Organic carbon mobilization in a Bangladesh aquifer explained by seasonal monsoon-driven storativity changes

► Seasonal storativity changes in clay-peat layers affect groundwater chemistry. ► During dry season, DOC is released and total mineralization increases. ► In rainy season, recharge leads to dilution and decrease of total mineralization. ► Depth-dependent As-distribution confirms DOC-driven reduction of iron hydroxides. ► Seasonal peak arsenic concentration follows peak DOC with an unexplained time lag.