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Methylated arsenicals, whether herbicides or defoliants, are sprayed on <br />plant surfaces. They can reach the soil during application or can be washed <br />from the plants. Additional arsenic enters soils by exchange from the roots <br />or when dead plant materials decay (Hood 1985). Cacodylic acid and sodium <br />cacodylate are nonselective herbicides used in at least 82 products to <br />eliminate weeds and grasses around trees and shrubs, and to eradicate <br />vegetation from rights-of-ways and other noncrop areas (Hood 1985). Normal <br />application rates of various organoarsenicals for crop and noncrop purposes <br />rarely exceed 5 kg/ha (Woolson 1975). Under recommended treatment levels, <br />organoarsenical soil residues were not toxic to crops, and those tested <br />(soybean, beet, wheat) were more resistant to organoarsenicals than to <br />comparable levels of inorganic arsenicals (Woolson 1975). <br />Air concentrations up to 3.9 ug As/m3 near gold mining operations were <br />associated with adverse effects on vegetation; higher concentrations of 19 to <br />69 ug As/m , near a coal fired power plant in Czechoslovakia, produced <br />measurable contamination in soils and vegetation in a 6-km radius (NRCC <br />1978). <br />The phytotoxic actions of inorganic and organic arsenicals are different, <br />and each is significantly modified by physical processes. The primary mode of <br />action of arsenite in plants is inhibition of light activation, probably <br />through interference with the pentose phosphate pathway (Marques and Anderson <br />1986). Arsenites penetrate the plant cuticle to a greater degree than <br />arsenates (NAS 1977). One of the first indications of plant injury by sodium <br />arsenite is wilting caused by loss of turgor, whereas stress due to sodium <br />arsenate does not involve rapid loss of turgor (NAS 1977). Organoarsenicals, <br />such as cacodylic acid, enter plants mostly by absorption of sprays; uptake <br />from the soil contributes only a minor fraction (Hood 1985). The <br />phytotoxicity of organoarsenical herbicides is characterized by chlorosis, <br />cessation of growth, gradual browning, dehydration, and death (NAS 1977). In <br />general, plants cease to grow and develop after the roots have absorbed much <br />arsenic (NRCC 1978). Plants can absorb arsenic through the roots and foliage, <br />although translocation is species dependent. Concentrations of arsenic in <br />plants correlate highly and consistently with water extractable soil arsenic, <br />and usually poorly with total soil arsenic (NRCC 1978). For example, <br />concentrations of arsenic in corn (Zea mays) grown in calcareous soils for 25 <br />days were significantly correlated with the soil water extractable arsenic <br />fraction, but not other fractions; extractable phosphorus was correlated <br />positively to both arsenic in corn and to the water soluble arsenic fraction <br />(Sadiq 1986). In the moss Hylocomium splendens, arsenate accumulation from <br />solution was through living shoots, optimum uptake being between pH 3 and 5 <br />(Wells and Richardson 1985). Some plants, such as beets (Beta vulgaris), <br />accumulated arsenic more readily at elevated temperatures, but the addition of <br />phosphate fertilizers markedly depressed uptake (Merry et al. 1986). <br />Soils amended with arsenic-contaminated plant tissues were not measurably <br />affected in C02 evolution and nitrification, suggesting that the effects of <br />43