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0240 It Ammonia concentrations remained relatively stable downstream to Las Animas. Substantial increases in ammonia concentrations (625 percent) occurred between Las Animas and just downstream from John Martin Reservoir (fig. 25). The occurrence of high ammonia concentrations immediately downstream from John Martin Reservoir, particularly during winter, is suggestive of anoxic conditions in John Martin Reservoir. As a result, the reduction of nitrogen compounds, such as nitrate, may occur in the reser- voir. Concentrations of ammonia decreased down- stream from the Below John Martin Reservoir station because of nitrification and assimilation of ammonia. At most stations, the median ammonia concen- trations were relatively constant throughout most of the year, with the exception of the station Below Pueblo Reservoir (fig. 26). Concentrations were elevated at the Below Pueblo Reservoir station during the snowmelt-runoff regime (May-June). Ammonia concentrations in the Arkansas River were low in comparison to stream-water-quality standards and drinking-water standards. There were no exceedances of the stream-water-quality standards for un-ionized ammonia. The chronic and acute stream- water-quality standards for un-ionized ammonia are 0.02 mgfL and 0.43 mgiL, respectively (Colorado Department of Health, 1994). It is important to recog- nize that the standards for ammonia are based on un- ionized ammonia and that data from this study are reported as total ammonia, which includes un-ionized and ionized ammonia. Analysis of total ammonia concentrations indicated that un-ionized ammonia concentrations were less than 10 percent of the total ammonia concentration. On the basis of this analysis, un-ionized ammonia concentrations in the has in did not exceed stream-water-quality standards. Nitrate Median nitrate concentrations in the upper basin ranged from 0.020 mgiL at Granite to 0.130 mglL at portland (fig. 27). California Gulch had the highest median nitrate concentration in the upper basin (0.51 mglL) and, along with Iowa Gulch and Empire Gulch, contributed to the elevated nitrate concentra- tion at Malta. A significant decrease (u=0.05) in nitrate concentration occurred just upstream from Granite as water released from Twin Lakes Reservoir diluted the flow in the Arkansas River. Concentrations of nitrate generally increased between Granite and Wellsville with significant (u=0.05) increases occur- ring between Buena Vista and Nathrop and between Salida and Wellsville. Wastewater facilities and nonpoint loading from livestock and septic systems probably contributed to the increases. As was the case with ammonia, nitrate concentrations decreased between Wellsville and Portland probably due to dilu- tion and assimilation of nitrogen by aquatic plants. Between Canon City and Portland, a threefold increase in median nitrate concentration (0.046 to 0.13 mglL) likely occurred because of discharge from the Fremont Sanitation Wastewater Treatment Facility and nonpoint sources such as agricultural return flow and livestock. Tributary inputs downstream from Malta did not contribute substantial amounts of nitrate to the upper Arkansas River. Median nitrate concentrations at main-stem sites in the lower Arkansas River Basin ranged from 0.20 to 2.4 mgIL (fig. 27). Nitrate concentrations immediately downstream from Pueblo Reservoir were larger than at Portland probably because of the same processes that resulted in increased ammonia concen- trations in the water released from Pueblo Reservoir. Median nitrate concentrations increased twofold between the Pueblo and Highway 227 sites because of inflow from Fountain Creek (fig. 27); the concentra- tions in Fountain Creek are elevated because of upstream wastewater discharges and irrigation return flows. The highest median nitrate concentration in the basin was at Fountain Creek (4.7 mg/L). Downstream to Avondale, median nitrate concentrations showed a steady and significant (u=0.05) increase. The increase can probably be attributed to wastewater discharges from Pueblo and agricultural and urban nonpoint- source runoff. Nitrate concentrations remained rela- tively constant downstream from Avondale to Catlin Dam, then increased downstream to La Junta. The increase observed at La Junta is probably attributable to irrigation return flows and wastewater discharge from the city of La Junta. Timpas Creek and Crooked Arroyo (pI. I), which largely consisted of irrigation return flow, had nitrate concentrations that ranged from about 2 mg/L to 6 mg/L. Two large feedlots that operate in the area also may have contributed nitrate to the river. The significant (u=O.05) decrease in nitrate concentrations at Below John Martin Reservoir was possibly caused by assimilation within the reservoir and chemical reduction or denitrification during periods of anoxia in the reservoir. Nitrate concentra- tions increased fivefold between the WATER QUALITY 53