Influence of monsoonal recharge on arsenic and dissolved organic matter in the Holocene and Pleistocene aquifers of the Bengal Basin.

Arsenic (As) mobilization in the Bengal Basin aquifers has been studied for several decades due to the complex redox bio-geochemistry, dynamic hydrogeology and complex nature of dissolved organic matter (DOM). Earlier studies have examined the changes in groundwater As in the dry season before monsoon and during the wet season after monsoonal recharge. To investigate the more immediate influence of recharge during the active monsoon period on As mobilization and DOM character, groundwater samples were analyzed in the pre-monsoon and during the active monsoon period. Groundwater samples were collected from shallow (<40 m) and deep (>40 m) tube-wells in West Bengal, India. Dissolved AsT in shallow groundwater ranged from 50 to 315 μg/L exceeding the WHO guideline of 10 μg/L. Shallow groundwater also showed high total dissolved nitrogen, carbon to nitrogen (C:N) <1, and humic-like DOM with a humic:protein ratio >1. By contrast, deep groundwaters contained AsT between 0.5 and 11 μg/L with carbonaceous and protein-like DOM, C:N >1, and humic:protein <1. Stable isotopes of δ18 O and δ2 H and Cl/Br results indicated three recharge scenarios in the shallow aquifer including direct recharge of dilute rainwater, evaporated surface water, and anthropogenically impacted surface water. Monsoonal recharge did not cause notable changes in AsT in deep or shallow groundwater, including two As hotspots in the Pleistocene aquifer. However, the monsoon did result in a two-fold decrease in SUVA254 , increase in nitrite and nitrate in the shallow groundwater. The DOM in the deep groundwater at the two As hotspots (with AsT 132 and 715 μg/L) had optical properties with much greater humic-like DOM than the surrounding groundwater, which had low AsT and highly protein-like DOM. Overall, these results support that protein-like DOM associated with low groundwater As concentrations and suggest that the monsoonal influence on nitrate and nitrite is limited to shallow aquifers.