Spatial Heterogeneity and Kinetic Regulation of Arsenic Dynamics in Mangrove Sediments: The Sundarbans, Bangladesh

Abstract

The biogeochemistry of arsenic (As) in sediments is regulated by multiple factors such as particle size, dissolved organic matter (DOM), iron mobilization, and sediment binding characteristics, among others. Understanding the heterogeneity of factors affecting As deposition and the kinetics of mobilization, both horizontally and vertically, across sediment depositional environments was investigated in Sundarban mangrove ecosystems, Bengal Delta, Bangladesh. Sediment cores were collected from 3 different Sundarbans locations and As concentration down the profiles were found to be more associated with elevated Fe and Mn than with organic matter (OM). At one site chosen for field monitoring, sediment cores, pore and surface water, and in situ diffusive gradients in thin films (DGT) measurements (which were used to model As sediment pore-water concentrations and resupply from the solid phase) were sampled from four different subhabitats. Coarse-textured riverbank sediment porewaters were high in As, but with a limited resupply of As from the solid phase compared to fine-textured and high organic matter content forest floor sediments, where porewater As was low, but with much higher As resupply. Depositional environment (overbank verses forest floor) and biological activity (input of OM from forest biomass) considerably affected As dynamics over very short spatial distances in the mosaic of microhabitats that constitute a mangrove ecosystem.

Salinization and arsenic contamination of surface water in southwest Bangladesh

“…Groundwater compositions are distinct from all surface water types and could have formed by mixing of dry season tidal channel water and rainwater followed by reduction. The anomalously high DOC in groundwater is likely preserved from the connate water, as soil porewater in the Sundarbans has similarly high DOC [ 12 , 49 ]. The DOC could also derive from surface sources such as freshwater ponds and shrimp ponds [ 50 – 52 ], which have high measured DOC concentrations (Fig.…”

Sources of salinity and arsenic in groundwater in southwest Bangladesh

“…For example, one tubewell may yield saline water, while a tubewell next to it drilled to roughly the same depth yields fresh water. Such local-scale heterogeneity may not be unexpected for non-conservative, redox-sensitive elements such as As (e.g., [ 16 , 18 , 45 ]), but it is perhaps more surprising for conservative elements in a Holocene-age aquifer. This suggests that the spatial distributions of salinity and As concentrations cannot be distinguished from a random distribution.…”

“…Arsenic dissolved in groundwater in south Asia is sourced from subsurface sediments, where it is sorbed onto Hydrous Ferric Oxides (HFOs; [ 18 , 19 ]). Many previous studies have concluded that reductive dissolution of HFOs driven by microbial metabolism of organic matter results in increased concentrations of dissolved arsenic in shallow groundwaters in Bangladesh (e.g., [ 7 ]).…”

Preliminary Evidence of Transport‐Limited Chemical Weathering and Element Immobility in the Ganges Tidal Delta Plain of Bangladesh

“…Saturation index calculations show that rice paddy soil and tidal channel sediment extracts are always saturated in hydroxyapatite and kaolinite and usually saturated in illite, gibbsite, smectite minerals beidellite and saponite, and dolomite ± quartz and calcite. Extracts are undersaturated in hydrous ferric oxyhydroxides that often control the mobility of arsenic and other elements in this region (Hossain et al, 2012). This may be due to their absence in the solids, to disequilibrium, or to their abundance being less than required to saturate the volume of extract solution.…”