Arsenic is present in a variety of environmental media including minerals, rocks, sedimentary deposits, soil, water and plants. It tends to be transported and exchanged among these environmental components. Arsenic can be released to the environment through human activities including mining exploitation, metal smelting, waste incineration, coal combustion and the use of pesticides, herbicides, crop desiccants, wood preservatives, and food additives for livestock containing arsenic compounds. Bioaccumulation of arsenic occurs in some aquatic organisms. The existence of a biological methylation/demethylation cycle for arsenic results in the presence of various forms of both organic and inorganic arsenic in the environment. Arsenic in water can undergo a complex series of transformations, including oxidation-reduction reactions, ligand exchange, precipitation and biotransformation. Acid mine drainages as well as mine tailings are of great concern due to their extremely high arsenic concentrations observed and reported.