Redox trapping of arsenic in hyporheic zones modified by silicate weathering beneath floodplains

Katrina Jewell, Kimberly D. Myers, Mehtaz Lipsi, Saddam Hossain, Saugata Datta, M. Bayani Cardenas, Jacqueline Aitkenhead-Peterson, Tom Varner, Kyungwon Kwak, Anne Raymond, Syed Humayun Akhter, Kazi M. Ahmed, Peter S.K. Knappett

Research output: Contribution to journalArticlepeer-review

1 Citation (Scopus)

Abstract

Groundwater containing high concentrations of dissolved arsenic (As) and iron (Fe(II)) discharges to rivers across the Ganges-Brahmaputra-Meghna delta. Observed Fe(III)-oxyhydroxide (FeOOH)–As deposits lining the riverbanks of the Meghna River may have been created by bidirectional mixing in the hyporheic zone (HZ) from ocean tides. This process has been named the Natural Reactive Barrier (NRB). Sedimentary organic carbon (SOC) is deposited annually on floodplains. Floodwaters that infiltrate through this layer may chemically transform the groundwater prior to discharging through the HZ in ways that influence the capture and retention of As in the NRB. The goal of this study is to understand how the interaction of these two scales of river-groundwater mixing influence the fate of As trapped within an NRB. Monitoring wells were installed to 1–17 m depth, up to 100 m distance from the river's edge during the dry season on the East (Site 1) and West (Site 2) sides of the river. They were sampled during the dry season (January) under gaining river conditions. The physical properties and elemental composition of the sediment was described by hand observation and hand-held X-Ray Fluorescence (XRF), respectively. Mixing with river water was quantified using the sum of charge of major cations (TC). Site 1 has a sloping bank that is only partially inundated during the wet season. The aquifer is composed of homogeneous sand. Site 2 is flat and therefore fully inundated in the wet season. The aquifer is composed of sand with thin (1–20 cm thick) clay layers. Both sites generate the dissolved products of FeOOH-reduction coupled to organic carbon oxidation, and silicate weathering beneath the floodplain. These products are dissolved Fe, As, silica, bicarbonate, calcium and phosphate. This chemistry is conducive to the formation of crystalline iron oxide minerals such as goethite which may co-precipitate with As, trapping it long-term.

Original languageEnglish
Article number105831
JournalApplied Geochemistry
Volume159
DOIs
Publication statusPublished - 2023 Dec
Externally publishedYes

Bibliographical note

Publisher Copyright:
© 2023 Elsevier Ltd

Keywords

  • Arsenic (As)
  • Bioturbation
  • Hyporheic zone (HZ)
  • Iron oxides
  • Natural reactive barrier (NRB)
  • Silicate weathering
  • Tides

ASJC Scopus subject areas

  • Environmental Chemistry
  • Pollution
  • Geochemistry and Petrology

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