2010-11-22_REVISION - M2009023 (7)

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. e The portion of the La Plata River basin in Colorado does not contain seleniferous-rich soils or bedrock, for example there is no Mancos Shale within the basin. The soils that are currently irrigated on Red Mesa are deposits that originated in the Monument Valley and are not rich in selenium (E. Bikis, Wright Water Engineers, pers. comm. 2004). Most selenium in fish and wildlife tissues is the result of dietary uptake (Lemly 1996b). Because of food chain bioaccumulation of selenium in aquatic systems, water concentrations of selenium cannot be used alone, to assess potential hazard. Furthermore, seasonally high water flows can dilute selenium concentrations confounding interpretation of effects on fish and wildlife reproduction and survival depending on when water samples were taken. Thus sediment and biotic analysis is critical to understanding selenium effects on fish and wildlife. Because selenium bioaccumulates in the aquatic food chain, an inorganic selenium level as low as 2 µg/L or an organic level of I gg/L in water may be of concern. Organic selenium may be derived from living or dying plants and animals as free seleno-amino acids (Lemly 1996b). Sediment levels above 4 gg/g may be at levels considered toxic to fish and wildlife (USDI 1998). Guidelines for toxic levels of selenium in fish and wildlife are discussed below and can be found in the references cited. Selenium toxicity thresholds in fish and wildlife tissues are as follows: 4 pg/g dry weight in whole body fish, 8 gg/g dry weight muscle tissue, 10 µg/g for fish eggs, 3 gg/g for invertebrates (eaten by fish and wildlife) (Lemly 1996b), and 6 pg/g for waterbird eggs (USDI 1998). Selenium concentrations in whole body fish above 4 gg/g dry weight have been associated with mortality, reduced growth, and reproductive failure (Hamilton et al. 2002a, Hamilton et al. 2002b, Hamilton et al. 2002c, Hilton et al. 1980, Hodson and Hilton 1983, Ogle and Knight 1989, Cleveland et al. 1993, Lemly 1996a, Lemly 1996b, Hamilton et al. 1998, and USDI 1998). Sediments and biota associated with the San Juan River have shown elevated selenium levels. Composite fish samples were collected during the DOI study from six reaches of the San Juan River in spring 1990 and from seven reaches in fall 1990. Each composite sample typically consisted of five individuals of a single species. Composite samples of common carp (Cyprinus carpio) and flannelmouth sucker (Catostomus latipinnis) were collected from each reach during each sampling period. In addition, six channel catfish (Ictalurus punctatus) composite samples were collected during the two sampling periods in reaches where the species was encountered. The highest concentrations of selenium in common carp and flannelmouth sucker occurred in the river from Bloomfield to Farmington, New Mexico (Blanchard et al. 1993). Subsequent investigations (Wilson et al. 1995, Thomas et al. 1998) have detected elevated levels of selenium in habitats associated with irrigation drainage returns and in the Mancos River. Selenium levels in whole body fish occasionally exceeded the toxic effects threshold of 4 µg/g dry weight (Lemly 1996b) and may pose a threat to predatory fish that consistently feed in the regions with elevated selenium. Thomas et al. (1998) found that: "Selenium concentrations in algae, odonates, and mosquitofish collected from both irrigation-drain and pond habitats underlain by Cretaceous soils were significantly greater than in those collected from similar habitats underlain by non-Cretaceous soils. 22