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National Irrigation Water Quality Program Guidelines carcinogenic in many mammal species (Eisler 1988,1994). However, beneficial effects have been reported in tadpoles, silkworm, rats, goats, and pigs at low dietary concentrations (Eisler 1988). Mammals with arsenic deficiencies display poor growth, reduced survival, and inhibited reproduction, whereas low doses of arsenic actually stimulate growth in plants and animals (Eisler 1994). Arsenic's toxicity and bioavailability may vary significantly, depending on the chemical forms and routes of exposure. In general, inorganic arsenic compounds are more toxic than organic compounds, and As (Ul) is more toxic than As (V) (Eisler 1988,1994). Hence, the natural conversion of As (ITI) to As (V), which is favored in most aquatic environ- ments (Manahan 1989), somewhat reduces the overall hazard of this element. It should be noted, though, that most dietary studies rely on only a single species of arsenic-generally inorganic-and that such studies thus do not reflect the diversity of arsenic species present in the environment. The varying effects of different arsenic compounds should be considered before using experimental data to assess the toxicity of arsenic in the environment. In the aquatic environment, adverse effects of arsenic have been reported at a wide range of concentrations in water, sediment, and diets. Suter and Mabrey (1994) evaluated a series of toxicological benchmarks for screening various contaminants for their potential effects on aquatic biota. In addition to the national ambient water quality (NAWQ) criteria, they provided secondary acute and chronic values, lowest chronic values (including those for fish, daphnids, nondaphnid invertebrates, aquatic plants, and all organisms), test EC20s (concen- trations that cause observable ill effects in 20 percent of specimens), sensitive species test EC20s, and population EC20s. These data were used to establish the general biotic effect levels presented in table 1. As listed there, UNo effect" is the lowest chronic value for all organisms; uToxicity threshold" is the NAWQ CfJ I chronic criterion (if established) or the secondary chronic value; and -Level of concern" is the range between the two other values. I I Field Cases I I I I I Though arsenic is ubiquitous in the environment, the incidence of wildlife poisoning by arsenic is relatively rare (Eisler 1988). Sandhu (1971) reported an intensive fish kill in a reservoir at Orangeburg, South Carolina, after aerial spraying of arsenic defoliants in a nearby cotton field. The arsenic concentration in the water was elevated to 2,500 Ilg/L, and catfish in the reservoir were reported to contain 5 and 12 mg As/kg in skeletal muscle after 5-hour and 7-week exposures (weight basis not specified). Arsenic is also relatively persistent in the aquatic environment. Tanner and Clayton (1990) reported elevated concentrations of arsenic in macrophytes (193-1,200 mg/kg dw) and surficial sediments (540-780 mg/kg dw) in Lake Rotoroa, New Zealand, 24 years after an application of sodium arsenite herbicide; arsenic levels in a nearby reference lake (Lake Rotokauri) were <20 mg/kg dw in macro- phytes and 16.5-40 mg/kg dw in sediments. (Note, however, that the ureference lake" had arsenic concentrations in the sediments that are in the middle of the levels of concern in table 1, and the detection limit for the macro- phyte datum was four times the toxicity threshold for plants in table 1. Alternatively, the uliving" macrophytes had arsenic con- centrations of between 39 and 240 times the toxicity threshold and are obviously tolerant species.) I I I I I I I I Natural sources, such as hot springs and volcanic activity, also contribute to elevated levels of arsenic in the environment. Lacayo et al. (1992) determined arsenic levels in water, fish, and sediments from XolotlAn, Managua, Nicaragua, a lake which contained high levels of arsenic from such sources. I I I