Groundwater Chemistry and Arsenic Enrichment of the Ganges River Basin Aquifer Systems

Abstract

The Ganges river basin aquifers provide one of the most prolific groundwater reservoirs of the South Asia, specifically northern India and Bangladesh. The thick aquifers serve as a perennial source for fresh, potable groundwater and stretch from the Himalayan foothills (of the north-central India) up to the Bay of Bengal (to the east), and are confined along the Himalayan foredeep between the Main Frontal Thrust (MFT) to the north and the cratonic outcrops to the south. The basin is underlain by a main shallow semi-confined (yet interconnected) aquifer system, hosting hydrochemically heterogeneous groundwater facies, along with the existence of deeper isolated aquifers. In the central Gangetic basin (CGB), the aquifers can be classified to be of Pre-Cenozoic (PC) lithology, piedmont deposit (PD), younger alluvial (YA), and older alluvium (OA). The lower Gangetic basin (LGB) is predominantly composed of YA and OA, which also forms the most extensive and prolific aquifers. Recharge of groundwater in these aquifers has taken place from meteoric inflow or partially evaporated surface water. However, in recent times, irrigational return flow also acts as major source of recharge. The groundwaters’ composition is predominated by recent-aged, Ca–HCO₃ facies, with some aquifers hosting varied hydrochemical facies, ranging from Ca–Na–${{\text{HCO}}_{3}}^{ - }$–Cl to Na–${{\text{HCO}}_{3}}^{ - }$ types, indicating hydrogeochemical evolution through longer water–rock interaction. While most of the solutes for the YA groundwater are derived from weathering of young, Himalayan carbonate dissolution, many of the PD, PC, and OA in CGB and LGB groundwater have evolved by silicate weathering of silicate-rich other lithotypes and temporally longer evolutionary processes. Groundwater redox ranges from oxic to methanic, with indefinite spatial and depth variance, and is dominated by metal-reducing conditions. The coexisting redox-sensitive solutes, e.g., As(III), Fe(II), NH_3(dis), elevated HS⁻ indicate disequilibrium in an overall reducing, post-oxic condition, with overlapped redox zones. More than 75% of groundwater for the YA and PD, north of the 22.75° latitude, are found to have dissolved As ≥0.01 mg/L, with negligibly elevated concentrations recorded in the OA and PC groundwater. Dissolution/mobilization of arsenic can be caused by reduction of Fe(III), as a consequence of the coupled Fe–S redox cycles, and this mechanism is regarded as the dominant process of As release in the basin groundwater. However, processes like ion exchange or replacement, driven by competitive anions, introduced from active water–rock interactions, or from nutrients sourced from agricultural practices, can also act as potential triggers for As mobilization in groundwater.