Advection of surface-derived organic carbon fuels microbial reduction in Bangladesh groundwater

Abstract

Chronic exposure to arsenic (As) by drinking shallow groundwater causes widespread disease in Bangladesh and neighboring countries. The release of As naturally present in sediment to groundwater has been linked to the reductive dissolution of iron oxides coupled to the microbial respiration of organic carbon (OC). The source of OC driving this microbial reduction-carbon deposited with the sediments or exogenous carbon transported by groundwater-is still debated despite its importance in regulating aquifer redox status and groundwater As levels. Here, we used the radiocarbon ( 14 C) signature of microbial DNA isolated from groundwater samples to determine the relative importance of surface and sediment-derived OC. Three DNA samples collected from the shallow, high-As aquifer and one sample from the underlying, low-As aquifer were consistently younger than the total sediment carbon, by as much as several thousand years. This difference and the dominance of heterotrophic microorganisms implies that younger, surface-derived OC is advected within the aquifer, albeit more slowly than groundwater, and represents a critical pool of OC for aquifer microbial communities. The vertical profile shows that downward transport of dissolved OC is occurring on anthropogenic timescales, but bomb 14 C-labeled dissolved OC has not yet accumulated in DNA and is not fueling reduction. These results indicate that advected OC controls aquifer redox status and confirm that As release is a natural process that predates human perturbations to groundwater flow. Anthropogenic perturbations, however, could affect groundwater redox conditions and As levels in the future.A quifer redox status is a major factor affecting groundwater composition and its suitability for human consumption. The most egregious example is the dire health impact of elevated levels of arsenic (As) in groundwater drawn with inexpensive handpumped tube wells by more than 100 million villagers across South, Southeast, and East Asia (1, 2). Aquifer redox status is largely controlled by microbial respiration of organic carbon (OC) coupled to the utilization of terminal electron acceptors. Reduction of iron (Fe) oxides containing As coupled with the oxidation of OC is the dominant process causing the accumulation of As in groundwater (1), but there is no consensus on the source of OC that fuels these transformations. Such information is critical to understanding subsurface carbon cycling, redox reactions, and the vulnerability of groundwater aquifers to perturbation.Human perturbations have long been suggested to exacerbate the problem of elevated As in groundwater in several ways. Given the connectivity between contaminated surface waters and aquifers, widespread pumping could redistribute As from the surface to depth (3, 4) or between aquifers (5). This pumping could potentially also deliver reactive OC from ponds and latrines to depth, causing microbial reduction and As release (3, 4). Fluorescent spectra of dissolved organic carbon (DOC), correlations between sedimentary ...

Characterization and microbial utilization of dissolved lipid organic fraction in arsenic impacted aquifers (India)

“…In this context, our investigations suggest complex biogeochemical interactions between microbial communities, OM sources and redox conditions in the BDP aquifers in Nadia district in West Bengal (Routh et al, 2005;Routh and Hjelmquist, 2011;Ghosh et al, 2014). This is broadly consistent with other investigations in aquifers located in West Bengal (Rowland et al, 2009;Héry et al, 2010) and Bangladesh (Radloff et al, 2007;Rowland et al, 2009;Maldenov et al, 2010;Dhar et al, 2011;Mailloux et al, 2013). Notably, the presence of arsenite oxidizing crude oil degrading bacteria such as Polymorphum gilvum, Acinetobacter calcoaceticus along with Fe(III) reducer Rhodoferax ferrireducens and nitrate reducers (Alkalilimnicola ehrlichi, Azoarcus and Nitrospira; Ghosh et al, 2014) in GSA groundwater indicate the large microbial diversity.…”

“…In recent years, several studies have indicated that in situ sedimentary OM in BDP aquifers is utilized by the phylogenetically diverse groups of bacteria (Radloff et al, 2007;Mailloux et al, 2013). These trends are similar to other biogeochemical investigations, which indicate breakdown of OM by Fe(III) reducers (Tuccillo et al, 1999;Chapelle et al, 2002), sulfate reducers (Cravo-Laureau et al, 2005), nitrate reducers (Ehrenreich et al, 2000), and methanotrophs (Townsend et al, 2003) in sedimentary environments.…”

Redox zonation and oscillation in the hyporheic zone of the Ganges-Brahmaputra-Meghna Delta: Implications for the fate of groundwater arsenic during discharge

“…The hydrological and biogeochemical processes resulting in elevated As content in shallow groundwater of the Ganges-Brahmaputra-Meghna Delta (GBMD) have been extensively investigated for more than a decade (Nickson et al, 1998; Harvey et al, 2002; Horneman et al, 2004; McArthur et al, 2004; Zheng et al, 2004; Polizzotto et al, 2005; Charlet and Polya, 2006; Stute et al, 2007; van Geen et al, 2008; Polya and Charlet, 2009; Fendorf et al, 2010; Mladenov et al, 2010; Neumann et al, 2010; Mailloux et al, 2013). There is broad consensus that microbial reduction of Fe(III) oxides containing As results in the mobilization of As into the dissolved phase in the presence of labile organic carbon, while the source of organic carbon and the impact of human activities on the enrichment of As in groundwater still remain controversial.…”

Groundwater Arsenic Contamination in Semi-Urban Areas of Tando Muhammad Khan District: A Case Study from Deltaic Flood Plain of Sindh, Pakistan

“…As a result, water from such lakes flow toward sand prone point bars which borders the oxbows. Since these lakes are rich in natural organic matter (Ravenscroft et al, 2001;Harvey et al, 2002Harvey et al, , 2006Islam et al, 2004;McArthur et al, 2004;Meharg et al, 2006;Postma et al, 2007;Neumann et al, 2010;Mailloux et al, 2013;Desbarats et al, 2014) anoxic condition is developed due to bacteria mediated organic matter decomposition and iron (FeOOH) reduction (Bauer & Blodau, 2006). Consequently, arsenic is released from the host sediments into the groundwater.…”