Environmental Earth Sciences Progress Report 2020 and Outlook 2021.

The present editorial 2020 continues the series of status reports in Environmental Earth Sciences (EES) in previous years 2017 and 2019 (Kolditz et al. in Environ Earth Sci 77: 8, 2018, Kolditz et al. in Environ Earth Sci 79: 11, 2020). The year 2020 coming to an end was heavily influenced by the COVID-19 pandemic affecting all areas of life including research work and, therefore, scientific publishing as well ("Introduction"). One bright spot which shows longevity of journals that produce a quality product is that Environmental Earth Sciences (EES) is celebrating its 45th anniversary of publication. To this extent EES continues the tradition to honor the most cited papers contributing to the 2020 Impact Factor (IF) ("Highly and most cited topics") and provide information on the current status of EES as well as an outlook to 2021 ("Progress report").

Origin of groundwater in Hanoi, Vietnam, revealed by environmental isotopes.

In 2015 and 2016, groundwater samples were collected in Hanoi to analyse the isotopic composition (δ2H, δ18O and 3H) and elucidate the relationship between groundwater and surface water, as well as the origin of the groundwater. The values for δ18O and δ2H indicate that the groundwater originated from evaporated meteoric water and the isotope enrichment is due to the evaporation of shallow groundwater. Evaporation is the primary process affecting stable isotope signatures. Water samples collected from both Holocene and Pleistocene aquifers are more depleted in the heavy isotopes 18O and 2H than the rainfall in the area. This indicates that part of the groundwater is paleo-groundwater or may be caused by the altitude effect due to recharge at a higher elevation. The results also show the close interaction between two granular aquifers and the Red River. Furthermore, the contribution of modern groundwater could be observed by the appearance of tritium in both aquifers. The presence of tritium indicates that originally tritium-free groundwater from the margins of the basin has been diluted by young water. The results of this study might help managers to evaluate the origin and reserves of groundwater more accurately.

Cultivation and functional characterization of 79 planctomycetes uncovers their unique biology.

When it comes to the discovery and analysis of yet uncharted bacterial traits, pure cultures are essential as only these allow detailed morphological and physiological characterization as well as genetic manipulation. However, microbiologists are struggling to isolate and maintain the majority of bacterial strains, as mimicking their native environmental niches adequately can be a challenging task. Here, we report the diversity-driven cultivation, characterization and genome sequencing of 79 bacterial strains from all major taxonomic clades of the conspicuous bacterial phylum Planctomycetes. The samples were derived from different aquatic environments but close relatives could be isolated from geographically distinct regions and structurally diverse habitats, implying that 'everything is everywhere'. With the discovery of lateral budding in 'Kolteria novifilia' and the capability of the members of the Saltatorellus clade to divide by binary fission as well as budding, we identified previously unknown modes of bacterial cell division. Alongside unobserved aspects of cell signalling and small-molecule production, our findings demonstrate that exploration beyond the well-established model organisms has the potential to increase our knowledge of bacterial diversity. We illustrate how 'microbial dark matter' can be accessed by cultivation techniques, expanding the organismic background for small-molecule research and drug-target detection.

Estimating groundwater recharge for Hanoi, Vietnam.

Determining groundwater recharge is in particular in areas with Monsoon rainfall a challenge. Several methods were used to estimate groundwater recharge for the first time ever for the urban area of Hanoi city, Vietnam. Water table fluctuation method (WTF), hydrograph analyses including recession curve displacement, graphical separation, the Automated Web GIS-Base Hydrology Analysis Tool (WHAT), and empirical formulas were utilized. The mean recharge is approximately 340 mm/year accounting for 20% of precipitation in the period 1996 to 2009 with a certain decrease during this period. Baseflow separation methods show the highest values, whereas the displacement recession curve obtains the lowest recharge. The WTF methods approximate values are close to the mean value of all other proxies. The differences between all proxies are around 10%, and Spearman correlation is statistically significant. This indicates that these methods can be used to estimate recharge for this area. With long-term data, however, the results of WTF appear more consistent and more reasonable than the other approaches.

Pb remobilization by bacterially mediated dissolution of pyromorphite Pb5(PO4)3Cl in presence of phosphate-solubilizing Pseudomonas putida.

Remediation of lead (Pb)-contaminated sites with phosphate amendments is one of the best studied and cost-effective methods for in situ immobilization. In this treatment, a very stable mineral, pyromorphite Pb5(PO4)3Cl, is formed. Several studies propose to improve this treatment method with the addition of phosphate-solubilizing bacteria (PSB). The effect of bacteria on solubilization of pyromorphite is unknown. In this study, the effect of the soil microorganisms on the stability of pyromorphite Pb5(PO4)3Cl has been investigated in a set of batch solution experiments. The mineral was reacted with Pseudomonas putida, a common soil microorganism. Dissolution of pyromorphite was enhanced by the presence of P. putida, resulting in an elevated Pb concentration in the solution. This occurred even when the bacteria were provided with an additional source of phosphate in the solution. Pyromorphite has been shown to be a potential source of nutrient phosphorus for common soil bacteria. Thus, the use of PSB in remediation treatments of Pb contaminated sites may have adverse long-term impacts on Pb immobilization. Conscious phosphate management is suggested for long-term sustainability of the in situ Pb immobilization by pyromorphite formation.

Surface complexation and oxidation of Sn(II) by nanomagnetite.

The long-lived fission product 126Sn is of substantial interest in the context of nuclear waste disposal in deep underground repositories. However, the prevalent redox state, the aqueous speciation as well as the reactions at the mineral-water interface under the expected anoxic and reducing conditions are a matter of debate. We therefore investigated the reaction of Sn(II) with a relevant redox-reactive mineral, magnetite (Fe(II)Fe(III)2O4) at <2 ppmv O2, and monitored Sn uptake as a function of pH and time. Tin redox state and local structure were investigated by Sn­K X-ray absorption spectroscopy (XAS). We observed a rapid uptake (<30 min) and oxidation of Sn(II) to Sn(IV) by magnetite. The local structure determined by XAS showed two Sn­Fe distances of about 3.15 and 3.60 Å in line with edge and corner sharing arrangements between octahedrally coordinated Sn(IV) and the magnetite surface, indicative of formation of tetradentate inner-sphere complexes between pH 3 and 9. Based on the EXAFS-derived surface structure, we could successfully model the sorption data with two different complexes, (Magn_sO)4Sn(IV)(OH)2­2 (logK(2,0)(­2) −14.97 ± 0.35) prevailing from pH 2 to 9, and (Magn_sO)4Sn(IV)(OH)2Fe (logK(2,1)(0) −17.72 ± 0.50), which forms at pH > 9 by coadsorption of Fe(II), thereby increasing sorption at this high pH.

Effect of microbial siderophore DFO-B on Cd accumulation by Thlaspi caerulescens hyperaccumulator in the presence of zeolite.

Hyperaccumulators are grown in contaminated soil and water in order that contaminants are taken up and accumulated. Transport of metals from soil to plant is initially dependent on the solubility and mobility of metals in soil solution which is controlled by soil and metal properties and plant physiology. Complexation with organic and inorganic ligands may increase mobility and availability of metals for plants. In this work the influence of desferrioxamine-B (DFO-B), which naturally is produced in the rhizosphere, and zeolite on Cd accumulation in root and shoot of Thlaspi caerulescens (Cd hyperaccumulator) was investigated. Plants were grown in pots with clean quartz sand, amended with 1% zeolite; treatment solutions included 0, 10, and 100 µM Cd and 70 µM DFO-B. Addition of zeolite to the quartz sand significantly reduced Cd concentration in plant tissues and translocation from root to shoot. On contrary, DFO-B considerably enhanced Cd sorption by roots and translocation to aerial part of plants. Treating the plants with zeolite and DFO-B together at 10 µM Cd resulted in reduction of the bioaccumulation factor but enhancement of Cd translocation from root to shoot at the rate of 13%. In contrast, at 100 µM Cd in the solution both bioaccumulation and translocation factors decreased. Total metal accumulation as a key factor for evaluating the efficiency of phytoremediation was highly influenced by treatments. Presence of zeolite in pots significantly decreased total Cd accumulation by plants, whereas, DFO-B clearly enhanced it.

EXAFS and DFT investigations of uranyl arsenate complexes in aqueous solution.

Uranium and arsenic often co-occur in nature, for example, in acid mine drainage waters. Interaction with arsenic is thus important to understand uranium mobility in aqueous solutions. For the present study, EXAFS spectroscopy was used to investigate the formation and identify the structure of aqueous uranyl arsenate species at pH 2. The nearest U-As distance of 3.39 Å, observed in shock-frozen liquid samples, was significantly shorter than that observed in solid uranyl arsenate minerals. The shorter bond length indicated that the solution contained a bidentate-coordinated species, in contrast to the monodentate coordination in solid uranyl arsenate minerals. The U-As coordination number of 1.6 implied that two uranyl arsenate species with U:As ratios of 1:1 and 1:2 formed in nearly equal proportions and that the hydrated uranyl ion was present only as a minor component. The two uranyl arsenate species could not be differentiated spectroscopically, since their U-As distances were equal. A comparison based on DFT modeling indicated for both the 1:1 and the 1:2 species, that the bidentate arsenates were bound to uranium with one of the binding oxygen atoms being protonated. Based on the present spectroscopic study, the two species that will have to be considered in acidic uranium-arsenic-rich solutions are thus UO(2)H(2)AsO(4)(+), and UO(2)(H(2)AsO(4))(2)(0).

Discrimination of thioarsenites and thioarsenates by X-ray absorption spectroscopy.

Soluble arsenic-sulfur compounds play important roles in the biogeochemistry of arsenic in sulfidic waters but conflicting analytical evidence identifies them as either thioarsenates (= As(V)-sulfur species) or thioarsenites (= As(III)-sulfur species). Here, we present the first characterization of thioarsenates (mono-, di-, and tetrathioarsenate) by X-ray absorption spectroscopy and demonstrate that their spectra are distinctly different from those of As(III)-sulfur species, as well as from arsenite and arsenate. The absorption near edge energy decreases in the order arsenate > thioarsenates > arsenite > As(III)-sulfur species, and individual thioarsenates differ by 1 eV per sulfur atom. Fitted As(V)-S and As(V)-O bond distances in thioarsenates (2.13-2.18 A and 1.70 A, respectively) are significantly shorter than the corresponding As(III)-S and As(III)-O bond distances in As(III)-S species (2.24-2.34 A and 1.78 A, respectively). Finally, we demonstrate that thioarsenates can be identified by principal component analysis in mixtures containing As(III)-sulfur species. This capability is used to study the spontaneous reduction of tetrathioarsenate to As(III)-sulfur species (possibly trithioarsenite) upon acidification from pH 9.5 to 2.8.

Estimating groundwater mixing and origin in an overexploited aquifer in Guanajuato, Mexico, using stable isotopes (strontium-87, carbon-13, deuterium and oxygen-18).

Stable Isotopes (strontium-87, deuterium and oxygen-18, carbon-13) have been used to reveal different sources of groundwater and mixing processes in the aquifer of the Silao-Romita Valley in the state of Guanajuato, Mexico. Calcite dissolution appeared to be the main process of strontium release leading to relatively equal (87)Sr/(86)Sr ratios of 0.7042-0.7062 throughout the study area which could be confirmed by samples of carbonate rocks having similar Sr ratios (0.7041-0.7073). delta(13)C values (-11.91- -6.87 per thousand VPDB) of groundwaters confirmed the solution of carbonates but indicated furthermore influences of soil-CO(2). Deuterium and (18)O contents showed a relatively narrow range of-80.1- -70.0 per thousand VSMOW and -10.2- -8.8 per thousand, VSMOW, respectively but are affected by evaporation and mixing processes. The use of delta(13)C together with (87)Sr/(86)Sr revealed three possible sources: (i) carbonate-controlled waters showing generally higher Sr-concentrations, (ii) fissure waters with low-strontium contents and (iii) infiltrating water which is characterized by low delta(13)C and (87)Sr/(86)Sr ratios. The third component is affected by evaporation processes taking place before and during infiltration which might be increased by extraction and reinfiltration (irrigation return flow).

Volatile metals and metalloids in hydrothermal gases.

Volatile metals and metalloids were sampled from hot springs, fumaroles, and a hydrothermally influenced wetland in Yellowstone National Park. The sampling was based on diffusion through gas sampling chambers. Collected gases were stabilized by dissolution and oxidation in 1:100 diluted NaOCl. Special procedures were developed to analyze the oxidized samples by GF-AAS and HG-AAS. For ICP-MS, samples had to be blank-corrected for polyatomic isotope interferences, especially by 23Na35Cl+ and 23Na37Cl+ on 58Ni and 60Ni and by 40Ar23Na+ on 63Cu. From the concentrations trapped in solution, net diffusion rates were calculated by Fick's first law. The highest concentrations reached a maximum of 8 g/m3 for volatile silicon. Volatile nickel, tungsten, zinc, copper, and molybdenum, previously only known from anthropogenic sources, occurred naturally in the hydrothermal gases in ranges of tens to hundreds of microg/m3. Replicate measurements indicated significant temporal variations in concentrations, probably the result of complex changes in the hydrothermal regime as well as varying microbial activity. Global correlations between gaseous and superficial aqueous phases were missing.

Development of a robust technique for sampling volatile metal(loid)s in wetlands.

The formation of volatile organic and inorganic metals and metalloids in aquatic environments is a known, but not very intensively investigated, process. Several techniques have been developed over the past 10 years to determine these trace components. These techniques are of limited use in wetland environments, where samples have to be taken from the soil-water interface, and require an immediate sample analysis due to thermodynamic instabilities of the volatile metal(loid)s. This paper presents an innovative sampling technique for total concentrations of volatile metal(loid)s in wetlands, based on an in situ gas-water separation via a porous PTFE membrane and stabilising the volatile metal(loid)s in a liquid sorbent (NaOCl solution). Samples may thus be collected even at remote sites, where longer storage times have to be accounted for. The sampling system was tested by means of a laboratory facility simulating the generation of arsine and dimethyl arsine under abiotic conditions as well as under field conditions. Results for sampling efficiency, reproducibility, and long-term storage are presented. Application of the sampling system in the field is shown.