WSP07510

• File Number: 8200.760
• Description: Yampa River General
• State: CO
• Basin: Yampa/White
• Water Division: 6
• Date: 1/1/1981
• Author: USGS
• Title: Assessments of Impacts of Proposed Coal-Resource and Related Economic Development on Water Resources - Yampa River Basin - Colorado and Wyoming - A Summary - 1981
• Water Supply Pro - Doc Type: Report/Study

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either a smaller scale or combined with one of the
other technologies. 9A~-lhFough cooling is a
technology with some favorable aspects (small
rates of evaporation, small capital cost, and small
plant-efficiency losses), as well as some un-
favorable aspects (damage to organisms from the
pumps and condensers and ecological changes due
to increased water temperatures). It is included in
this analysis to facilitate the comparison of a range
of development alternatives. In this manner, com-
binations of those options can be determined,
which would be a realistic compromise among
various economic, environmental, and social
objectives.
For the second analysis. the cooling mechanism
was assumed to he a cooling pond in the shape of a
long sinuous canal covering 4,230 acres (l.71O ha)
(fig. 3B). By simulation of the cooling process in
this pond, considering the range of conditions
prevailing in the Yampa River basin, the estimated
annual evaporation from the pond would be 4l,500
acre-ft (51.2 million m3) (table 6). This evaporation
would be larger than for once-through cooling,
because this value includes both the natural and
the waste-heat-induced evaporation from the pond.
To compensate for the 41,500 acre-ft (51.2 mil-
lion m3) evaporative loss, a volume of water would
need to be withdrawn from the river. If the volume
of withdrawn water equaled the evaporation, then
the cooling water would accumulate dissolved
solids at a rate of 7,850 tons (7.120 t) per year. To
avoid this, an amount of water equal to 10 percent
of the evaporation rate would need to be constantly
removed from the cooling system and returned to
the river (this is called blowdown). Consequently,
the withdrawal rate would need to be 10 percent
greater than the evaporation rate. The annual
withdrawals would total 45, 700 acre-ft (56.3 million
m') and the dissolved-solids load of the river would
be unchanged due to this water use. The flow of the
Yampa River would be diminished by 3 percent,
and the dissolved-aolids concentration would in-
crease by 4 mg/L (table 6).
Compared with once-through cooling, the effects
on the river's biota using a cooling-pond system
would be very small. The withdrawal of water
would be 2 percent of that needed for once-through
cooling, and the blowdown would amount to 0.4
percent of the total annual flow of the Yampa River
near Maybell. However, a pond system would af-
fect the terrestrial habitat by converting 4,230
acres (1,710 ha) from land to water. It also would

have some effects on local climate, due to the vast
amount of heat and vapor transferred to the air
over this area. Once-through cooling, by com-
parison, would spread the effects over a stream
reach many miles in length.
A wet-cooling water tower rejects waste heat by
bringing the cooling water into contact with mov-
ing air as the water falls over baffles, causing it to
form small droplets. The water is taken from a col-
lecting basin at the base of the cooling tower, pas-
sed through the powerplant condenser, and
pumped to the top of the tower. As the water falls
through the tower, it moves past a large volume of
air driven either by fans or by natural buoyancy.
During this period of contact. some of the water
evaporates. In addition, the water is cooled by the
transfer of heat (conduction), The air leaving the
tower generally is saturated and is at a greater
temperature than it was when it entered the tower.
Because part of the water is evaporated as it
passes through the tower and a small amount
drifts out of the tower structure, water must con-
stantly be added to the system to keep the volume
of water in the system from decreasing. In addition,
some water must be added to prevent an excessive
increase of dissolved solids in the system (fig. 3C).
In a cooling tower, the fraction of waste heat that
is rejected by evaporation is an increasing function
of the ambient air temperature and a decreasing
function of the relative humidity and altitude. By
calculating the rate of evaporation for the range of
conditions typical of the Yamps River basin, the
annual rate of evaporation would be 40,400 acre-ft
(49.8 million m3) (table 6). This blowdown would
equal about 10 percent of the evaporation, so the
total rate of annual withdrawal from the river
would be 44,400 acre-ft (54.7 million m3).
Assuming an operating system with no direct
discharge of residuals to the river, the blowdown
(4,040 acre-ft or 5.0 million m3 per year) could be
used for transporting ash from the powerplant
boilers for discharge into a system of evaporation
ponds. For such a system, there is an unresolved
question as to the ultimate fate of the residue in the
evaporating ponds. Whether these residuals
(predominantly salts and sulfur compounds) can
be prevented from leaching back into the nearby
ground water or surface waters is unknown. An
ongoing study at the Hayden powerplant may
provide some site-specific information on the possi-
ble long-term effects of this form of disposal (S. R.
Ellis, written commun., 1978).

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