A review of arsenic (III) in groundwater

Abstract

Recent improvements in sample collection and analytical techniques have suggested that As(III) is more prevalent in groundwater than previously believed. Indeed, reducing conditions in alluvial aquifers supplying single families may result in significant exposures to naturally occurring As(III). These results are noteworthy because As(III) is both more toxic and more mobile in the environment than As(V). The literature contains contradictory information concerning the appropriate preservation and analytical techniques for determining As(III). It appears that several previously reported occurrences of As(V) may have been predominantly As(III), but the samples were either not preserved or analyzed properly. For example, separation of arsenic species by ion exchange is apparently necessary to obtain reliable analytical results for certain environmental samples. The problems encountered with investigating As(III) in the environment are due to the complex series of geochemical reactions undergone by arsenic. The complexity of these reactions and the variable experimental conditions used by different investigators have resulted in widely different conclusions concerning both the nature of arsenic adsorption reactions and reaction kinetics. Moreover, it appears that biological reactions may play a role in certain ecosystems. In general, the mechanism promoting the mobility of As(III) in groundwater is the onset of reducing conditions in alluvium in which iron oxides have sorbed arsenic. Such conditions may result in concentrations of arsenic in groundwater as high as several hundred micrograms per liter.