The fate of arsenic in groundwater discharged to the Meghna River, Bangladesh
Michelle Berube A , Katrina Jewell B , Kimberly D. Myers C , Peter S. K. Knappett C F , Pin Shuai C , Abrar Hossain D , Mehtaz Lipsi D , Sadam Hossain D , Alamgir Hossain D , Jacqueline Aitkenhead-Peterson E , Kazi M. Ahmed D and Saugata Datta A
+ Author Affiliations
- Author Affiliations
A Department of Geological Sciences, 108 Thompson Hall, Kansas State University, Manhattan, KS 66506, USA.
B Water Management and Hydrologic Sciences Program, 611 Ross Street, Texas A&M University, College Station, TX 77845, USA.
C Department Geology and Geophysics, Texas A&M University, College Station, TX 77845, USA.
D Department Geology, University of Dhaka, Dhaka 1000, Bangladesh.
E Department Soil and Crop Science, Texas A&M University, College Station, TX 77845, USA.
F Corresponding author. Email: knappett@tamu.edu
Environmental Chemistry 15(2) 29-45 https://doi.org/10.1071/EN17104
Submitted: 31 May 2017 Accepted: 13 October 2017 Published: 23 May 2018
Environmental context. Arsenic contamination of groundwater is a major environmental problem in many areas of the world. In south-east Asia, iron-rich reducing groundwater mixes with oxidising river water in hyporheic zones, precipitating iron oxides. These oxides can act as a natural reactive barrier capable of accumulating elevated solid-phase concentrations of arsenic.
Abstract. Shallow, anoxic aquifers within the Ganges–Brahmaputra–Meghna Delta (GBMD) commonly contain elevated concentrations of arsenic (As), iron (Fe) and manganese (Mn). Highly enriched solid-phase concentrations of these elements have been observed within sediments lining the banks of the Meghna River. This zone has been described as a Natural Reactive Barrier (NRB). The impact of hydrological processes on NRB formation, such as transient river levels, which drive mixing between rivers and aquifers, is poorly understood. We evaluated the impact of groundwater flow dynamics on hydrobiogeochemical processes that led to the formation of an Fe- and Mn-rich NRB containing enriched As, within a riverbank aquifer along the Meghna River. The NRB dimensions were mapped using four complementary elemental analysis methods on sediment cores: X-ray fluorescence (XRF), aqua regia bulk extraction, and HCl and sodium phosphate leaching. It extended from 1.2 to 2.4 m in depth up to 15 m from the river’s edge. The accumulated As was advected to the NRB from offsite and released locally in response to mixing with aged river water. Nearly all of the As was subsequently deposited within the NRB before discharging to the Meghna. Significant FeII release to the aqueous phase was observed within the NRB. This indicates the NRB is a dynamic zone defined by the interplay between oxidative and reductive processes, causing the NRB to grow and recede in response to rapid and seasonal hydrologic processes. This implies that natural and artificially induced changes in river stages and groundwater-tables will impact where As accumulates and is released to aquifers.
Additional keywords: groundwater-surface interactions, hyporheic zone, iron curtain, natural reactive barrier, NRB.
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