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in soil varied from 6.5 years for arsenic trioxide to 16 years for lead <br />arsenate (NRCC 1978). <br />In water, arsenic occurs in both inorganic and organic forms, and in <br />dissolved and gaseous states (EPA 1980). The form of arsenic in water depends <br />on Eh, pH, organic content, suspended solids, dissolved oxygen, and other <br />variables (EPA 1985). Arsenic in water exists primarily as a dissolved ionic <br />species; particulates account for less than 1% of the total measurable arsenic <br />(Maher 1985a). Arsenic is rarely found in water in the elemental state (0), <br />and is found in the -3 state only at extremely low Eh values (Lima et al. <br />1984). Common forms of arsenic encountered in water are arsenate, arsenite, <br />methanearsonic acid, and dimethylarsinic acid (EPA 1985). The formation of <br />inorganic pentavalent arsenic, the most common species in water, is favored <br />under conditions of high dissolved oxygen, basic pH, high Eh, and reduced <br />content of organic material; reverse conditions usually favor the formation of <br />arsenates and arsenic sulfides (NRCC 1978; Pershagen and Vahter 1979; EPA <br />1980), although some arsenite is attributed to biological activity (Maher <br />1985a). Water temperature seems to affect arsenic species composition in the <br />estuary of the River Beaulieu in the United Kingdom, where reduced and <br />methylated species predominate during warmer months and inorganic As during <br />colder months; the appearance of methylated arsenicals during the warmer <br />months is attributed both to bacterial and abiotic release from decaying <br />plankton and to grazing by zooplankton (Howard et al. 1984). Also <br />contributing to higher water or mobile levels are the natural levels of <br />polyvalent anions, especially phosphate species. Phosphate, for example, <br />displaces arsenic held by humic acids, and sorbs strongly on the hydrous <br />oxides of arsenates (Thanabalasingam and Pickering 1986). <br />Physical processes play a key role in governing arsenic bioavailability <br />in aquatic environments. For example, arsenates are readily sorbed by <br />colloidal humic material under conditions of high organic content, low pH, low <br />phosphate, and low mineral content (EPA 1980; Thanabalasingam and Pickering <br />1986). Arsenates also coprecipitate with, or adsorb on, hydrous iron oxides <br />and form insoluble precipitates with calcium, sulfur, aluminum, and barium <br />compounds (EPA 1980). Removal of arsenic from seawater by iron hydroxide <br />scavenging seems t?3be a prelgminant factor in certain estuaries. The process <br />involves both As and As and results in a measurable increase in arsenic <br />levels in particulate matter, especially at low salinities (Sloot et al. <br />1985). Arsenic sulfides are comparatively insoluble under conditions <br />prevalent in anaerobic aqueous and sedimentary media containing hydrogen <br />sulfide; accordingly, these compounds may accumulate as precipitates and thus <br />remove arsenic from the aqueous environment. In the absence of hydrogen <br />sulfide, these sulfides decompose within several days to form arsenic oxides, <br />sulfur, and hydrogen sulfide (NAS 1977). <br />In reduced environments, such as sediments, arsenate is reduced to <br />arsenite and methylated to methylarsinic acid or dimethylarsenic acids: these <br />compounds may be further methylated to trimethylarsine or reduced to <br />8