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

effluent at site 6 (about 140 times) compared to site 3
(about 40 times). Furthermore, some of the effects of
photosynthesis at site 6 possibly were caused by
discharge of sewage outfall from SSRWWTP, much
smaller sewage discharges at Hayden and Milner, and
agricultural effluent.
Downstream from site 6, the effects of photo-
synthesis decreased to Yampa River near Maybell
(site 8) and further to Yampa River above Little Snake
River (site 9). Values of pH decreased slightly to 8.74
at both sites. Dissolved oxygen concentrations
became less oversaturated (120 percent at site 8 and
105 percent at site 9), and the river became only
slightly undersaturated with COZ (87 percent of satu-
ration at site 8 and 86 percent at site 9).
Apparently, respiration plus oxidation of plant
material and organic matter (approximately the reverse
of equation 1; Drever, 1982) were more dominant than
photosynthesis at Yampa River at Deerlodge Park (site
11) and at Yampa River at mouth (Echo Park) (site 12).
This conclusion is based on the smaller pH value
measured at site 11 (8.51) and site 12 (8.60), which
resulted from oversaturation of C02 (189 percent at
site 11 and 137 percent at site 12). The apparently
abrupt shift to dominance of respiration plus oxidation
at site 11 compared to relative balance between photo-
synthesis and respiration plus oxidation at sites 8 and 9
possibly resulted (1) from inflow of nutrient-poor
organic matter from the Little Snake River (site 10),
(2) from arrival of water with larger concentrations of
oxidizable organic matter, possibly flushed from the
land surface by rain in the basin on the previous day,
(3) from a reduction in aquatic-plant biomass and
productivity between sites 9 and 11, or (4) from a
combination of these factors. Slight oversaturation of
the river water with dissolved oxygen (109 percent at
site 11 and 103 percent at site 12) indicates that less
photosynthesis occurred at sites 11 and 12 than at
upstream sites.
Except for dissolved nitrogen as ammonia plus
organic nitrogen, concentrations of dissolved nutrients
(table 3) were small at Yampa River and tributary sites
during August 16-19, 1999. Downstream from Yampa
River below Stagecoach Reservoir (site 1), concentra-
tions of dissolved ammonia were less than 0.02 mg/L
as N, and concentrations of dissolved nitrite plus
nitrate were less than 0.05 mg/L as N. Concentrations
of dissolved nitrogen as ammonia plus organic
nitrogen generally decreased from 0.36 mg/L as N at
Yampa River below Stagecoach Reservoir (site 1) to
0.18 mg/L as N at Yampa River at mouth (site 12).
These relations imply that most of the dissolved
nitrogen in the Yampa River during this sampling
consisted of organic (reduced) nitrogen. Downstream
from Yampa River above Elk River (site 3) concentra-
tions of dissolved phosphorus were less than
0.05 mg/L as P, and concentrations of dissolved ortho-
phosphate were less than 0.01 mg/L as P. Bacterial
mineralization (oxidation) of reduced nitrogen and
phosphorus provided most of the nutrients for photo-
synthesis downstream from sites 3 and 6.
Calculations using PHREEQC indicate that
samples from all Yampa River sites, the Williams Fork
(site 7), and the Little Snake River (site 10) were
substantially oversaturated with calcite (CaC03)
(fig. 5). This relation implies that these waters could
not dissolve calcite and possibly were precipitating it.
Only water from the Elk River (site 4) was undersatu-
rated with calcite.
Geochemical calculations indicate that most of
the variation in pH between Yampa River sites was
caused by differences in PCp2 (the effective partial
pressure of C02 gas on the solution) and degree of
oversaturation with calcite. The geochemical reaction
model PHREEQC was used to simulate the effect of
allowing the samples collected from Yampa River sites
to equilibrate with ambient PCp2 (0.00033 times
ambient atmospheric pressure). The resulting pH
values from the simulations are in the narrow range
from 8.55 to 8.80 (fig. 6) with a relative distribution
very similar to that of alkalinity concentrations (fig. 4).
Once the dominating effects of undersaturation and
oversaturation with C02 were removed, pH was
largely a function of, and was proportional to, alka-
linity.
Simulated equilibration with atmospheric PCo
had the greatest effect on the sample from site 3 2
(which was most undersaturated with dissolved C02
because of photosynthesis), decreasing the pH value
by 0.51 unit. The simulated equilibrium also decreased
the pH of water from site 5 (by 0.15 unit) and from site
6 (by 0.21 unit). Water from these sites also was
undersaturated with C02 (fig. 5). Simulated equilibra-
tion with atmospheric C02 substantially increased the
pH value of samples from sites that were substantially
oversaturated with COZ (fig. 5): site 1 (by 0.34 unit);
site 11 (by 0.23 unit); and site 12 (by O.ll unit). Values
for pH at sites 8 and 9 were decreased slightly (by 0.06
unit) by the simulation because they were close to
equilibrium with atmospheric C02 (fig. 5).
INTERPRETATION OF DATA COLLECTED FOR THIS STUDY 11