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<br />--'''' <br /> <br />0021&9 <br /> <br />effluent at site 6 (about ]40 times) compared to site 3 <br />(about 40 times), Furthermore, some of the effects of <br />photosynthesis at site 6 possibly were caused by <br />discharge of sewage outfall from SSRWWTP, much <br />smaller sewage discharges at Hayden and Mi]ner, and <br />agricu]tural effluent. <br />Downstream from site 6, the effects of photo- <br />synthesis decreased to Yampa River near Maybell <br />(site 8) and further to Yampa River above Little Snake <br />River (site 9), Values of pH decreased slightly to 8,74 <br />at both sites, Dissolved oxygen concentrations <br />became less oversaturated (120 percent at site 8 and <br />105 percent at site 9), and the river became only <br />slightly undersaturated with CO2 (87 percent of satu- <br />ration at site 8 and 86 percent at site 9), <br />Apparently, respiration plus oxidation of plant <br />material and organic matter (approximately the reverse <br />of equation I; Drever, 1982) were more dominant than <br />photosynthesis at Yampa River at Deerlodge Park (site <br />II) and at Yampa River at mouth (Echo Park) (site 12), <br />This conclusion is based on the smaller pH value <br />measured at site II (8,51) and site] 2 (8,60), which <br />resulted from oversaturation of CO2 (189 percent at <br />site 11 and 137 percent at site 12), The apparently <br />abrupt shift to dominance of respiration plus oxidation <br />at site II compared to relative balance between photo- <br />synthesis and respiration plus oxidation at sites 8 and 9 <br />possibly resulted (I) from inflow of nutrient-poor <br />organic matter from the Little Snake River (site 10), <br />(2) from arrival of water with larger concentratious of <br />oxidizable organic matter, possibly flushed from the <br />land surface by rain in the basin on the previous day, <br />(3) from a reduction in aquatic-plant biomass and <br />productivity between sites 9 and 11, or (4) from a <br />combination of these factors, Slight oversaturation of <br />the river water with dissolved oxygen (109 percent at <br />site II and 103 percent at site 12) indicates that less <br />photosynthesis occurred at sites II and 12 than at <br />upstream sites. <br />Except for dissolved nitrogen as ammonia plus <br />organic nitrogen, concentrations of dissolved nutrients <br />(table 3) were small at Yampa River and tributary sites <br />during August ]6-19, 1999, Downstream from Yampa <br />River below Stagecoach Reservoir (site I), concentra- <br />tions of dissolved ammonia were less than 0,02 mg/L <br />as N, and concentrations of dissolved nitrite plus <br />nitrate were less than 0,05 mgfL as N, Concentrations <br />of dissolved nitrogen as ammonia plus organic <br />nitrogen generally decreased from 0,36 mgfL as N at <br />Yampa River below Stagecoach Reservoir (site I) to <br /> <br />0.18 mg/L as N at Yampa River at mouth (site 12), <br />These relations imply that most of the dissolved <br />nitrogen in the Yampa River during this sampling <br />consisted of organic (reduced) nitrogen, Downstream <br />from Yampa River above Elk River (site 3) concentra- <br />tions of dissolved phosphorus were less than <br />0,05 mg/L as P, and concentrations of dissolved ortho- <br />phosphate were less than 0,01 mg/L as p, Bacterial <br />mineralization (oxidation) of reduced nitrogen and <br />phosphorus provided most of the nutrients for photo- <br />synthesis downstream from sites 3 and 6, <br />Calculations using PHREEQC indicate that <br />samples from all Yampa River sites, the Williams Fork <br />(site 7), and the Little Snake River (site 10) were <br />substantially oversaturated with calcite (CaC03) <br />(fig, 5), This relation implies that these waters could <br />not dissolve calcite and possibly were precipitating it. <br />Only water from the Elk River (site 4) was undersatu- <br />rated with calcite, <br />Geochemica] calculations indicate that most of <br />the variation in pH between Yampa River sites was <br />caused by differences in Peo2 (the effective partial <br />pressure of CO2 gas on the solution) and degree of <br />oversaturation with calcite, The geochemical reaction <br />model PHREEQC was used to simulate the effect of <br />allowing the samples collected from Yampa River sites <br />to equilibrate with ambient P CO2 (0,00033 times <br />ambient atmospheric pressure), The resulting pH <br />values from the simulations are in the narrow range <br />from 8,55 to 8,80 (fig, 6) with a relative distribution <br />very similar to that of a]kalinity concentrations (fig, 4), <br />Once the dominating effects of undersaturation and <br />oversaturation with CO2 were removed, pH was <br />large]y a function of, and was proportional to, alka- <br />linity, <br />Simulated equilibration with atmospheric Peo <br />had the greatest effect on the sample from site 3 2 <br />(which was most undersaturated with dissolved CO2 <br />because of photosynthesis), decreasing the pH value <br />by 0,5] unit. The simulated equilibrium also decreased <br />the pH of water from site 5 (by 0,15 unit) and from site <br />6 (by 0.21 unit), Water from these sites also was <br />undersaturated with CO2 (fig, 5), Simulated equilibra- <br />tion with atmospheric CO2 substantially increased the <br />pH value of samples from sites that were substantially <br />oversaturated with CO2 (fig, 5): site I (by 0.34 unit); <br />site 11 (by 0,23 unit); and site 12 (by 0,11 unit), Values <br />for pH at sites 8 and 9 were decreased slightly (by 0,06 <br />unit) by the simulation because they were close to <br />equilibrium with atmospheric CO2 (fig, 5), <br /> <br /> <br />INTERPRETATION OF DATA COLLECTED FOR THIS STUDY 11 <br />