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

9.3
9.2
9.1
H 9.0
Z
0
~ 8.9
a
Z
~ 8.8
Z
= 8.~
8.6
8.5
84
- -~-- Measured pH
3
--
--0--- Simulated pH at equilibrium with atmospheric carbon dioxide
- ~-- Simulated pH at equilibrium with atmospheric carbon dioxide and calcite
~~
I
I ~
I ~
I ~
I ~
i
11
I Note: number by symbol is site number in tables 1 and 3
~
I
I 1
I 1
I ~
I ~
I 1
I
1 ~ ~~
i

6
- ~ ~
- ~~/// -'18 ---------~ Qi- 1;
.- j O is.

.
----
a ~;
-°------------------ a
,
2 ~, --,-

i ,
6 __ ~ 12
i
~ 0----
---.-. i ,,,
~ ~~
1 i~
~ 11 ~~~~
3
5
`\\~~~
~
--~-~------ 9i~-~ i ------
6
1
~ __-
12
230 220 200 180 160 140 120 100 80 60 40 20
DISTANCE FROM MOUTH OF YAMPA RIVER, IN RIVER MILES
Figure 6. Measured and simulated pH at Yampa River sites, August 16-19, 1999.
Further hypothetical simulations using
PHREEQC to allow samples collected from Yampa
River sites to precipitate enough calcite to attain equi-
librium with that mineral further decreased pH values
between 0.13 and 0.31 unit (compared to pH at equi-
librium with atmospheric Pip) (fig. 6). These simula-
tions indicate that pH for Yampa River water would be
restricted to the narrow range from 8.42 to 8.50 if it
was in equilibrium with calcite and atmospheric COZ.
Diurnal Measurements
Measurements at Yampa River above Elk River
(site 3 in fig. 1) during August 23-24, 1999, indicated
strong diurnal fluctuations in pH and dissolved oxygen
concentration (fig. 7) as a result of dominance by
photosynthesis during daylight and by respiration plus
oxidation of organic matter during night. Values for
pH ranged from a peak of 9.07 at 1500 hours in the
afternoon of August 23 to a minimum of 7.92 at
0300 hours the next morning (a diurnal change of
1.15 units). Dissolved oxygen concentrations peaked
at 178 percent of saturation at the same time pH
peaked and attained a minimum of 66 percent of satu-
ration at the time of minimum pH. (Because the degree
of saturation of dissolved oxygen is dependent on
water temperature, minimum and maximum concen-
tration of dissolved oxygen does not exactly corre-
spond to minimum and maximum concentration.)
Specific conductance remained relatively constant at
331 to 343 µS/cm over the 23-hour period. The abrupt
decrease in pH value from 9.07 at 1500 hours to 8.84
at 1600 hours apparently was caused by a 15-minute
cloudy period between those measurements; this
temporary inhibition of photosynthesis probably
prevented pH from reaching its maximum potential
that afternoon at this site, a conclusion supported by a
pH of 9.20 at 1600 hours on August 18, 1999.
Diurnal measurements at Yampa River below
Craig (site 6 in fig. 1) during August 23-24, 1999
(fig. 8) also indicated substantial effects of photosyn-
thesis and respiration plus oxidation of organic matter,
although not as strongly as at Yampa River above
Elk River. Values for pH peaked at 8.78 at 1800 hours
on August 23 and attained a minimum of 8.17 at
0600 hours the next day (a diurnal change of 0.61
unit). Dissolved oxygen concentration peaked at 140
percent of saturation at 1600 hours on August 23 and
attained a minimum of 78 percent of saturation at
0600 hours the next day. Specific conductance ranged
from 346 to 380 µS/cm.
12 Evaluation of Trends in pH in the Yampa River, Northwestern Colorado, 1950-2000