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as much CO2 gas was removed by photosynthesis at that site as was removed at Yampa River above Elk River on August 18, 1999. The gas-removal plots (fig. 16) indicate that pH would equal or exceed the Colo- rado water-quality standard of 9.0 if at least 54 percent of the calculated amount of CO2 gas removed from site 3 was removed from these samples. The pH predictions in figure 16 assume that relative proportions and concentrations of major ions (especially alkalinity) remain constant. To estimate the effect of increased salinity on maximum potential late- afternoon pH, the synoptic sample collected at Yampa River at Deerlodge Park (site 11, fig. 1) on August 18, 1999, was selected for simulations of increased salinity and photosynthetic removal of COz gas-conditions that might occur during prolonged drought. This sample was selected for these simula- tions because of its sampling location near the upstream boundary of Dinosaur National Monument and its sampling date during the time of the year when pH is at or near a seasonal maximum and when drought conditions are most likely to occur. PHREEQC was used to simulate 150 percent of constituent concentrations by evaporating one-third of the water in the sample. This concentrated sample was simulated to attain equilibrium with ambient atmo- spheric PCp2, followed by stepwise removal of 209 µmol/L of CO2 gas (100 percent of the amount of CO2 gas removed at Yampa River above Elk River on 9.5 9.4 ~ 9.3 H Z ~ 9.2 Q 9.1 D Z ~ 9.0 z 8.9 2 Q 8.8 8.7 8.6 0 20 40 60 August 18, 1999). The simulation was repeated to simulate a 200-percent concentration of the original sample (by removing one-half of the sample water) and stepwise removal of 209 µmol/Lof CO2 gas. The results of these simulations (fig. 17) show that, relative to sampled concentrations with 209 µrrlol/L of COZ gas removed (the lower curve), a 150-percent increase in constituent concentrations would cause a pH about 0.07 unit higher and a 200-percent increase (the upper curve) would cause a pH about 0.13 unit higher. Furthermore, the original sample would have a pH of 9.0 at about 60 percent of CO2 gas removal, whereas, at 150 percent of sampled concentrations, pH would be 9.0 at about 34 percent of CO2 gas removal, and, at 200 percent of sampled concentrations, pH would be 9.0 at about 10 percent of COZ gas removal. These results indicate that drought conditions potentially could cause late-afternoon pH to sustain values above the Colorado water-quality standard of 9.0. Maximum potential late-afternoon pH (fig. 16) would be 0.03 to 0.06 unit higher for samples collected during August 16-19, 1999, than for samples collected during March 13-16, 2000, despite greater alkalinity during the latter period. The higher simu- lated pH during August 1999 can be attributed to the greater amount of CO2 gas removed to simulate photo- synthesis and the smaller solubility of dissolved CO2 gas in the warmer water. ----a----~ Sampled concentrations (table 3) -~ ^ ----~- -- 1.5 times sampled concentrations ___-.^-"' ----~-- -- 2.0 times sam led concentrations "^ ~ ~". -.. ---~--- -,~---"- --~ Colorado water-quality standard - ~--- " ,,^--- " 209 micromoles per ~-"-- liter of carbon dioxide ~,.-- gas removed at Yampa River above Elk River •"~ 80 100 120 140 160 CARBON DIOXIDE GAS REMOVED AS PERCENTAGE OF CARBON DIOXIDE GAS REMOVED AT YAMPA RIVER ABOVE ELK RIVER Figure 17. Simulated maximum potential pH for water sample collected at the Yampa River at Deerlodge Park on August 18, 1999, at varying salinities. 24 Evaluation of Trends in pH in the Yampa River, Northwestern Colorado, 1950-2000