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BACKGROUND CONCENTRATIONS GENERAL In abundance of elements, arsenic ranks 20th in the earth's crust (1.5 to 2 mg/kg) 14th in seawater, and 12th in the human body (Woolson 1975). It occurs in various forms, including inorganic and organic compounds and trivalent and pentavalent states (Pershagen and Vahter 1979). In aquatic environments, higher arsenic concentrations are reported in hot springs, in groundwaters from areas of thermal activity or in areas containing rocks with high arsenic content, and in some waters with high dissolved salt content (NAS 1977). Most of the other elevated values reported in lakes, rivers, and sediments are probably due to anthropogenic sources, which include smelting and mining operations; combustion of fossil fuel; arsenical grasshopper baits; synthetic detergent and sewage sludge wastes; and arsenical defoliants, herbicides, and pesticides (NAS 1977). Most living organisms normally contain measurable concentrations of arsenic, but except for marine biota, these are usually less than 1 mg/kg fresh weight. Marine organisms, especially crustaceans, may contain more than 100 mg As/kg dry weight, usually as arsenobetaine, a water soluble organoarsenical that poses little risk to the organism or its consumer. Plants and animals collected from naturally arseniferous areas or near anthropogenic sources may contain significantly elevated tissue residues of arsenic. Additional and more detailed information on background concentrations of arsenic in abiotic and living resources was given by NAS (1977), Hall et al. (1978), NRCC (1978), EPA (1980), Jenkins (1980), and Eisler (1981). NONBIOLOGICAL SAMPLES Arsenic is a major constituent of at least 245 mineral species, of which arsenopyrite is the most common (NAS 1977). In general, background concentrations of arsenic are 0.2 to 15 mg/kg in the lithosphere, 0.005 to 0.1 ug/m in air, <10 ug/l in water, and <15 mg/kg in soil (NRCC 1978). The commercial use and production of arsenic compounds have raised local concentrations in the environment far above the natural background concentrations (Table 1). Weathering of rocks and soils adds about 45,000 tons of arsenic to the oceans annually, accounting for less than 0.01 mg/l on a global basis (NRCC 1978). However, arsenic inputs to oceans increased during the past century 14