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<' ) ( / 0247 from Pueblo to Las Animas and frequently exceeded the stream-water-quality standard (10 mglL) at Catlin Dam and at Las Animas. Most exceedances occurred during the low-flow regime. Dissolved-manganese and dissolved-iron concentrations generally were small at most sites; however, during the low-flow regime, substantial increases in dissolved-manganese and dissolved-iron concentrations were measured immedi- ately downstream from John Martin Reservoir. Anoxic conditions in John Martin Reservoir during the winter are suspected to result in the dissolution of iron and manganese from bottom sediments, thereby resulting in increased concentrations in the water column. Concentrations of most total-recoverable trace elements generally increased from Pueblo to Las Animas and were largest during snowmelt runoff and post-snowmelt runoff. Resuspension of fluvial sediment was presumed to be a major cause of the downstream increases. Additionally, a few water samples collected from major tributaries in the lower basin during storm-runoff events had very large concentrations of most trace elements. This indicates that tributary storm runoff probably is a major source of trace elements in the lower basin. Median ammonia concentrations in the Arkansas River ranged from a minimum of 0.0 10 mglL at Leadville to maximum of 0.145 mglL at the Below John Martin Reservoir site; median nitrate concentra- tions at main-stem sites ranged from a minimnm of 0.020 mgIL at Granite to a maximum of 2.4 nigIL at La Junta. High concentrations of ammonia down- stream from John Martin Reservoir, particularly during 10w'f1ow regimes, are indicative of anoxic conditions in the reservoir. Ammonia and nitrate concentrations generally increased downstream but were low in comparison to State and Federal standards. Nitrate concentrations were higher during low flow, whereas ammonia concentrations were generally similar throughout the year. Median total-phosphorus concentrations were relatively low in the upper basin, but they exceeded tbe Environmental Protection Agency recommenda- tion for total-pbosphorus concentration in rivers (0.10 mglL) at three sites in the lower basin. At Las Animas, which is immediately upstream from John Martin Reservoir, the median total-phosphorus concentration was 0.10 mgIL; the Environmental Protection Agency recommends that total-phosphorus concentrations in rivers entering a reservoir not exceed 0.05 mgIL. There was no distinct temporal pattern in the phosphorus concentrations. Uranium concentrations increased downstream in tbe Arkansas River, and large increases occurred in the lower basin. The use and reuse of irrigation water in the lower basin increase the amount of time tbe water can interact with soils and rock, resulting in increased concentrations of dissolved uranium. Instan- taneous dissol ved natural uranium concentrations exceeded the proposed drinking-water standard at two sites, once at the Las Animas site (36 mglL) and twice at the Below John Martin Reservoir site (29 mgIL). The median value for dissolved uranium at Below John Martin Reservoir also exceeded the proposed drinking-water standard. Pesticides are widely used in agricultural areas and are of concern because of the potential to degrade water quality. Most of tbe pesticide samples submitted for analyses during the study period were reported as having concentrations less than analytical reporting limits. The herbicide 2,4-0 was detected at each of the five sampled main-stem sites but at concentrations less than maximum contaminant levels. Suspended-sediment concentrations in the Arkansas River upstream from Pueblo Reservoir were relatively low but increased significantly downstream. Median suspended-sediment concentrations increased significantly between Parkdale and Portland and between Pueblo and Catlin Dam. The downstream increases are attributable to changes in geology, tribu- tary inflow, and resuspension of fluvial sediment. During periods of elevated runoff, suspended-sedi- ment concentrations can increase by several times; tbe largest measured concentration (1,650 mglL) in the upper basin occurred in a storm-runoff sample collected at Parkdale. Runoff from thunderstorms also can have a large effect on suspended-sediment concen- trations in the lower Arkansas River. Fecal-coliform, fecal-streptococci, and total- coliform bacteria concentrations in the Arkansas River increased downstream; concentrations were relatively low in comparison to stream-water-quality standards. Fecal-coliform concentrations were substantially larger in the lower basin. The highest median fecal- coliform concentration of 162 colonies/lOO mL occurred at Las Animas, which was less than the 2,000 colonies/lOO mL standard. SUMMARY 67