Reversible adsorption and flushing of arsenic in a shallow, Holocene aquifer of Bangladesh

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 3H–3He 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.

The Impact of Aquifer Flushing on Groundwater Arsenic Across a 35‐km Transect Perpendicular to the Upper Brahmaputra River in Assam, India

“…Using a somewhat more involved method to displace As from adsorption sites, van Geen et al () showed that this type of relation between exchangeable As in Holocene sediment and groundwater As can be attributed to equilibrium exchange. Sizeable concentrations of exchangeable As in shallow gray sands have been confirmed by a series of push‐pull experiments in Bangladesh during which low‐As water was pumped into a high‐As aquifer and vice versa (Radloff et al, ). The broad increase in both groundwater As concentrations and exchangeable As levels in gray sediment from the river to the hills can therefore plausibly be attributed to some form of equilibration, even if the sediment is of Pleistocene rather than Holocene age.…”

Evidence of decoupling between arsenic and phosphate in shallow groundwater of Bangladesh and potential implications

“…However, long-term trends in As concentrations, both increases and decreases, have been documented over a period of 8 years for a number of wells within a relatively small region of West Bengal, India (McArthur et al, 2010). Some shorter time series have shown significant seasonal variability in groundwater composition, particularly in shallow aquifers, but by and large variations in groundwater As have been much more limited (BGS/DPHE, 2001; Cheng et al, 2005; Dhar et al, 2008; Bhattacharya et al, 2011; Planar-Friedrich et al, 2012; Neidhardt et al, 2013; Radloff et al, this volume). Only for large perturbations induced by local pumping in the vicinity of a spatial gradient have larger variations in groundwater As concentrations been reported (McArthur et al, 2010; Jessen et al, 2012; Neidhardt et al, 2013).…”

“…The muted response of As concentrations relative to other groundwater constituents reported in the present study over a period of two years is therefore consistent with previous findings. The characteristic buffering of As concentrations in the face of groundwater movement has been attributed to adsorptive exchange with a large pool residing on aquifer sands (Dhar et al, 2008; Radloff et al, this volume). A contributing explanation that limits variations in groundwater As concentrations suggested by the present study is the formation of sulfides across a sharp redox-interface that dampens the effect of a vertical displacement in the concentration gradient.…”

Geochemical Evolution of Groundwater Flowing Through Arsenic Source Sediments in an Aquifer System of West Bengal, India

“…Different schools of thought concerning As and OC sources have emerged. With respect to As sources, proponents of the “aquifer” hypothesis (Neumann et al, ; Postma et al, ; Radloff et al, ; Stute et al, ; Swartz et al, ; van Geen et al, ) contend that the source of As is dispersed within the aquifer itself. Proponents of the “aquitard” hypothesis (Desbarats et al, ; Polizzotto et al, ) hold that the As source is localized and situated within fine‐grained sedimentary units from which it is mobilized and transported to adjacent aquifers.…”