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adding arsenic to soils does not influence the decomposition rate of plant tissues by soil microorganisms (Wang et al. 1984). The half-life of cacodylic acid is about 20 days in untreated soils and 31 days in arsenic-amended soils (Hood 1985). Estimates of the half-life of inorganic arsenicals in soils are much longer, ranging from 6.5 years for arsenic trioxide to 16 years for lead arsenate (NRCC 1978). Data on arsenic effects to soil biota and insects are limited. In general, soil microorganisms are capable of tolerating and metabolizing relatively high concentrations of arsenic (Wang et al. 1984). This adaptation seems usually to be due to decreased permeability of the microorganism to arsenic (NAS 1977). Tolerant soil microbiota can withstand concentrations up to 1,600 mg/kg; however, growth and metabolism were reduced in sensitive species at 375 mg As/kg and, at 150 to 165 mg As/kg, soils were devoid of earthworms and showed diminished quantities of bacteria and protozoan (NRCC 1978). Honeybees (Apis mellifera) that were killed accidentally by As+ spray dusting contained 4 to 5 ug As per bee (NAS 1977)--equivalent to 21 to 31 mg/kg body weight (Table 3). Larvae of the western spruce budworm (Choristoneura occidentalis) continued to feed on As+3-contaminated vegetation until a threshold level of about 2,300 to 3,300 mg As/kg dry weight whole larvae was reached; death then sometimes occurred (Table 3; Robertson and McLean 1985). Larvae that had accumulated sufficient energy reserves completed the first stage of metamorphosis, Yyt developed into pupae of subnormal weight; larvae containing <2,600 mg As /kg ultimately developed into adults of less than normal weight, and some containing >2,600 mg/kg dry weight died as pupae (Robertson and McLean 1985). AQUATIC BIOTA Adverse effects of arsenicals on aquatic organisms have been reported at concentrations of 19 to 48 ug/l in water, 120 mg/kg in diets, and 1.3 to 5 mg/kg fresh weight in tissues (Table 4). The most sensitive aquatic species tested showing adverse effects were three specks of marine algae, which showed reduced growth in the range of 19 to 22 ug As /1; developing embryos of the narrow-mouthed toad (GastrophrynUP. carolenensis), of which 50% were dead or malformed in 7 days at 40 ug As /l; and a freshwater alga ( enedesmus obliquus), in which growth was inhibited 50% in 14 days at 48 ug As /1 (Table 4). Chronic studies with mass cultures of natural phytoplankton communiti?? exposed to low levels of arsenate (1.0 to 15.2 ug/1) showed that As differentially inhibits certain plants, causing a marked change in species composition, succession, and predator-prey relations; the significance of these changes on carbon transfer between trophic levels is unknown (Sanders and Cibik 1985; Sanders 1986). Adverse biological effects have also ?een documented at water concentrations of 75 to 100 ug As/1. At 75 ug As+ /l, growth and biomass in freshwater and marine algae was reduced; at 85 to 88 44