GROUNDWATER FLOW AND ARSENIC DISTRIBUTION IN THE BENGAL BASIN
We show that the very subdued basin topography is sufficient to generate a hierarchical system of groundwater flow. Basin-margin, As-free regions are the sources for deep groundwater, as corroborated by a vertical profile of groundwater age established in SE Bangladesh in which age is invariant with depth at >100 m. The models illustrate the variety of patterns of As distribution in BAS, as individually described in published studies. These patterns are all consistent with and predictable from the SiHA hypothesis, which explains (i) the low background level of As in groundwater at >150 m, other than exceptionally where (ii) absence of silt-clay layers has enabled deeper penetration of As-bearing groundwater; (iii) the sharp transition from As-bearing to low-As groundwater in depth profile; (iv) consistently excessive As at shallow depth in zones of relative groundwater stagnancy; (v) regions where shallow groundwater As is at low background level, beneath and adjacent to sites where topography imposes greater hydraulic gradients and effective groundwater flushing; (vi) steep lateral gradients in As concentration at the boundaries of groundwater discharge zones, including where the juxtaposition of older and younger groundwater, previously paradoxical, is an expected outcome under SiHA.
We conclude that As has been restricted to shallow levels of BAS since the early stages of basin/aquifer development, and will remain so for as long as high-stand sedimentation continues in the basin. Deeper levels of the aquifer will remain at a low background As level, unless As is re-distributed by excessive pumping.
*SiHA: Silt-clay layers impose Hierarchical groundwater flow, constraining Arsenic progression