That gives either arsenic substituted iron sulfide or arsenopyrite. These formations are stable unless they are disturbed or exposed to oxygen. These kinds of depositions are in 2 impervious layers under modern delta formations of the Gangetic plain. One within 15 to 30 meters depth and the other one below 100 meters depths. The Himalayas have pyrites and sedimentary formations as it is marine in nature origin.
Marine conditions are the ultimate resting place for metals or elements or compounds.
So the arsenic is certainly a geologic source because no organic arsenic compounds were found at high concentration. These aquifers are also hydraulically connected to the major streams in Bangladesh, especially the Ganges in the Northwestern region of Bangladesh. Ground water recharge is low due to less rainfall and upstream diversion of Ganges water by India. During the dry season, the water table falls to below more than 25-30 feet. So, these two-fold reactions released the arsenic in the water.