Mobilization of Arsenic During One-Year Incubations of Grey Aquifer Sands from Araihazar, Bangladesh

Abstract

Elevated As concentrations in shallow groundwater pose a major health threat in Bangladesh and similarly affected countries, yet there is little consensus on the mechanism of As release to groundwater or how it might be influenced by human activities. In this study, the rate of As release was measured directly with incubations lasting 11 months, using sediment and groundwater collected simultaneously in Bangladesh and maintained under anaerobic conditions throughout. Groundwater and grey sediment were collected as diluted slurries between 5 and 38 m depth, a range over which ambient groundwater As concentrations increased from 20 to 100 μg L −1 . Arsenic was released to groundwater in slurries from 5 and 12 m depth at a relatively constant rate of 21 ± 4 (2 σ) and 23 ± 6 μg As kg −1 yr −1 , respectively. Amendment with a modest level of acetate increased the rate of As release only at 12 m (82 ± 18 μg kg −1 yr −1 ). Although the groundwater As concentration was initially highest at 38 m depth, no release of As was observed. These results indicate that the spatial distribution of dissolved As in Bangladesh and local rates of release to groundwater are not necessarily linked. Iron release during the incubations did not occur concurrently with As release, providing further confirmation that the two processes are not directly coupled. Small periodic additions of oxygen suppressed the release of As from sediments at all three depths, which supports the notion that anoxia is a prerequisite for accumulation of As in Bangladesh groundwater.

Microbial Mineral Weathering for Nutrient Acquisition Releases Arsenic

“…Importantly, we believe that ancillary arsenic release has been obscured by the overprint of iron cycling. Laboratory and field experiments are showing that Fe and As release are decoupled (17,33,42) and that arsenic release occurs at a constant rate across various depositional environments (39). These observations are consistent with our novel mechanism that can occur under all redox conditions.…”

“…Collecting microbially pristine sediment samples from Bangladesh aquifers is difficult and requires novel techniques (33, 41) compared to those traditionally utilized (16,29). Traditional drilling techniques are not available, and extensive work has been undertaken with the local drillers to obtain pristine samples that are not contaminated by drilling fluid (33,41). Briefly, during drilling a core or evacuated cylinder is lowered through the drill casing to collect sediment from below the casing.…”

Anthropogenic influences on groundwater arsenic concentrations in Bangladesh

“…A variety of previous laboratory incubation studies [22][23][24][25] have found that the addition of labile organic carbon, or BDOC, mobilizes arsenic from Bengali aquifer sediments. Under a pond that loses ∼1.5 cm day −1 of water to the subsurface, sampling lysimeters show that dissolved arsenic increases with depth, although, at the deepest sampling point, arsenic has not reached the peak concentration measured in the aquifer (Fig.…”

Arsenic retention and transport behavior in the presence of typical anionic and nonionic surfactants

“…Both processes are to a great extent influenced by coexisting substances such as natural organic matters (NOM) and anions. It has been well documented that such substances may reduce arsenic adsorption and enhance arsenic transport through adsorption competition, complexation reactions, anion exchange and electrostatic repulsion (Barringer et al, 2011;Dias et al, 2009;Garcia-Sanchez et al, 2010;Garcia et al, 2007;Guo et al, 2014;He et al, 2010;Radloff et al, 2007;Reza et al, 2010;Serfes et al, 2003;Zheng et al, 2004). Thus, through such mechanisms, surfactant molecules may potentially influence arsenic adsorption and transport behavior in the environment.…”