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In an effort to combat the threat of drought, China constructed the South-to-North Water Transfer Project (SNWTP), the biggest water transfer project in terms of volume with the largest beneficiary population in the world. Reports have shown that massive water diversion projects have had detrimental environmental consequences including water quality decline and freshwater habitat degradation. However, few reports have assessed the impact of the transfer project on sediment quality, which is highly susceptible to allogenic and local anthropogenic pollution. We examined the distribution characteristics of Cd, Cr, Cu, Ni, Pb and Zn in surface sediment of the largest reservoir along the East Route of SNWTP, Nansihu Lake, followed by positive matrix factorization (PMF) to determine their potential sources. We utilized enrichment factor, multiple sediment quality guidelines (SQGs), and potential ecological risk index (RI) to determine metal accumulation or pollution risk. The results show the mean concentrations of Cr, Cu, Pb, Zn were slightly lower than in samples collected in 2003, 2010 and 2012, while the mean concentrations of Cr and Ni were significantly higher than samples from previous years. Among the six metals, Cr, Cu and Ni are of higher ecological risk according to SQGs; but Cd is of higher ecological risk according to RI. PMF analysis shows that industrial production and shipping are important sources of Cr, Cu, and Ni. PMF analysis also shows that a considerable amount of trace metals, especially Cd, Cr, Pb and Zn, mainly comes from the use of pesticide fertilizers and biomass sources in farmland, and may partly enter Nansihu Lake from SNWTP. This study reveals the possible sources of trace metals to the Nansihu Lake which is part of SNWTP; the results of the study may serve as a reference for better understanding the impact of future water diversion projects on metals distribution. Unlabelled Image • Water transfer is likely a new source of trace metals in sediment of Nansihu Lake. • Industry and agricultural sources contributed to metals in the lake significantly. • Pollutions are expected to occur especially from Cr, Cu, Ni and Pb according to SQGs. • Cd posed a considerable potential ecological risk according to RI. • SQGs and RI are short for pollution or ecological risk assessment methods. [ABSTRACT FROM AUTHOR]

Abstract: The aggregate-based structure of soils imparts physical heterogeneity that gives rise to variation in microbial and chemical processes which influence the speciation and retention of trace elements such as As. To examine the impact of distributed redox conditions on the fate of As in soils, we imposed various redox treatments upon constructed soil aggregates composed of ferrihydrite- and birnessite-coated sands presorbed with As(V) and inoculation with the dissimilatory metal reducing bacterium Shewanella sp. ANA-3. Aeration of the advecting solution surrounding the aggregates was varied to simulate environmental conditions. We find that diffusion-limited transport within high dissolved organic carbon environments allows reducing conditions to persist in the interior of aggregates despite aerated advecting external solutes, causing As, Mn, and Fe to migrate from the reduced aggregate interiors to the aerated exterior region. Upon transitioning to anoxic conditions in the external solutes, pulses of As, Mn and Fe are released into the advecting solution, while, conversely, a transition to aerated conditions in the exterior resulted in a cessation of As, Mn, and Fe release. Importantly, we find that As(III) oxidation by birnessite is appreciable only in the presence of O2; oxidation of As(III) to As(V) by Mn-oxides ceases under anaerobic conditions apparently as a result of microbially mediated Mn(IV/III) reduction. Our results demonstrate the importance of considering redox conditions and the physical complexity of soils in determining As dynamics, where redox transitions can either enhance or inhibit As release due to speciation shifts in both sorbents (solubilization versus precipitation of Fe and Mn oxides) and sorbates (As). [Copyright &y& Elsevier]

Abstract: Manganese (Mn) and iron (Fe) oxides are ubiquitous solids in terrestrial systems that have high sorptive capacities for many trace metals, including arsenic (As). Although numerous studies have characterized the effects of As adsorption onto Fe and Mn oxides individually, the fate of arsenic within mixed systems representative of natural environments has not been completely resolved. Here, we examine oxidation and competitive retention of As on goethite and birnessite using a Donnan reactor, where each oxide is isolated by a semi-permeable membrane through which arsenic can migrate. To initiate the Donnan reactor experiments, As(III) was simultaneously added to both chambers. Arsenic(III) injected into the birnessite chamber is rapidly oxidized to As(V) and then slowly redistributes across both chambers, while that added to the goethite chamber undergoes rapid adsorption; the adsorbed As(III) on goethite subsequently undergoes desorption and diffusion into the birnessite chamber followed by oxidation to As(V). With increased reaction time, As(V) is generated and preferentially partitioned onto goethite due to higher adsorption affinity compared to birnessite. Furthermore, the dissolved concentration of As(V) is controlled by the adsorption capacity of the goethite surface, which when saturated, leads to increased aqueous As concentrations; despite an increase in As(V) loading on birnessite with increasing initial As(III) concentration, the resulting aqueous As(V) concentration increase appreciably once the goethite surface is saturated. Our findings show that Mn oxides in soils act as a temporary sorbent of As, but operate primarily as strong oxidants responsible for transformation of As(III) to As(V), which can then strongly adsorb on the surrounding Fe oxide matrix. [Copyright &y& Elsevier]

The role of Fe oxyhydroxides dynamic on metal bioavailability was studied in the Rio Doce estuary after the largest mining disaster in the world. Soon after the disaster in 2015, metals were associated with Fe oxyhydroxides under a redox-active estuarine environment. Our results indicate that organic matter inputs from plant colonization on deposited tailings over estuarine soils led to a reductive dissolution of Fe oxyhydroxides within two years. Soil pseudo-total Fe content decreased by 70% between 2015 and 2017, while the total metal contents (Cr, Cu, Ni, Pb, and Zn) decreased by 79% in the soil. The losses of Fe and metals coupled to changes in Fe oxides crystallinity reveal a future ephemeral control of Fe oxyhydroxides over metal immobilization. Our results suggest a potential chronic contamination at the estuary and points to an aggravating scenario for the following years due to the increasing dominance of poorly crystalline Fe oxyhydroxides. • Massive Fe and metal losses took place within two years after the Samarco disaster. • Inputs of organic matter increase the dissolution of Fe oxides within mine tailings. • Loss in the crystallinity of Fe oxides weakens the soil capacity to immobilize metals. • A chronic contamination is expected at Rio Doce estuary for the following years. [ABSTRACT FROM AUTHOR]

• The Fe-rich tailings adsorbed high Mn contents in its way to the estuary. • Redox fluctuations led to precipitation of Mn oxides and poorly crystalline Fe oxides. • Both Mn and Fe reduction in estuarine soils increased Mn contents in water. • Dissolved Mn increased in 880% posing a risk to environmental and human health. Manganese (Mn) is an abundant element in terrestrial and coastal ecosystems and an essential micronutrient in the metabolic processes of plants and animals. Mn is generally not considered a potentially toxic element due to its low content in both soil and water. However, in coastal ecosystems, the Mn dynamic (commonly associated with the Fe cycle) is mostly controlled by redox processes. Here, we assessed the potential contamination of the Rio Doce estuary (SE Brazil) by Mn after the world's largest mine tailings dam collapse, potentially resulting in chronic exposure to local wildlife and humans. Estuarine soils, water, and fish were collected and analyzed seven days after the arrival of the tailings in 2015 and again two years after the dam collapse in 2017. Using a suite of solid-phase analyses including X-ray absorption spectroscopy and sequential extractions, our results indicated that a large quantity of MnII arrived in the estuary in 2015 bound to Fe oxyhydroxides. Over time, dissolved Mn and Fe were released from soils when FeIII oxyhydroxides underwent reductive dissolution. Due to seasonal redox oscillations, both Fe and Mn were then re-oxidized to FeIII, MnIII, and MnIV and re-precipitated as poorly crystalline Fe oxyhydroxides and poorly crystalline Mn oxides. In 2017, redox conditions (Eh: −47 ± 83 mV; pH: 6.7 ± 0.5) favorable to both Fe and Mn reduction led to an increase (~880%) of dissolved Mn (average for 2015: 66 ± 130 µg L-1; 2017: 582 ± 626 µg L-1) in water and a decrease (~75%, 2015: 547 ± 498 mg kg−1; 2017: 135 ± 80 mg kg−1) in the total Mn content in soils. The crystalline Fe oxyhydroxides content significantly decreased while the fraction of poorly ordered Fe oxides increased in the soils limiting the role of Fe in Mn retention. The high concentration of dissolved Mn found within the estuary two years after the arrival of mine tailings indicates a possible chronic contamination scenario, which is supported by the high levels of Mn in two species of fish living in the estuary. Our work suggests a high risk to estuarine biota and human health due to the rapid Fe and Mn biogeochemical dynamic within the impacted estuary. [ABSTRACT FROM AUTHOR]

Abstract: Arsenic is contaminating the groundwater of Holocene aquifers throughout South and Southeast Asia. To examine the biogeochemical and hydrological processes influencing dissolved concentrations and transport of As within soils/sediments in the Mekong River delta, a ∼50km2 field site was established near Phnom Penh, Cambodia, where aqueous As concentrations are dangerously high and where groundwater retrieval for irrigation is minimal. Dissolved As concentrations vary spatially, ranging up to 1300μg/L in aquifer groundwater and up to 600μg/L in surficial clay pore water. Groundwaters with high As concentrations are reducing with negligible dissolved O2 and high concentrations of Fe(II), , and dissolved organic C. Within near-surface environments, these conditions are most pronounced in sediments underlying permanent wetlands, often found within oxbow channels near the Mekong River. There, labile C, co-deposited with As-bearing Fe (hydr)oxides under reducing conditions, drives the reductive mobilization (inclusive of Fe and As reduction) of As. Here, conditions are described under which As is mobilized from these sediments, and near-surface As release is linked to aquifer contamination over long time periods (100s to 1000s of years). Site biogeochemistry is coupled with extensive hydrologic measurements, and, accordingly, a comprehensive interpretation of spatial As release and transport within a calibrated hydraulic flow-field is provided of an As-contaminated aquifer that is representative of those found throughout South and Southeast Asia. [Copyright &y& Elsevier]

Wetting dry soil triggers emission of nitric oxide (NO), which can harm air quality and influence ecosystem N balance. NO released after wetting may be derived from two primary mechanisms: (1) abiotic decomposition of nitrite (NO 2 −) present in the dry soil and (2) biologically-mediated production of NO. The relative contributions of these mechanisms to NO emission are challenging to partition. To differentiate the two mechanisms, we made continuous observations of NO emission from a range of soils that varied in texture, pH, and exchangeable cations. We coupled NO gas exchange measurements with automated, non-destructive high-resolution sampling of inorganic N species via microdialysis. We identified two distinct phases of NO production (1) an initial phase that reached its maximum less than 30 min after rewetting; and (2) a second phase lasting up to 36 h. The magnitude of the initial phase of NO production was correlated with the initial nitrous acid concentration (calculated from NO 2 − concentrations and soil pH) and soil exchangeable calcium. The second phase of NO emission coincided with a peak in NO 2 − that preceded NO 3 − accumulation. Together, these results suggest that NO emissions following wetting are defined by an initial pulse of abiotic NO formation mediated by soil acidity at mineral surfaces, followed by a prolonged pulse of NO emission driven by a lag between the biological processes of ammonia oxidation and nitrite oxidation. These results can inform mechanistic models of NO emissions from soils to improve predictions of the terrestrial N cycle and atmospheric chemistry. • After wetting dry soil, nitric oxide (NO) is emitted in two distinct phases. • Chemodenitrification of NO 2 − drives NO emission immediately after wetting. • NO 2 − build up contributes to NO emission later after wetting. [ABSTRACT FROM AUTHOR]

Mining rare earth elements (REEs) can release large amounts of metal(loid)-rich dust, which can pose significant health risks to local residents. However, compared to other types of particulates, toxicity of mining dust has been largely overlooked. To provide experimental evidence on toxicity of REE mine dust, the study assessed the oxidative stress potential and genotoxicity of inhalable particles collected in a REE mining area, and associated toxicological response with source compositions. Both source types (i.e., mine and tailing area) and distances from source (i.e., industrial and residential areas) were considered when selecting the 44 sampling sites. The particle samples contained 2.3–3.5 folds higher concentrations of tested metal(loid)s than background concentrations in soil. Specially, elevated Fe, REEs, Cd, Pb were found. In spite of low cytotoxicity in lung epithelial A549 cells, there was increased cellular ROS production by of particle exposure. Samples with higher mining-originated source contributions (Provenance Index <0.3) had higher cellular ROS production (1.72 fold, 95%CI: 1.66–1.79 fold) than samples with lower mining contributions (1.58 fold, 95%CI: 1.52–1.65 fold). The factors soil (~46%), mine (~22%), and heavy metal (~20%) sources were recognized by source apportionment analysis as the main contributors to cellular ROS production; importantly, mine and heavy metal sources counted more in industrial samples. While samples generated genotoxicity, there were no differences in DNA damage between the location groups of sampling. Collectively, the results indicate that particles in mining areas may cause ROS production and DNA damage in lung cells depending on mine dust. Coupled with the long-range transportation potential of mine dust, safety measures on open pit and dust disposal sites should be adopted. Unlabelled Image • Particles around rare earth mine and tailing areas were rich in metallic components. • The particles induced cellular ROS production and genotoxicity. • Particles collected in industrial areas induced more cellular ROS than in residential areas. • Soil , mine and heavy metal sources were the main contributors to ROS. [ABSTRACT FROM AUTHOR]