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northeastern Utah was identified as one area needing further study. The middle Green River provides sensi- tive habitats for four endangered fish species, Colorado squawfish (Ptychocheilus Lucius), razorback sucker (Xyrauchen texanus), bonytail (Gila elegans), and hump- back chub (Gila cypha). The major causes for the decline of endangered fishes in the upper Colorado River basin include physical and water quality factors such as dams, which reduce water temperature, de- crease turbidity, alter seasonal and annual flow pat- terns, and reduce habitat for adult and larval stages of fish such as spawning bars and flooded bottomlands (U.S. Fish and Wildlife Service, 1987). The middle Green River is defined as the drainage area consisting of the Green River and its tributaries between Flaming Gorge Dam and the City of Green River, Utah (Fig. 1). A DOl irrigation project near Jensen, Utah, returns water to the Stewart Lake Waterfowl Management Area and the Green River. Extensive irrigation outside the DOl project influences water quality in Ashley Creek and the Duchesne River, tributaries of the Green River downstream from Stewart Lake. Also down- stream, the Ouray NWR uses surface and groundwater for irrigation. Analysis of water, bottom sediment, and biological tissue collected from the middle Green River basin in the Ashley Creek-Stewart Lake area and at Ouray NWR since 1986 have confirmed the presence of boron, selenium, and zinc at concentrations that could be potentially harmful to fish and wildlife (Stephens et a!., 1988; 1992; Peltz and Waddell, 1991). Previous research has shown that selenium and other inorganics were acutely toxic to Colorado squawfish, razorback sucker, and bony tail at concentrations close to those in certain habitats in the middle Green River and in the San Juan River of New Mexico (Hamilton, 1995; Buhl and Hamilton, 1996; Buhl, 1997; Hamilton and Buhl, 1997a; 1997b). Acute toxicity tests with envi- ronmental mixtures of inorganics simulating conditions at Stewart Lake and Ashley Creek in reconstituted Green River water and at several locations on the San Juan River in reconstituted river water showed high hazards for larval endangered fish. In a review of potential effects of selenium on endangered fish in the Colorado River basin, Hamilton (1998) concluded that selenium was. a contributing factor to the decline of endangered fish. A recent assessment of selenium con- centrations in wild larval stages of razorback sucker from the middle Green River showed that selenium was elevated in larvae from the Ashley Creek-Stewart Lake area (Hamilton et a!., 1998). Chronic toxicity studies were conducted with endan- gered larval razorback sucker and bony tail to deter- mine the influence of an inorganic mixture of elements derived from irrigation return flows entering the Green River from Ashley Creek. SIMULATING IRRIGATION WATER TOXICITY 49 MATERIALS AND METHODS Razorback sucker and bonytail were acquired as eggs from Dexter National Fish Hatchery (NFH), New Mex- ico. The brood stock for razorback sucker and bonytail were adults from Lake Mohave, Arizona-Nevada. Eggs were acclimated to well water at Yankton, South Dakota, and hatched in a heath fish egg incubator. After hatching, larvae were held in a 46 L aquarium in well water and fed live nauplii of brine shrimp (Artemia spJ and a commercial salmon diet (Biodiet, BioProd- ucts, Inc.) ad libitum. Water characteristics of well water were hardness 930-1020 mg/L as CaC03, alka- linity 247-270 mg/L as CaC03, and pH 7.7-7.8. Experimental Design, Exposure Water and System Partial life-cycle chronic toxicity studies were con- ducted by exposing larvae to constant concentrations of a waterborne mixture of inorganics for 90 days in a flow-through proportional diluter system (Mount and Brungs, 1967). The diluter delivered 1 L of reconsti- tuted water to each exposure <\.quarium every 15 min. Nominal exposure concentrations were OX, IX, 2X, 4X, 8X, and 16X, where IX represented the environmental mixture measured at the mouth of Ashley Creek on 08/27/86 (Stephens et a!., 1988). Exposures were conducted in reconstituted water simulating the average environmental concentrations of cations and anions, minus trace elements, in the middle Green River at the Jensen gaging station of the U.S. Geological Survey (46 mg/L calcium, 20 mg/L magnesium, 49 mg/L sodium, 159 mg/L sulfate, 23 mg/L chloride, and 130 mg/L bicarbonate) (ReMil- lard et a!., 1989). This reconstituted water was prepared by mixing high-purity water from a softener-reverse osmosis-de ionization system of water treatment with four salts (calcium chloride, calcium sulfate dihydrate, magnesium sulfate heptahydrate, and sodium bicarbon- ate). The salts were dissolved and" mixed in 5700 L polyethylene storage tanks equipped with mixing pumps and a repressurization system (Hamilton et a!., 1989). Water quality characteristics were measured weekly according to standard methods (American Public Health Association et a!., 1992) (Table I). The photope- riod was controlled by a timed, artificial lighting system (Drummond and Dawson, 1970) and simulated condi- tions in the middle Green River basin during March to July 1986. Oxygen concentrations in exposure water were supplemented by passing compressed air from an oil-less air compressor through air stones. Exposures were conducted in 46 L replicate aquaria divided by partitions into one 23 L residue chamber and two 11.5 L growth chambers. Residue chambers