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I I I I I I I I I I I I I I I I I I I 18 2.3 WATER QUALITY The water quality of the San Juan River, from its impoundment behind Navajo Dam to its confluence with Lake Powell, is influenced by both natural and anthropogenic factors, exhibiting the results of these influences both longitudinally and seasonally as it flows through the habitat occupied by the endangered fish species. Of particular concern for this Implementation Program are the levels and effects of elemental contaminants such as selenium, the levels of which are influenced by natural baseline conditions as well as agricultural development within the Basin. The San Juan Basin is considered naturally seleniferous, as is much of the western United States. Historic data concerning concentrations of selenium in the mainstem of the San Juan River indicate a general increase in levels with distance downstream from Archuleta, New Mexico (downstream of Navajo Dam) to Bluff, Utah, (<1 ug/l to 4 ug/l). Tributaries to the San Juan carry higher concentrations of selenium than found in the mainstem river immediately upstream from their confluence with the San Juan; although these levels are diluted by the flow of the San Juan, the net effect is a gradual accumulation of the element in the river's flow as it travels downstream. Increased selenium concentrations may also result from the introduction of groundwater to the mainstem of the river along its course. Irrigated agriculture is known to contribute the element to the river via surface and subsurface return flows through three potential avenues: 1) concentration of selenium in the irrigation water by evaporation, 2) selenium pickup from the soils that are irrigated, and 3) selenium pickup in the shale beds underlying the irrigated areas. Development of the oil and gas resources of the Basin, as well as other factors including but not limited to urban runoff, domestic and industrial sewage effluents, and spillage of petroleum and petroleum products, have contributed to the presence of polycyclic aromatic hydrocarbons in the biota of the San Juan River. Sampling of fish species from the San Juan River in 1990 and 1991 suggest that aquatic organisms are being exposed to high levels of hydrocarbons such as naphthalene, benzo(a)pyrene, and phenanthrene. Other environmental contaminants, their individual effects on the endangered fish species of the San Juan River, or their synergistic or antagonistic effects in the presence of naturally occurring or introduced elements or compounds have not been the subject of site or species specific investigations.