]).
Dipterans were usually found associated with
organic debris during the winter and late sum-
mer. The genera of macroinvertebrates found on
this substrate indicated that the upper Arkansas
River contained rocks and had a moderate-to-
fast streamflow (Hart and Fuller, 1974 1701 ).
This accurately describes the river in the study
area.

Water Chemistry

The major ion properties of the water in the
study section of the Arkansas River are similar to
most high mountain streams in Colorado. The
concentration of any particular element or com-
pound usually increases below pollution sources
and decreases below freshening inflows
(LaBounty et al., 1975 1941 ).

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