Geochemical Evolution of Groundwater Flowing Through Arsenic Source Sediments in an Aquifer System of West Bengal, India

The source of geogenic arsenic (As) contaminating a shallow aquifer in West Bengal was traced to fine-grained sediments deposited in an abandoned river channel. Along with As-bearing phases, these sediments contain 0.46% codeposited organic carbon. The release of As and the geochemistry of groundwater within the channel-fill deposits is investigated using a detailed mass balance model supported by aqueous, solid-phase, and mineralogical data. The model describes the evolution of groundwater chemistry along a flow path extending from its recharge in an abandoned channel pond, through the channel fill, to the underlying aquifer. Variations in groundwater composition are explained in terms of mineral weathering of host sediments driven by organic carbon decay. Arsenic is released through the reductive dissolution of goethite and the weathering of chlorite. Concomitantly, some As is sequestered in precipitating vivianite. These competing processes reach equilibrium deeper in the channel-fill sequence as groundwater As concentrations stabilize. The model yields estimates of mineral reaction (or precipitation) rates including rates of organic carbon oxidation (1.15 mmol CL-1 a(-1)) and net As release (4.57 x 10(-4) mmol L-1 a(-1)). Fine-grained, slightly permeable, deposits such as channel fill containing reactive organic carbon and As-bearing goethite and phyllosilicates are centers of intense chemical weathering conducive to As mobilization.