Microbially Mediated Release of As from Mekong Delta Peat Sediments.

Peat layers within alluvial sediments are considered effective arsenic (As) sinks under reducing conditions due to the binding of As(III) to thiol groups in natural organic matter (NOM) and the formation of As-bearing sulfide phases. However, their possible role as sources of As for anoxic groundwaters remains unexplored. Here, we perform laboratory experiments to provide evidence for the role of a sediment peat layer in releasing As. Our results show that the peat layer, deposited about 8,000 years ago in a paleomangrove environment in the nascent Mekong Delta, could be a source of As to porewater under reducing conditions. X-ray absorption spectroscopy (XAS) analysis of the peat confirmed that As was bound to NOM thiol groups and incorporated into pyrite. Nitrate was detected in peat layer porewater, and flow-through and batch experiments evidenced the release of As from NOM and pyrite in the presence of nitrate. Based on poisoning experiments, we propose that the microbially mediated oxidation of arsenic-rich pyrite and organic matter coupled to nitrate reduction releases arsenic from this peat. Although peat layers have been proposed as As sinks in earlier studies, we show here their potential to release depositional- and/or diagenetically-accumulated As.

Arsenic Methylation Dynamics in a Rice Paddy Soil Anaerobic Enrichment Culture.

Methylated arsenic (As) species represent a significant fraction of the As accumulating in rice grains, and there are geographic patterns in the abundance of methylated arsenic in rice that are not understood. The microorganisms driving As biomethylation in paddy environments, and thus the soil conditions conducive to the accumulation of methylated arsenic, are unknown. We tested the hypothesis that sulfate-reducing bacteria (SRB) are key drivers of arsenic methylation in metabolically versatile mixed anaerobic enrichments from a Mekong Delta paddy soil. We used molybdate and monofluorophosphate as inhibitors of sulfate reduction to evaluate the contribution of SRB to arsenic biomethylation, and developed degenerate primers for the amplification of arsM genes to identify methylating organisms. Enrichment cultures converted 63% of arsenite into methylated products, with dimethylarsinic acid as the major product. While molybdate inhibited As biomethylation, this effect was unrelated to its inhibition of sulfate reduction and instead inhibited the methylation pathway. Based on arsM sequences and the physiological response of cultures to media conditions, we propose that amino acid fermenting organisms are potential drivers of As methylation in the enrichments. The lack of a demethylating capacity may have contributed to the robust methylation efficiencies in this mixed culture.

Groundwater in Southern Vietnam: Understanding geochemical processes to better preserve the critical water resource.

A thorough understanding of groundwater geochemical characteristics and dominant hydro(bio)geochemical processes in the aquifers is valuable for sustainable groundwater protection. With this respect, this study provides a comprehensive assessment of hydrogeochemical characteristics of groundwater in sedimentary aquifers of the Southern region of Vietnam. The dataset comprised 291 water samples collected in rainy and dry seasons from 155 wells, and their chemical compositions of dissolved ions (Ca2+, Mg2+, Na+, K+, HCO3-, Cl-, SO42-, NO3-, NH4+, Fe, total dissolved solids) and pH. We calculated the groundwater quality index to determine the suitability of groundwater for drinking purposes. Accordingly, about 47% of groundwater samples range from poor to unsuitable quality for drinking purposes, in which total dissolved solid (TDS) and high iron concentrations are primary factors. We also examined hydrogeochemical characteristics by multivariate statistical analyses (Hierarchical Cluster Analysis and Principal Component Analysis). The results demonstrated four groups of water: fresh groundwater (TDS < 1 g/L) in the highland (Group 1), lowland fresh-to-saline groundwater (2a), lowland saline groundwater (2b), and lowland saline/acidic groundwater (2c). Although the highland area is currently not impacted by salinization, the contamination by nitrate and chloride associated with a dense urban population and agricultural practices is deteriorating its fresh groundwater resources. On the other hand, the lowland area of Southern Vietnam is highly vulnerable to seawater intrusion (groups 2b and 2c). Only 34% out of 191 samples collected in the lowland area of southern Vietnam remained fresh. In this light, this study depicted the specific geographical location for various groundwater groups in Southern Vietnam. This finding is significant to assist water scientists and decision-makers in implementing targetted groundwater management measures as prevention and protection strategies should be tailored to groundwater geochemical characteristics and the dominant hydro(bio)geochemical processes.

Intensified salinity intrusion in coastal aquifers due to groundwater overextraction: a case study in the Mekong Delta, Vietnam.

Groundwater salinization is one of the most severe environmental problems in coastal aquifers worldwide, causing exceeding salinity in groundwater supply systems for many purposes. High salinity concentration in groundwater can be detected several kilometers inland and may result in an increased risk for coastal water supply systems and human health problems. This study investigates the impacts of groundwater pumping practices and regional groundwater flow dynamics on groundwater flow and salinity intrusion in the coastal aquifers of the Vietnamese Mekong Delta using the SEAWAT model-a variable-density groundwater flow and solute transport model. The model was constructed in three dimensions (3D) and accounted for multi-aquifers, variation of groundwater levels in neighboring areas, pumping, and paleo-salinity. Model calibration was carried for 13 years (2000 to 2012), and validation was conducted for 4 years (2013 to 2016). The best-calibrated model was used to develop prediction models for the next 14 years (2017 to 2030). Six future scenarios were introduced based on pumping rates and regional groundwater levels. Modeling results revealed that groundwater pumping activities and variation of regional groundwater flow systems strongly influence groundwater level depletion and saline movement from upper layers to lower layers. High salinity (>2.0 g/L) was expected to expand downward up to 150 m in depth and 2000 m toward surrounding areas in the next 14 years under increasing groundwater pumping capacity. A slight recovery in water level was also observed with decreasing groundwater exploitation. The reduction in the pumping rate from both local and regional scales will be necessary to recover groundwater levels and protect fresh aquifers from expanding paleo-saline in groundwater.

Associations between inorganic arsenic in rice and groundwater arsenic in the Mekong Delta.

There is growing concern regarding human dietary exposure to arsenic (As) via consumption of rice. The concentration and speciation of As in rice are highly variable, and models describing rice As speciation as a function of environmental covariates remain elusive. We conducted a survey of paddy rice and soil in the Mekong Delta with the objective of linking patterns in rice As content to soil chemical variables or hydrogeological parameters. The sum of As species (ΣAs) in husked rice averaged 243 µg/kg and the average inorganic As (iAs) content was 84%. There was no relationship found between rice As concentration or speciation and As levels in soil. However, mean As concentrations in groundwater near rice sampling locations were strongly correlated with grain ΣAs and iAs over a large part of the study region, despite the fact that groundwater is not commonly used for rice paddy irrigation in this region. We hypothesize that surficial sediments with high concentrations of soluble and plant-available As also serve as sources of arsenic to downgradient shallow aquifers, explaining the observed associations between rice and groundwater As. This study suggests that shallow groundwater As concentrations may serve as a useful indicator for locations at risk of elevated iAs concentrations in rice.