However, natural geological changes are presumed to be the primary reason for arsenic contamination. The basis for such belief lies with the studies conducted in West Bengal, India, having similar alluvial deposits. Arsenic contamination in groundwater is due to the various natural geological processes that exist in the geological environment. The source of arsenic in sediments is mainly the parent rock materials from which it is derived. Arsenic associated with sediment particles can be a major source of arsenic contamination when particles are detached and carried as sediments during erosion.
Sediments can contain substantial amounts of total arsenic.
During the formation of sedimentary rocks, arsenic is carried down by precipitation of iron hydroxides and sulphides. In a moist climate, arsenic sulphides are easily oxidized, become water-soluble, are washed out of the sediment particles by meteoric precipitation, and are transported with run off. All of these reactions control mobilization and accumulation of arsenic in the environment. A biotic reaction between arsenic species and the substrates on the species and the substrates on the sediment surface, as well as physical disturbance of sediments, all play very important roles in controlling the mobilization of arsenic. In nature, arsenic bearing minerals undergo oxidation and release arsenic to water.