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<br />002533
<br />The profiles of computed organic-nitrogen concentrations (fig. 16) fit
<br />the average observed organic nitrogen within data and model accuracies. The
<br />computed ammonia-nitrogen concentrations (fig. 17) are larger than the
<br />observed concentrations except at site YM-3. The rapid disappearance of
<br />ammonia downstream from site YM-3 is a phenomenon observed on all Colorado
<br />streams surveyed by the Colorado Department of Health (R. D. Anderson, written
<br />commun., 1976). The computed profiles of concentrations of .nitrite-plus~
<br />nitrate nitrogen (fig. 18) from sites YM-O to YM-5 agree within 20 percent of
<br />the observed values, but downstream from site YM-5 the computed values were
<br />consistently larger than observed values. Computed versus observed total-
<br />nitrogen concentrations are shown on figure 19. Observed concentrations were
<br />20 to 40 percent smaller than concentrations computed by both models
<br />downstream from site YM-8.
<br />
<br />The comparison of the results of computed concentrations of organic
<br />nitrogen, ammonia nitrogen, nitrite nitrogen, and ,nitrate nitrogen agree
<br />closely for the U.s. Geological Survey and Pioneer-I models from approximately
<br />site YM-01 to site YM-8. For the reach downstream from' site YM-8, some
<br />variations in the computed values are noted. The larger computed concentra-
<br />tions of nitrite-pIus-nitrate nitrogen for the Pioneer-I model can be ,explain-
<br />ed by its accumulation of nitrogen in the nitrate form. The reason for the
<br />differences in computed organic-nitrogen and ammonia-nitrogen concentrations
<br />between the two models is not known. Some additional tests need to be made
<br />with each model to determine reasons for these differences. Computed total
<br />nitrogen, which was modeled as a conservative constituent, agreed closely be-
<br />tween the two models along the entire study reach (fig.'19).
<br />
<br />There are several possible explanations of the poor fit of computed
<br />versus observed ammonia nitrogen (fig. 17). Willingham (1976) reportei that
<br />ammonia nitrogen in aqueous solutions exists in two states, ionized NH4 and
<br />nonionized NH3' In its nonionized (NH3) state, ammonia can escape as a gas
<br />from water. According to Willi~gham (1976), the partitioning of total ammonia
<br />between non ionized NH3 or NH4 forms is dependent primarily on pH and
<br />temperature conditions. The estimated ranges of ammonia as NH3 during the 24-
<br />hour sampling period are shown in table 3. The percentage of nonionized NH3
<br />in most instances is less than 15 percent, which suggests that the loss of
<br />nitrogen as gaseous ammonia to the atmosphere is small. A second explanation
<br />is the use of the ammonia nitrogen by plants in the stream environment, for,
<br />example, by the different algal forms. Kittrell (1969) reported that ammonia
<br />nitrogen can be assimilated by algae and changed to organic nitrogen by algae.
<br />It was noted that DO is not utilized in this process. Kittrell also pointed
<br />out that organic nitrogen, changed to ammonia nitrogen and oxidized to nitrate
<br />nitrogen, can be assimilated quickly by algae, which reconvert the nitrogen to
<br />protein as an organic-nitrogen form. Dissolved oxygen is used in this latter
<br />process. Because of the small oxygen sag in any part of the study reach
<br />(fig. 15), it is assumed for the Yampa River that nitrogen most probably is
<br />being lost as ammonia. The amount of DO used by the oxidation of organic and
<br />ammonia nitrogen to nitrate nitrogen is accounted for by both models, with a
<br />rate of 4.57 units of DO for each unit of nitrogen oxidized.
<br />
<br />28
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