ARSENIC MOBILIZATION AND RETENTION CAUSED BY PARTIAL REDOX EQUILIBRIUM IN DEEPER GROUNDWATER OF THE WESTERN BENGAL BASIN, WEST BENGAL, INDIA

A detailed regional-scale hydrological study in a ~21,000-km² area of the Indian state of West Bengal suggests the absence of complete equilibrium among various redox processes. The study area includes parts of four arsenic-affected districts (Murshidabad, Nadia, North and South 24 Parganas) east of the main distributary of the River Ganges. The lack of equilibrium, which may in part be an artifact of extensive pumping for irrigation and municipal use, is believed to be a primary control on arsenic (As) mobilization and retention in deeper water of the main semiconfined regional aquifer.

The suboxic main-aquifer water exhibits overlapping redox zones (postoxic, sulfidic and methanic). The redox processes are depth-dependent and hydrostratigraphically variable. The observed Eh values of the main aquifer water samples range from 311.42 to -40.16 mV with a mean of 80.79 mV, thus falling mainly in the calculated Eh range of the Fe(II)/Fe(III) couple and approaching As(III)/As(V). This suggests that Fe(III) reduction is the dominant redox process in deep groundwater of the western Bengal basin. However, the co-existence of O₂, NH₄⁺, Fe(II), As(III), V, SO₄^2-, and CH₄, in various wells indicates that the aquifer is not at redox equilibrium. Moreover, observed spatial trends in values of δ³⁴S_SO4, δ¹³C_DIC and the As(III)/total As ratio suggest the possibility of oxidation in micro-scale environments.

Elevated dissolved As is related to reductive dissolution of Fe(III), but is influenced by coupled Fe-S redox cycles. Arsenic does not show good correlation with most solutes at the regional scale, which suggests involvement of multiple processes in As cycling, including reoxidation. Such oxidation may be caused by re-circulation of water by extensive deep irrigational pumping, thus drawing more aerated water from shallower depths. Partial redox equilibrium could lead to the existence of As in solution even in sulfidic and methanogenic zones by remobilizing the sequestered or re-adsorbed As from sulfide precipitates or residual metal (oxy)hydroxides.