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26 Water Quantitv The environmental baseline for water quantity includes all historical depletions in the Upper Basin. depletions resulting from projects which have previously undergone section 7 consultption. and depletions resulting from projects contemporaneous with this consultation. Water Qualitv Some of the contaminants of concern within waters of the Upper Basin include heavy metals. selenium. salts. PAHs. and pesticides. Selenium is of particular concern because of its documented effects on fish (and wildlife) reproduction. Many chemical. physical. and biological factors affect the toxicity of environmental contaminants to biological organisms. Chemical and physical factors include contaminant type. chemical species or form. pH. water temperature. dissolved oxygen. hardness. salinity. and multiple-chemical exposure (antagonism and synergism). Duration of exposure. quantity of contaminant. and exposure pathways from the environment to the organism also affect toxicity. Some trace elements are beneficial to organisms at low concentrations but may be toxic at higher concentrations. Biological and physiological factors affecting toxicity include species. age. sex. and health of the organism. Selenium concentrations can be elevated in areas where irrigation occurs on soils which are derived from or which overlie Upper Cretaceous marine sediments. Percolation of irrigation water through these soils and sediments leaches selenium into receiving waters. Qther sources of selenium include powerplant fly ash and oil refineries. Water depletions. by reducing dilution effects. have increased the concentrations of selenium and other contaminants. Recent United States Geological Survey data collected in the Colorado River from 1990 to 1996 show that selenium concentrations under existing conditions (current level of development. not section 7 baseline) have been as high as 2 pg/L in the 15-mile reach and as high as 6 pg/L below the 15-mile reach. In 1995. Colorado's Water Quality Control Commission reduced the chro~ic selenium standard from 17 pg/L to 5 pg/L. The Service recommended the level be lowered to 2 fJg/L. Phvsical Habitat Water depletions. by affecting the quantity and timing of flows. have reduced the ability of the river to create and maintain habitats and have reduced the frequency and duration of availablllty of certain habitats.