ACUTE TOXICITY OF SELENIUM (IV) AND SELENIUM (VI) TO FRESHWATER ORGANISMS

The primary sources of selenium in the environment are geologic and industrial. Selenium closely resembles sulfur chemically, and sulfur or sul­fide deposits of bismuth, copper, iron, lead, mercury, silver, and zinc some- times contain as much as 20 percent selenium. It occurs in several oxidation states, -4, -2, 0, +2, +4, +6; however, the +2 oxidation state is not known in nature. Geologic formations sometimes containing selenium include sand­ stones, limestones, and shales. Sandstones containing >100 mg•L-l selenium have been found in Wyoming. A limestone region of South Dakota contains >40 mg•L-l selenium in chalky shales and marls. Phosphate rocks associated with limestone may contain from l to 300 mg·L-l selenium, suggesting the occurrence of selenium in phosphate fertilizers. Shales have been mainly responsible for cases of selenium poisoning in animals in the United States. Industry releases selenium to the environment through combustion of coal and fuel oil, nonferrous smelting and refining processes, metal refining, and glass manufacturing. Domestic coal average 3.2 mg,L-l selenium. Coal combus­tion accounted for 62 percent of the total industrial emission of selenium in 1970. Smelting and refining of nonferrous metals produces slag heaps and tailing dumps containing high concentrations of selenium. Solid wastes from metal mining and milling may be a more serious source of selenium input to the environment than atmospheric fallout from base metal smelting and refining