9431

• UCREFRP Catalog Number: 9431
• Author: Chafin, D. T.
• Title: Evaluation of Trends in pH in the Yampa River, Northwestern Colorado, 1950-2000.
• USFW Year: 2002.
• USFW - Doc Type: Denver.
• Copyright Material: NO

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~ •••••a•• •• Simulated pH at equilibrium with atmospheric C02
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- Simulated pH at equilibrium with atmospheric COp and calcite
~ Note: number by symbol is site number in tables 1 and 3
•1
200 180 160 140 120 100 80 60 40 20
DISTANCE FROM MOUTH OF YAMPA RIVER, IN RIVER MILES
7.6
230 220
Figure 15. Measured and simulated pH at Yampa River sites, March 13-16, 2000.
site 3, 209 µmoUL of C02 gas was removed from river
water at site 3 by photosynthesis on August 18, 1999,
and 172 µmol/L was removed on March 13, 2000.
PHREEQC was then used to simulate equilibration of
synoptic samples from sites 8, 9, 11, and 12 (fig. 1)
with ambient atmospheric Pao (corrected for eleva-
tion), followed by stepwise removal of 209 µmol/L of
COZ gas from synoptic samples collected from those
sites during August 16-19, 1999, and of 172 µmoUL
of COz gas from synoptic samples collected during
March 13-16, 2000. These simulations predict pH
(fig. 16) that would result from removal of the same
amounts of C02 gas (relative to ambient C02 concen-
trations) as was removed from site 3 by photosynthesis
during each sampling period. For conditions at site 3
to encroach to a given point in the lower basin, nutrient
loads in the upper basin would have to increase so that
algae proliferated to its maximum density in the river
to that point. A deterministic, periphyton-algae, river
model would be required to predict the degree of
photosynthesis in the lower basin, taking into account
nutrient loads, exposure to and penetration of light,
channel stability, and other river characteristics.
Therefore, the following simulations should not be
interpreted to indicate that conditions at site 3 could
actually develop at sites 8, 9, 11, and 12. For example,
productivity downstream of site 11 might be limited
by shading of much of the river in Yampa Canyon.
Results of the simulations are shown in figure 16.
Estimates of maximum potential late-afternoon
pH resulting from the simulations (fig. 16) indicate
that pH hypothetically could reach values of 9.10 to
9.12 in the lower Yampa River Basin at sites 8, 9, 11,
and 12 (fig. 16) during lowland runoff in late March.
USGS records indicate that pH at Yampa River near
Maybell was 9.1 at 1200 hours on March 17, 1992,
indicating a similar amount of photosynthesis as at
Yampa River above Elk River on March 13, 2000. The
measured pH of 9.01 at Yampa River near Maybell on
March 15, 2000 (an overcast day), indicates that about
115 µmol/L of CO2 gas (67 percent of the 172 µmol/L
removed from water at site 3 on March 13, 2000) was
removed from that water. The gas-removal plots
(fig. 16) indicate that pH would equal or exceed the
Colorado water-quality standard of 9.0 if at least
67 percent of the calculated amount of COZ gas
removed from site 3 was removed from these samples.
Hypothetically, pH could reach values of 9.14 to
9.18 at sites 8, 9, 11, and 12 during late summer
(fig. 16). USGS records indicate that pH at Yampa
River near Maybell was 9.1 at 1430 hours on
September 18, 1984, indicating that about 75 percent
22 Evaluation of Trends in pH in the Yampa River, Northwestern Colorado, 1950-2000