Mobilisation of arsenic in sedimentary aquifers may be, in part, a result of changes in the geohydrochemical environment due to agricultural irrigation. In the deeper subsurface, elevated arsenic concentration is associated with compaction caused by groundwater withdrawal. An understanding of geochemical principles is essential for an explanation of the mobilisation of arsenic by natural processes. Along with supergenic mobilisation of elements arsenic is transferred from one phase to another or within a heterogeneous phase such as soil by microbially mediated processes. Aerobic bacteria and fungi often transform arsenic into coordination complexes that are more mobile in soil water than the uncomplexed cation.
Movement is a strong function of speciation and soil type.
Soil pH also influences arsenic mobility. The West Bengal Government's investigation revealed that a 450 km long layer of arsenic rich silt clay is lying between the depths of 70 and 200 feet below the surface of the upper deltaic plain of river Bhagirathi. All these zones are located between the Ganges-Bhagirathi river and the western border of Bangladesh. The sediments on both sides of the border have the same depositional history and geological environment. The area is a part of the Ganga-Brahmaputra delta.