Arsenic (As), a toxic element is a concern for the habitants in and around the coastal areas of West Bengal (India) where world's largest Sundarbans mangrove is situated. Little is known about the potential of these mangroves in storing As within their biomass and transporting to the Bay of Bengal. A comprehensive yearly data of above ground and below ground biomass (AGB, BGB), and exchange fluxes like litter fall, plant uptake, sedimentary diffusion/advection, and suspended particle deposition were used in a box model for constructing As budget for the Sundarbans mangroves. About 80% of total As was stored in AGB while As stock in sediment was several hundred times higher than in the AGB and BGB indicating poor bioaccumulation and sequestration capacity of the mangroves, which was further supported by higher As loss though litterfall (16.8 μg As m−2 month−1) compared to gain through plant uptake (0.05 μg As m−2 month−1). About 65% of the river-discharged As exported to the Bay of Bengal, the rest amounting to 67.2 × 103 kg yr−1 remained in the mangrove estuaries. Although ecotoxicological indexes confirmed low As pollution impact in the Sundarbans, mass budget revealed net As addition in the estuarine ecosystem (67.2 Mg As yr−1), mainly derived from natural and anthropogenic sources such as, contamination via atmospheric dust deposition. Overall reservoir-based mass budget showed weak As sequestration capacity by these mangroves. The approach developed for As in this study could be applied to other major metals to estimate metal sequestration and conservation potential by the Sundarbans mangroves. © 2019 Elsevier B.V.