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Sulfates occur naturally in waters, particularly in the Western United States. as a result of leach- ings from gypsum and other common minerals. They may occur as the final oxidized stage of sulfides. sulfites, and thiosulfates. McKee and Wolf 11963) 11051 cite an example of iron pyrite (FeS,) being leached from abandoned coal mines and the sulfide ions bein9 converted in surface streams to sulfates. This reaction prob- ably occurs in the mine drainage discharges enterin9 the upper Arkansas River. Durin9 the peak spring runoff, sulfate levels in the river were less than 64 mg/L, but during low flow and limited dilution. sulfate concentrations reached 143 mg/L downstream of the Califor- nia Gulch inflow. Sulfate concentrations aver- aged 360 mg/L and 591 mg/L in the Leadville Drain and California Gulch. respectively. Hart et al. (1945) 1711 stated that 95 percent of the waters in the United States that contained good game fish populations had sulfate concentra- tions of less than 90 mg/L. It is not likely that sulfates ever reach concentrations that would be harmful to aquatic life in the upper Arkansas river. Chloride concentrations in the upper Arkansas River were always low and much below any levels that have demonstrated toxic effects on aquatic organisms. Chlorides may be of natural mineral origin. or derived from: la) salts spread on fields for agricultural purposes, (b) human or animal sewage. or Ic) industrial effluents (McKee and Wolf, 1963 11051 I. The highest chloride levels were found in California Gulch. but these concentrations were much less than the 400 mg/L cited by McKee and Wolf as be- ing toxic to trout. The Leadville sewage treat- ment plant discharges into California Gulch and is a likely SOurce of chlorides. Phosphorus was almost always found in greater concentrations below the inflow of California Gulch than at any of the other stations. This in- dicates the contribution of phosphorus com- pounds by California Gulch and, ultimately, the Leadville sewage treatment plant. High flows in the river, particularly during the spring runoff. reduced most phosphorus concentrations to below detectable limits. Phosphorus concentra- tions found during the low-flow period of the winter indicated that ample amounts are available for primary production. Low total phosphorus concentrations observed during the summer and fall may indicate uptake by algae. Cole (1979) 1281 suggests that total phosphorus, and not the orthophosphate com- pound, is the best indication of available phosphorus because of the rapid conversion from organic to inorganic forms. The Leadville Drainage Tunnel and California Gulch were obvious contributors of nitrogen to the upper Arkansas River. The influence of these inflows is generally less apparent during high riverflows, but an increase in flooded areas (e.g., marshes and farmland) and the resulting runoff could maintain high nitrogen levels in the river. A rapid decrease in nitrogen concentration follows the spring runoff. when the stream is more stable and plants utilize the nitrogen IReid, 1961 11281), The predominant form of nitro- gen usable in primary production is nitrate. The upper Arkansas River generally had a higher nitrate content during the winter months INo- vember-April) than during other seasons of the year, due to the low dilution factor and very little uptake by algae for primary production. The mean nitrate concentration during the winter was 0.17 mg/L, and during the remainder of the year, 0.07 mg/L. Heavy Metals As in most field studies, only total heavy metal concentrations were addressed in this study. At times, and particularly during high-flow periods, water samples contained metal-laden sediments in suspension. Even though this results in the detection of high concentrations of heavy metals, these metals would not be entirely in an ionic or labile form which would provide the greatest toxicity threat to aquatic organisms. There may be harmful concentrations of other toxic substances not analyzed for in the Arkan- sas River which could be contributed from drain- ages such as Ihe Leadville Drainage Tunnel. California Gulch, Tennessee Creek. Iowa Gulch, and other tributaries. However. based on resources available, results reported herein are sufficient to satisfy the inIended goals of this study of providing a base of information to use to evaluate the effects of construction and future water diversions. 36