Irrigation drainage studies conducted under the National Irrigation Water Quality Program in the middle Green River basin, Utah, by the U.S. Fish and Wildlife Service, U.S. Geological Survey, and Bureau of Reclamation from 1986 to 1989 detected significantly high concentrations of selenium in water, sediment, and biota (Stephens et al., 1988; Peltz and Waddell, 1991; Stephens et al., 1992). At Stewart Lake selenium entered the lake through subsurface drains. Selenium concentrations in the drains were consistently elevated (up to 140 µg/L), while selenium concentrations in water at the lake outlet ranged from 1 to 12 µg/L. Selenium concentrations in 98% of composite whole- body fish samples collected in Stewart Lake during 1986, 1988, and 1989 were substantially elevated above 4 µg/g dry weight (dw). In 1991 fish were collected from 5 reaches of the Green River to determine whether selenium concentrations were elevated in fish along reaches receiving selenium laden surface or subsurface water. Selenium concentrations in fish varied spatially, with the highest concentrations detected in fish near the Stewart Lake/Ashley Creek and Jensen areas (Waddell and Wiens, 1994). Eighty percent (80%) of these samples exceeded 4 µg/g dw. To determine selenium concentrations in endangered fish Waddell and May (1995) developed a non-lethal tissue collection method. Mean selenium concentrations in razorback suckers were 33.0 µg/g dw at Ashley Creek ( n = 12 ), and 9.7 dw µg/g at the spawning site ( n = 9 ). Ninety-five percent (95%) of samples exceeded 4 µg/g dw. REMEDIATION ACTIVITIES AT STEWART LAKE AND SELENIUM REDUCTIONS IN BIOTA Nathan Darnall 1, Bruce Waddell 1, and Elise Boeke 1 Studies conducted in identified elevated selenium concentrations in water, sediments and biota at Stewart Lake, in northeastern Utah. Since 1997, remediation actions, based on an adaptive management approach, have been implemented at Stewart Lake to reduce selenium in water, sediments, and biota. These included diversion of drainwater around the lake, addition of fresh water, ability to completely drain the lake, and oxidation of sediments. Selenium concentrations were monitored in whole-body adult common carp (Cyprinus carpio) at selected sites within Stewart Lake and in the Green River from 1995 to 2001 to determine spatial and temporal trends, and to assess threats to endangered fish. Three composite samples of carp were collected at two primary locations in Stewart Lake during all seven years, and at ten primary sites along the Green River during six of the seven years. Years 1995 to 1997 were considered pre- remediation and years 1998 to 2001 were considered post-remediation for temporal analysis. Spatial differences in selenium were identified within Stewart Lake with highest concentrations near irrigation drains and seeps. Selenium was also spatially different along the Green River with highest concentrations adjacent to Stewart Lake. Selenium concentrations were lower post-remediation at each of the two primary sites in Stewart Lake (26% and 36% reduction), and for most Green River sites (up to 45% reduction). After four years of remediation, selenium was still bio-available at Stewart Lake, and rapidly accumulated in plankton and in fish tissue. Selenium concentrations in razorback suckers stocked into Stewart Lake increased 10-fold over a 32 day period from 0.9 to 9.0 µg/g dry weight during Results from this monitoring show that selenium has declined in biota at some sites; however, this decline may be due in large part to water management within Stewart Lake that has reduced lake availability to adult fish and likely reduced exported contaminated food to the Green River, rather than significant reduction of selenium in sediment. One hundred and sixty-seven composite, whole-body common carp samples were collected at six locations within Stewart Lake between 1995 and The geometric mean selenium concentration for all samples was 17.5 µg/g dry weight. Only two locations, North Overlook ( n = 56 ) and Outlet ( n = 66 ), were sampled during all seven years, and therefore, provide opportunity for temporal analysis between pre- and post-remediation. Because selenium concentrations were significantly different between the two locations ( F 1,120 = 30.22; P = ), analysis for temporal trends was performed for each site. Selenium concentrations were significantly different between pre- and post-remediation at both the North Overlook ( F 1,54 = 22.21; P = ) and at the Outlet ( F 1,64 = 24.64; P = ), and geometric mean selenium concentrations decreased 28.9% and 39.4% respectively (Figure 2). Selenium concentrations declined in carp at Stewart Lake after remediation. However, selenium was still bio-available at Stewart Lake, and quickly accumulated in fish and plankton tissues once organisms entered Stewart Lake. In 2000, 145 hatchery reared razorback suckers (Xyrauchen texanus) were stocked into Stewart Lake. Muscle plugs were collected from 5 fish prior to stocking and analyzed for selenium with a mean concentration of 0.9 µg/g dw. After only 32 days, the mean selenium concentration increased 10-fold to 9.0 µg/g dw (Figure 3). Selenium concentrations also remained elevated in most plankton samples after remediation. Excluding 5 plankton samples collected in 2000 at or near the Inlet Channel, the geometric mean selenium concentration decreased slightly between pre- and post-remediation (16.3 and 14.3 µg/g dw, respectively); and one-way ANOVA detected no significant difference between the two periods ( F 1,21 = 1.54; P = ) (Figure 4). Even those samples collected near the Inlet, where sediments are less contaminated and water was predominately from the Green River, had elevated selenium concentrations. Three composite, whole-body common carp samples were collected from 10 locations on the Green River near Stewart Lake in 1995 to determine the spatial extent of selenium contamination. This historical data, along with more recent data, has shown the influence of Stewart Lake and Ashley Creek as sources of selenium to the Green River, with highest concentrations at the site adjacent to Stewart Lake (relative river kilometer zero) and decreasing with distance upstream and downstream (Figure 6). Spatial distribution of selenium in Green River fish Selenium concentrations declined in Green River carp after remediation. Abstract Introduction Study area Methods Remediation Non-lethal muscle plugs were collected opportunistically from endangered fish in the Green River and in Stewart Lake. Selenium concentrations were significantly higher in Colorado pikeminnow (Ptychocheilus lucius) sampled from Stewart Lake when compared to fish in the Green River near Stewart Lake ( F 1,28 = 8.20; P = ) (Figure 5; aqua bars). Fish sampled further from Stewart Lake generally had lower selenium concentrations. Geometric mean selenium concentration in Colorado pikeminnow from the Green River reach near Stewart Lake (relative river kilometers ˉ4 to 23) declined between pre- and post-remediation (5.2 and 3.8 µg/g dw, respectively) but the decline was not quite significant ( F 1,22 = 3.52; P = ). Selenium concentrations declined in endangered fish after remediation. Conclusions Fish were collected at 6 locations within Stewart Lake using trammel nets or Fyke nets, and at 12 sites on the Green River by electro-fishing. Usually, 3 composite samples consisting each of 5 common carp of similar size were collected per location. Common carp samples were analyzed for selenium at Geochemical and Environmental Research Group via graphite furnace atomic absorption spectroscopy. Non-lethal muscle plugs were opportunistically collected from endangered fish, and analyzed for selenium at Columbia Environmental Research Center via neutron activation. Statistical analyses were performed with NCSS2001 (Number Cruncher Statistical Systems, Kaysville, UT), and significance was determined at the 0.05 level. Remediation activities began in the spring of 1997 when a new inlet channel was cut into Stewart Lake, allowing Green River water to flow through the lake during peak runoff. Later that same year, drainage channels were excavated in the lake bed to allow the lake to drain completely. Finally, irrigation return-flow drains (the major source of selenium to the lake) were diverted away from the lake and extended to the Green River during the winter of 1997 and spring of Even though the processes of flooding, draining, and drying the lake have continued since 1997, for the purposes of interpreting the biological data, 1998 represents the first full field season of new lake management with the sources of selenium removed. For statistical analysis, the following conventions have been used: 1 U.S. Fish and Wildlife Service 2369 West Orton Circle, Suite 50 West Valley City, UT Special thanks to Joe Gardner and Dave Naftz (U.S. Geological Survey) and Chris Cline (U.S. Fish and Wildlife Service) for technical assistance and editorial review.  Whole-body selenium concentrations decreased significantly in common carp at Stewart Lake and in the adjacent Green River after selenium-laden irrigation return flow drains were removed from Stewart Lake.  Selenium in endangered Colorado pikeminnow decreased slightly post-remediation.  In Stewart Lake, plankton still accumulated selenium to pre-remediation concentrations despite influx of clean Green River water.  Razorback suckers stocked into Stewart Lake in 2000 showed a 10-fold increase in selenium concentrations in one month.  Selenium is still bioavailable and readily accumulates in biota at Stewart Lake despite recent remediation activities. Reductions of selenium in biota may be attributable to the yearly filling and draining of the lake that limits fish exposure and subsequent selenium accumulation. Pre-remediationPost-remediation 1998, 1999, 2000, , 1996, 1997 Figure 3. Selenium concentrations (ppm, dw) in hatchery reared razorback suckers as a function of time from initial stocking into Stewart Lake during Peltz, L.A., and Waddell, B Physical, chemical and biological data for detailed study of irrigation drainage in the middle Green River basin, Utah, 1988_89, with selected data for 1982_87. U.S. Geological Survey Open_File Report 91_530, 213 pages. Stephens, D.W., Waddell, B., and Miller, J. B Reconnaissance investigation of water quality, bottom sediment, and biota associated with irrigation drainage in the middle Green River basin, Utah, 1986_1987. U.S. Geological Survey Water_Resources Investigations Report 88_4011, 70 pages. Stephens, D.W., Waddell, B., Peltz, L. A., and Miller, J Detailed study of selenium and selected elements in water, bottom sediment, and biota associated with irrigation drainage in the middle Green River basin, Utah, 1988_90. U.S. Geological Survey Water_Resources Investigations Report 92_4084, 164 pages. Waddell, B. and May, T Selenium concentrations in the razorback sucker (Xyrauchen texanus): substitution of non-lethal muscle plugs for muscle tissue in contaminant assessment. Archives of Environmental Contamination and Toxicology. 28: Waddell, B. and Wiens, C Monitoring of selenium in fish of the Green River, Utah. U.S. Fish and Wildlife Service. Salt Lake City, UT. Unpublished report. Figure 4. Selenium concentrations (ppm, dw) in plankton samples collected from various locations within Stewart Lake between 1995 and Literature Cited For the periods pre-remediation (1995 to 1997) and post-remediation (1998 to 2001), selenium concentrations declined in Green River common carp similar to that observed in Stewart Lake. For the site adjacent to Stewart Lake, the geometric mean selenium concentration decreased significantly from 18.3 to 9.6 µg/g dw ( F 1,20 = 8.77; P = ). The Mann-Kendall S statistic was used to determine trends in selenium concentrations over the seven years at each Green River location. Results of this test showed selenium concentrations were stable at 5 locations, but decreased at 3 locations at the 90% confidence level and at 2 additional locations at the 80% confidence level (Figure 6). Figure 2. Selenium concentrations (ppm, dw) in whole-body common carp samples collected from two locations in Stewart Lake from 1995 to The geometric mean selenium concentrations, shown in parentheses, declined significantly at both locations between pre- and post-remediation. Figure 6. Selenium concentrations (ppm, dw) in common carp collected from the Green River relative to distance upstream (depicted by - symbol), and downstream from Stewart Lake. For each location, the yellow bars on the left are pre-remediation (1995 to 1997), while bars on the right are post-remediation (1998 to 2001). Light yellow bars indicate stable trend, aqua and grey bars indicate decreasing trend at 90% and 80% confidence levels, respectively. Figure 5. Selenium concentrations (ppm, dw) in muscle plugs from Colorado pikeminnow in Stewart Lake and various reaches of the Green River relative to Stewart Lake. Geometric mean selenium concentrations shown in parentheses. Figure 1. Map of study location. Table 1. Location codes, names and relative river kilometers (from Stewart Lake).