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42 MAYLAND ET AL. Table 2-5. Selenium levels in tissues of cattle receiving a basal diet containing an esti- mated 0.16 mg Selkg or asludge-amended diet containing 1.7 mg Selkg (adapted from Boyer et al., 1981). Se in tissues, mg/kg wet basis Tissue Control Sludge-amended Kidney 1.35 1.55 Liver 0.42 1.05 Muscle 0.23 0.30 Spleen 0.36 0.39 Brain 0.20 0.23 provided 1.7 mg Se/kg of diet. Tissues obtained after slaughter of these animals showed small, but consistent, increases in Se concentration as a result of sludge supplementation, compared with tissues from animals on the basal diet (Table 2-5). Although the energy-depleted sludges are not likely to be used in livestock rations because of reduced animal performance, these wastes could be used as soil conditioners in gardening, landscaping, and cropping situations where they provide organic matter and some plant nutrients. However, the Se con- centrations found in municipal sludges tested to date are not threatening the quality of edible produce grown on sludge-amended soil. AGRONOMIC IMPACTS ON SELENIUM WEATHERING Crop-Fallow Versus Seleniferous Cretaceous Materials Humans have also impacted the natural weathering rate of seleniferous materials by choice of cropping systems. This has occurred in many areas of the Great Plains that once were rangelands, but now are largely in dryland grain production. This type of agriculture has been possible because of the adoption of acrop-fallow system. Fallowing in alternate years conserves extra water for use by the next year's grain crop. However, this cropping system, especially in years of above average precipitation, allows excess water to percolate beyond the root zone where it may contact the underlying Cretaceous geologic formations (Brown et al., 1983). The percolating water may then encounter impervious layers and move laterally to intersect the surface, forming a spring or seep. These seeps have a detrimental impact on the area because of their high salinity. The Great Plains area is underlain with Cretaceous geological materi- als, including formations that are classified as being in the Montana or Colorado groups (Donovan et al., 1981; Miller et al., 1981). The selenifer- ous groundwater in the northern Great Plains is associated with the dark or black shales of the Colorado group where the unweathered shale may con- tain 0.25 to 10.3 mg Se/kg. Groundwater samples from these dark shales have correspondingly high Se values. The highest Se values were typically found in thin glacial till deposits and weathered shale overlying relatively