Ground‐Water Geochemistry: Arsenic in Landfills

Abstract

ABSTRACTThree key hydrogeochemical environments based on the oxidation‐reduction state of ground water are defined by the presence or absence of dissolved oxygen and hydrogen sulfide. The mobility of arsenic is greatly influenced by these dissolved gases and the behavior of iron in these same environments.The main forms of arsenic in aqueous environments are arsenate and arsenite. Arsenate occurs in oxygenated waters but if colloidal ferric hydroxide is also present, the arsenate may be adsorbed and removed from solution. In strongly reducing waters containing hydrogen sulfide arsenic is removed from solution and precipitated either as arsenic sulfide, heavy‐metal sulfarsenite compounds, or coprecipitated with iron sulfides. In mildly reducing waters lacking both dissolved oxygen and hydrogen sulfide arsenic occurs as the arsenite ion. In waters of this type the mobility of both arsenic (as arsenite) and iron (as ferrous iron) are at a maximum. It must also be stressed that arsenite is 60 times more toxic to humans than arsenate.