Reversible adsorption and flushing of arsenic in a shallow, Holocene aquifer of Bangladesh. (February 2017)

• Record Type: Journal Article
• Authors: Radloff, Kathleen A.; Zheng, Yan; Stute, Martin; Weinman, Beth; Bostick, Benjamin; Mihajlov, Ivan; Bounds, Margaret; Rahman, M.Moshiur; Huq, M.Rezaul; Ahmed, Kazi M.; Schlosser, Peter; van Geen, Alexander
• Is Part Of: Applied geochemistry. Volume 77(2017:Feb.)
• Page Start: 142
• Page End: 157
• Publication Date: 2017-02
• Subjects: Arsenic -- Bangladesh -- Adsorption -- Hydrology -- Groundwater age dating -- Push–pull tests; Environmental geochemistry -- Periodicals; Water chemistry -- Periodicals; Geochemistry -- Social aspects -- Periodicals; Geochemistry -- Periodicals; 551.9
• DOI: 10.1016/j.apgeochem.2015.11.003
• Languages: English
• ISSNs: 0883-2927

Abstract

Abstract: The spatial heterogeneity of dissolved arsenic (As) concentrations in shallow groundwater of the Bengal Basin has been attributed to transport of As (and reactive carbon) from external sources or to the release of As from within grey sand formations. We explore the latter scenario in this detailed hydrological and geochemical study along a 300 m transect of a shallow aquifer extending from a groundwater recharge area within a sandy channel bar to its discharge into a nearby stream. Within the 10–20 m depth range, groundwater ages along the transect determined by the 3 H– 3 He method increase from <10 yr in the recharge area to a maximum of 40 yr towards the stream. Concentrations of groundwater As within the same grey sands increase from 10 to 100 to ∼500 μg/L along this transect. Evidence of reversible adsorption of As between the groundwater and sediment was obtained from a series of push–pull experiments, traditional batch adsorption experiments, and the accidental flooding of a shallow monitoring well. Assuming reversible adsorption and a distribution coefficient, Kd, of 0.15–1.5 L/kg inferred from these observations, a simple flushing model shows that the increase in As concentrations with depth and groundwater age at this site, and at other sites in the Bengal and Red River Basins, can be attributed to the evolution of the aquifer over 100–1000 years as aquifer sands are gradually flushed of their initial As content. A wide range of As concentrations can thus be maintained in groundwater with increases with depth governed by the history of flushing and local recharge rates, without external inputs of reactive carbon or As from other sources. Graphical abstract: Highlights: Push–pull tests show fast, reversible As adsorption in a shallow reducing aquifer. Distribution coefficients of 0.15–1.5 L/kg confirm a large source of As within the aquifer. As-sulfide precipitation slows As release and can disrupt adsorptive equilibrium. A flushing model can explain As distributions in similar aquifers elsewhere.