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I, 'I II 1 1 ,I, I 1 'I I I., . . ,'I '. . II . I 27 Increased loading of the San Juan River and its tributaries with soil salts and elemental contaminants from irrigation return flows, urban runoff, contaminated groundwater, mine waste runoff, brine salt discharges from oil and gas wells, and oil refinery wastes degrade water quality and may be contributing to the decline of Colorado squawfish and razorback sucker as well as other native fish species in the San Juan River basin. Water quality studies in the San Juan River basin have previously focused on trace-element residues and organochlorine in fish and wildlife. Comparison of data from 1980 and 1984 with 1973 data on elemental residues in the Farmington area indicates residual levels of many elements not only are elevated but may be gradually increasing. Data on the toxic effects of selected chemicals on endangered fish species are being collected and evaluated. However, the results of these studies may have limited application for the San Juan River. The investigations to date have only considered the toxicity of waterborne concentrations of inorganic selenium and other elements to the Colorado squawfish, razorback sucker, and the bony tail chub. These studies have not addressed the effects of long-term exposure to selenium (organo-selenium) in the diet of these species and potential impacts to survival, growth, and reproduction of endangered fish species or other components of the aquatic environment. As survival of other native fish species as well as aquatic invertebrates is essential to survival of the endangered fish species, investigations on the effects of various contaminants upon all components of the aquatic communities of the San Juan River basin are critical. Monitoring of existing water quality conditions in the San Juan River drainage is considered inadequate to provide the information upon which to base analysis of current land management practices (oil and gas development, agriculture, etc.) and flow regulation of the San Juan River and its tributaries, or to accurately predict environmental impacts from proposed development scenarios. Little is known concerning the tolerance levels of Colorado squawfish and razorback sucker to potential contaminants in the San Juan River. Criteria presently available for use in analysis of project impacts have been based on conditions not present in the river and were developed using other fish species. The concentration of contaminants that may impact the endangered fish species should be quantified, while considering the synergistic relationships of the particular water quality conditions of the river (i.e., determination of criteria levels for selected contaminants singly and in conjunction with other contaminants) . Since many of the contaminants are a part of the natural river system, elimination of these contaminants is not possible. Other contaminants derive completely or predominately from development activities and can be eliminated, or at least reduced to a level deemed unlikely to have chronic or acute affects upon members of