Phosphate immobilisation dynamics and interaction with arsenic sorption at redox transition zones in floodplain aquifers: Insights from the Red River Delta, Vietnam.

Although phosphate (PO43-) may play a decisive role in enriching toxic arsenic (As) in the groundwater of many Asian deltas, knowledge gaps exist regarding its interactions with As. This study investigates the simultaneous immobilisation of PO43- and As in aquifer sediments at a redox transition zone in the Red River Delta of Vietnam. The majority of PO43- and As was found to be structurally bound in layers of Fe(III)-(oxyhydr)oxide precipitates, indicating that their formation represents a dominant immobilisation mechanism. This immobilisation was also closely linked to sorption. In the surface sorbed sediment pools, the molar ratios of total P to As were one order of magnitude higher than found in groundwater, reflecting a preferential sorption of PO43- over As. However, this competitive sorption was largely dependent on the presence of Fe(III)-(oxyhydr)oxides. Ongoing contact of the aquifer sediments with iron-reducing groundwater resulted in the reductive dissolution of weakly crystalline Fe(III)-(oxyhydr)oxides, which was accompanied by decreased competition for sorption sites between PO43- and As. Our results emphasise that, to be successful in the medium and long term, remediation approaches and management strategies need to consider competitive sorption between PO43- and As and dynamics of the biogeochemical Fe-cycle.

Arsenic behavior in groundwater in Hanoi (Vietnam) influenced by a complex biogeochemical network of iron, methane, and sulfur cycling.

The fate of arsenic (As) in groundwater is determined by multiple interrelated microbial and abiotic processes that contribute to As (im)mobilization. Most studies to date have investigated individual processes related to As (im)mobilization rather than the complex networks present in situ. In this study, we used RNA-based microbial community analysis in combination with groundwater hydrogeochemical measurements to elucidate the behavior of As along a 2 km transect near Hanoi, Vietnam. The transect stretches from the riverbank across a strongly reducing and As-contaminated Holocene aquifer, followed by a redox transition zone (RTZ) and a Pleistocene aquifer, at which As concentrations are low. Our analyses revealed fermentation and methanogenesis as important processes providing electron donors, fueling the microbially mediated reductive dissolution of As-bearing Fe(III) minerals and ultimately promoting As mobilization. As a consequence of high CH4 concentrations, methanotrophs thrive across the Holocene aquifer and the redox transition zone. Finally, our results underline the role of SO42--reducing and putative Fe(II)-/As(III)-oxidizing bacteria as a sink for As, particularly at the RTZ. Overall, our results suggest that a complex network of microbial and biogeochemical processes has to be considered to better understand the biogeochemical behavior of As in groundwater.

Spatial and temporal evolution of groundwater arsenic contamination in the Red River delta, Vietnam: Interplay of mobilisation and retardation processes.

Geogenic arsenic (As) contamination of groundwater poses a major threat to global health, particularly in Asia. To mitigate this exposure, groundwater is increasingly extracted from low-As Pleistocene aquifers. This, however, disturbs groundwater flow and potentially draws high-As groundwater into low-As aquifers. Here we report a detailed characterisation of the Van Phuc aquifer in the Red River Delta region, Vietnam, where high-As groundwater from a Holocene aquifer is being drawn into a low-As Pleistocene aquifer. This study includes data from eight years (2010-2017) of groundwater observations to develop an understanding of the spatial and temporal evolution of the redox status and groundwater hydrochemistry. Arsenic concentrations were highly variable (0.5-510 µg/L) over spatial scales of <200 m. Five hydro(geo)chemical zones (indicated as A to E) were identified in the aquifer, each associated with specific As mobilisation and retardation processes. At the riverbank (zone A), As is mobilised from freshly deposited sediments where Fe(III)-reducing conditions occur. Arsenic is then transported across the Holocene aquifer (zone B), where the vertical intrusion of evaporative water, likely enriched in dissolved organic matter, promotes methanogenic conditions and further release of As (zone C). In the redox transition zone at the boundary of the two aquifers (zone D), groundwater arsenic concentrations decrease by sorption and incorporations onto Fe(II) carbonates and Fe(II)/Fe(III) (oxyhydr)oxides under reducing conditions. The sorption/incorporation of As onto Fe(III) minerals at the redox transition and in the Mn(IV)-reducing Pleistocene aquifer (zone E) has consistently kept As concentrations below 10 µg/L for the studied period of 2010-2017, and the location of the redox transition zone does not appear to have propagated significantly. Yet, the largest temporal hydrochemical changes were found in the Pleistocene aquifer caused by groundwater advection from the Holocene aquifer. This is critical and calls for detailed investigations.

Insights into arsenic retention dynamics of Pleistocene aquifer sediments by in situ sorption experiments.

The migration of arsenic (As) enriched groundwater into Pleistocene aquifers as a consequence of extensive groundwater abstraction represents an increasing threat to the precious water resources in Asian delta regions. Pleistocene aquifer sediments are typically rich in FeIII-(hydr)oxides and are capable to adsorb high amounts of As. This results in a pronounced accumulation of As in Pleistocene aquifers, where high As groundwater infiltrates from adjacent Holocene aquifers. However, As retention by Pleistocene aquifers over long-term time scales remains largely unknown. We studied As sorption in situ by placing natural Pleistocene sediments and pure mineral phases directly inside groundwater monitoring wells at a study site near Hanoi (Vietnam). This in situ exposure allows for constant flushing of the samples with unaltered groundwater and the establishment of undisturbed sorption equilibria similar to those in local aquifer sediments, which is not readily attainable in traditional laboratory sorption experiments. The groundwaters in our experimental wells were characterized by different As concentrations (0.01-6.63 µmol/L) and redox states, reaching from suboxic to anoxic conditions (Eh of +159 to -4 mV). Results show that adsorption is the dominant As retention mechanism, independent from the respective groundwater chemistry (i.e. concentrations of dissolved P, HCO3- and Si). Whilst most of the As sorbed within the first week, sorption further increased slowly but consistently by 6-189%, respectively, within six months. Hence, the As sorption behavior of Pleistocene aquifer sediments should be determined over longer periods to avoid an underestimation of the As sorption capacity. Accompanying desorption experiments revealed that about 51% of the sorbed As was remobilized within six months when exposed to low As groundwater. We therefore conclude that a considerable proportion of the As accumulated in the aquifer sediments is prone to remobilization once the As concentrations in migrating groundwater decline. Remobilization of As should be considered in local water management plans to avoid contamination of precious groundwater resources with this As legacy.

Evaluation of dioxin-like activities in settled house dust from Vietnamese E-waste recycling sites: relevance of polychlorinated/brominated dibenzo-p-dioxin/furans and dioxin-like PCBs.

Few studies have investigated the human exposure to the ensemble of dioxin-related compounds (DRCs) released from uncontrolled e-waste recycling, especially from a toxic effect standpoint. This study evaluated the TCDD toxic equivalents (TEQs) in persistent extracts of settled house dust from two Vietnamese e-waste recycling sites (EWRSs) using the Dioxin-Responsive Chemically Activated LUciferase gene eXpression assay (DR-CALUX), combined with chemical analysis of PCDD/Fs, DL-PCBs, PBDD/Fs, and monobromo PCDD/Fs to determine their TEQ contribution. The CALUX-TEQ levels in house dust ranged from 370 to 1000 pg g(-1) in the EWRSs, approximately 3.5-fold higher than in the urban control site. In EWRS house dust, the concentrations of the unregulated PBDFs were 7.7-63 ng g(-1), an order of magnitude higher than those of regulated DRCs (PCDD/Fs and DL-PCBs), and PBDFs were also principal CALUX-TEQ contributors (4.2-22%), comparable to PCDD/Fs (8.1-29%). The CALUX-TEQ contribution of DRCs varied, possibly depending on thermal processing activities (higher PCDD/F-TEQs) and PBDE content in the waste (higher PBDF-TEQs). However, the percentage of unknown dioxin-like activities was high in all dust samples, indicating large contribution from unidentified DRCs and/or synergy among contaminants. Estimates of TEQ intake from dust ingestion suggest that children in the EWRSs may be adversely affected by DRCs from dust.