WSP07478

• File Number: 8272.600.60
• Description: Colorado River Basin Salinity Control Program - Basin Member State Info - Utah
• Basin: Colorado Mainstem
• Water Division: 5
• Date: 3/1/1982
• Title: Salt Uptake in Natural Channels Traversing Mancos Shales in the Price River Basin - Utah
• Water Supply Pro - Doc Type: Report/Study

significsnt differences smong shsles and a
continuing significsnt difference between
rv treatments on the seventh day.

c:> The differences were probed once more by
~testing dissolution amounts during the second
Q040-day treatment period. The results in
Table 4.8 cover the entire 85-day period
and thus, according to the results reported
in Table 4,6, would be significant if s
constant dissolution were added during the
second 40-day period. Therefore, Hicks'
(1973) model was used to test for significant
differences among shales, between treatments,
and in interaction between the two. Again,
all three differences were found significant.

The results of these tests have impor-
tant implications. Dissolution rates vary
significantly among shales and with the
history of wetting and drying as the material
moves downstream. The many shale sources and
histories will make it very difficult to
estimate dissolution rates in a -given stream.
Also, the tendency of wetting and drying
cycles to increase dissolution would cause
more of the salts in the bed material of
ephemeral channels to be leached out before
the bed material reaches a larger stream.
Msterial directly entering a perennial stream
may move through the system with much more of
its salt content'in tact. These materials
may continue as an important salt aource
downstream on the Colorado River for years.

Time rates of dissolution

Whitmore (1976) found that when salt
dissolution rates are plotted against the

square root of time a broken curve of
the sort illustrated by Figure 4.16 results.
Accordingly, an attempt was made to fit the
dissolution data with a square root model of
the form:

C a K1 TO.5. . . . . . , . . . . . (4.3)

in which

C The specific conductance in
~mhos, at time T
T Time in minutes
Kl A dissolution

1 n order to determine the effect of
grain size on dissolution rates, accumulated
conductivities were also measured in the
laboratory for shale samples separated by
grain size with the results shown in Table
D.4. Equation 4.3 fit the data with a single
cons tant K1 rather than wi th the breakpoint
shown in Figure 4.16. Eighty percent of the
72-hour conductivity was obtained after a
mean of 9.4 hours, with a standard deviation
of 7.1 hours, as compared to the few minutes
found by Whitmore (1976) for Mancos soil.
The advanced weathering state of the channel
material used by Whitmore probably accounts
for the rapid dissolution that he observed.

The results of the student t-test
analysis for differences by grain size
of the 3D-second and 72-hour conductivity
va lues are presented in Table 4.9. The
significant increase in 3D-second dissolution
for smaller grain sizes is evidence that the
initial rate of salt dissolution increases
with partial surface area.

~ 2000 I
u
. K2
LO j
t\J 0 . \
@ 0. ,
1500 .'
Ul
0
.c
E
:t. 1000
~
>-
....
:> 500
~
::::>
0
z 0
0 0 100 200 300
u
SQUARE ROOT OF TIME (mino.l5)

Figure 4,16, Accumulated conductivity from laboratory salt dissolution.

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