In natural water arsenic participates in oxidation and reduction reactions, coagulation and adsorption. Adsorption of arsenic to fine particles in water and precipitation with aluminium or iron hydroxides causes arsenic to enter sediments. After some time, arsenic may dissolve once again consequential to reduction reactions.
Read more: http://www.lenntech.com/periodic/water/arsenic/arsenic-and-water.htm#ixzz3Yb7ADfPl