Pamela A. Reilly Paul M. Bradley Julia L. Barringer 2013 Arsenic (As) is a metalloid element (atomic number 33) with one naturally occurring isotope of atomic mass 75, and four oxidation states (-3, 0, +3, and +5) (Smedley and Kinniburgh, 2002). In the aqueous environment, the +3 and +5 oxidation states are most prevalent, as the oxyanions arsenite (H₃AsO₃ or H₂AsO₃^- at pH ~9-11) and arsenate (H₂AsO₄^- and HAsO₄^2- at pH ~4-10) (Smedley and Kinniburgh, 2002). In soils, arsine gases (containing As^3-) may be generated by fungi and other organisms (Woolson, 1977). The different forms of As have different toxicities, with arsine gas being the most toxic form. Of the inorganic oxyanions, arsenite is considered more toxic than arsenate, and the organic (methylated) arsenic forms are considered least toxic (for a detailed discussion of toxicity issues, the reader is referred to Mandal and Suzuki (2002)). Arsenic is a global health concern due to its toxicity and the fact that it occurs at unhealthful levels in water supplies, particularly groundwater, in more than 70 countries (Ravenscroft et al., 2009) on six continents. application/pdf 10.5772/55354 en InTech Arsenic in groundwater: a summary of sources and the biogeochemical and hydrogeologic factors affecting arsenic occurrence and mobility chapter