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0024 <II Hardness (as CaCO,) would not appear to con- tribute signilicantly to the protection 01 aquatic organisms from metal toxicity. However, the hardness in the river immediately downstream of the Leadville Drain and California Gulch is slight- ly higher in low-flow periods, during which metal concentrations in the river are highly correlated with inflow from these pollution sources. The higher hardness levels may protect aquatic or- ganisms from metal toxicity to some degree. I' Calcium concentration in the upper Arkansas River is always below 60 mg/L and highest levels occur during low-flow periods of the late fall and winter. Concentrations during the peak of the spring runoff average only 1 3.2 mglL. Calcium in water reduces the toxicity of heavy metals to fish; for example, mature fish have been killed by 0.1 mg/L lead in water containing only 1 mg/L calcium, but have not been harmed by the same amount of lead in water containing 50 mg/L calcium (McKee and Wolf, 1963 1105)). A concentration of 50 mg/L calcium has cancelled the toxic effect upon some fish of 2 mg/L zinc and 10 nig/L lead (Jones. 1 938 187)). When calcium concentrations are less than 5 mg/L. some species of aquatic insects are unable to harden their cuticles after molting; however. these same species are often found in abundance in waters that contain less than this amount of calcium (Hynes. 1954 183)). Calcium concentrations were always far less than known toxic levels and would not limit the production of aquatic fauna. \1 Magnesium and calcium are similar in chemical activity, particularly in the formation of car- bonate salts, and they are both necessary for biological processes in streams (Reid, 1961 (128)). Magnesium concentrations in the up- per Arkansas River did not exceed 27.3 mg/L and were usually below 20 mg/L downstream of the Leadville Drain and 10 mg/L downstream of the Lake Fork inflow. McKee and Wolf (1963) 1105) cite reports that state that to be toxic to aquatic organisms, magnesium in such com- pound forms as magnesium chloride, nitrate, or sulfate must be in concentrations exceeding 500 mg/L of magnesium. Sodium concentrations in the river were always below 7 mg/L. and were approximately 1 mg/L during the peak runoff. McKee and Wolf 119631 11051 conclude that the toxicity of sodium salt depends largely on the anion involved. the chromate being exceedingly toxic and the sulfate least so. They also cite data that recom- mend that drinking water of good quality may contain up to 10 mg/L of sodium, and that sodium chloride concentrations of less than 2000 mg/L in fresh water are recommended for the protection of fish-food organisms and fish fry. The sodium concentrations in the upper Arkansas River were always well below levels considered to be toxic to aquatic organisms. Potassium concentrations in the river were always below 3 mg/L. This is far below levels considered to be deleterious to aquatic life. McKee and Wolf (19631 11051 cited studies which show the toxic level of potassium to chir- onomid larvae to be 700 mglL and to trichop- teran larvae to the 1000 mg/L. The upper Arkansas River seldom contained car- bonate. Concentrations of carbonates in natural and polluted waters is a function of temperature, pH, cations, and other dissolved solids. In general. it may be expected that carbonates in themselves are not harmful to aquatic organ- isms. but their buffering action and effect upon pH may contribute to the toxicity of a high pH value. Like carbonates, the concentration of bicar- bonates in natural and polluted waters is a func- tion of temperature, pH, and concentrations of other dissolved solids. Bicarbonates may reach the water from many natural sources, including absorption of carbon dioxide from the air, lime- stone weathering, the decomposition of organic matter, or they may be discharged as a pollutant (McKee and Wolf, 1963 11051). In streams, the occurrence and abundance of components of the bicarbonate buffer system and the pH condition are determined primarily by current, biological processes, and the chemical nature of the substrate (Reid. 1961 11281). Other than the fact that excessive bicarbonates add to the salinity and total solid content of water. bicar- bonates are seldom considered to be detrimen- tal. Concentrations of bicarbonates in the Arkan- sas River never exceeded 137 mg/L above the confluence of Lake Creek and were considerably less below the confluence. These levels would not limit the production of aquatic macrofauna. 35