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.

Peat moss has been a standard carrier of inoculum for experimentation and in agriculture. Peat moss is, however, a non-renewable resource. Alternatively, biochar could serve as an inoculum carrier. Here, we tested the effect of biochar-based seed coatings as a carrier for the phosphorous-solubilizing Pseudomonas libanensis inoculum, on corn growth after soluble and insoluble P addition. The survival of P. libanensis was determined based on the measure of colony-forming units from samples of four inoculated guar gum-based biochar coatings and was compared to peat. Storage experiments were performed on inoculated biochars for 22 weeks at 25 °C and on coated corn seeds for 16 weeks at 4 °C. Seed coatings were prepared with inoculated and uninoculated biochars (100 seeds treatment⁻¹), and effects of these treatments are reported on indices of seed germination after 7 days. A greenhouse experiment investigated the effects of the inoculated and uninoculated biochar seed coating on corn plants. The parameters measured from the greenhouse-grown corn plants were germination, fresh weight, dry weight, height, root length, basal stem diameter, leaf area, chlorophyll content, and tissue phosphorous. Our results show that corn plants grown from seeds coated with a biochar from hardwood feedstock are 2 to 10 g heavier than controls and that controls are 4 to 26 % shorter than the plants grown from biochar-coated seeds, where soluble phosphorous is applied. Moreover, corn seeds that were coated with a biochar produced from softwood feedstock germinated more quickly, based on the speed of germination index. Overall, we show that a biochar-based seed coating can benefit sustainable agriculture by carrying P. libanensis and enhancing the growth of corn, but according to parametric statistical tests, it does so without increasing the phosphorous content of the plants.

Geogenic arsenic (As) contamination of groundwater is a health threat to millions of people worldwide, particularly in alluvial regions of South and Southeast Asia. Mitigation measures are often hindered by high heterogeneities in As concentrations, the cause(s) of which are elusive. Here we used a comprehensive suite of stable isotope analyses and hydrogeochemical parameters to shed light on the mechanisms in a typical high-As Holocene aquifer near Hanoi where groundwater is advected to a low-As Pleistocene aquifer. Carbon isotope signatures (δ13C-CH4, δ13C-DOC, δ13C-DIC) provided evidence that fermentation, methanogenesis and methanotrophy are actively contributing to the As heterogeneity. Methanogenesis occurred concurrently where As levels are high (>200 µg/L) and DOC-enriched aquitard pore water infiltrates into the aquifer. Along the flowpath to the Holocene/Pleistocene aquifer transition, methane oxidation causes a strong shift in δ13C-CH4 from -87‰ to +47‰, indicating high reactivity. These findings demonstrate a previously overlooked role of methane cycling and DOC infiltration in high-As aquifers.

Although arsenic (As) groundwater contamination in South and Southeast Asia is a threat to human health, mechanisms of its release from sediment to groundwater are still not fully understood. In many aquifers, Fe(III) minerals are often the main hosting phases for As and their stability is crucial for As mobility. Recently, a new mechanism for As mobilization into groundwater was proposed with methane (CH₄) serving as an electron donor for microbially mediated reductive dissolution of As-bearing Fe(III) minerals. To provide unequivocal evidence for the occurrence of Fe(III)-coupled methanotrophy, we incubated sediments from an As-contaminated aquifer in Hanoi (Vietnam) anoxically with isotopically labeled ¹³CH₄. Up to 35% of the available Fe(III) was reduced within 232 days with simultaneous production of ¹³CO₂ demonstrating anaerobic oxidation of ¹³CH₄ with Fe(III) as the electron acceptor. The microbial community at the end of the incubation was dominated by archaea affiliating with Candidatus Methanoperedens, implying its involvement in Fe(III)-dependent CH₄ oxidation. These results suggest that methanotrophs can contribute to dissolution of As-bearing Fe(III) minerals, which eventually leads to As-release into groundwater.