Geochemical accumulation and source tracing of heavy metals in arable soils from a black shale catchment, southwestern China.

Heavy metal enrichment in soils has been linked to the weathering of lithologies with high geochemical backgrounds, such as black shale. Therefore, this study conducted a typical sampling of surface soils in a black shale catchment in southwestern China to characterize the accumulation and sources of the heavy metals As, Cd, Co, Cr, Cu, Ni, Pb, Zn, Mo and Tl. Elevated concentrations of most heavy metals in the soils underlain by black shale are determined to exceed the regional soil background values, even the risk screening values, especially for Mo, As and Cd. Sequential extraction analysis, together with previous results, reveals that most heavy metals in soils are mainly bound in the residual fraction (> 65 %) as a result of the fixation of stable aluminosilicates (e.g., clay minerals). In contrast, Cd mainly occurs in relatively labile proportions as exchangeable (24.42 %), carbonate (24.48 %) and Fe/Mn oxide fractions (26.60 %) due to the non-specific adsorption of soil colloids and the precipitation of carbonates and Fe/Mn oxides. Pb isotopic tracing and APCS/MLR receptor model suggest that heavy metals in the urban surface soils (SG1) have a mixed source of black shale weathering, vehicle exhaust and agricultural input, while heavy metals in the rural surface soils (SG2) are a geogenic source of black shale weathering. Overall, this study provides new insights into contamination management, land use planning and health risk assessment in regions with high geochemical backgrounds.

A bibliometric and content analysis of research trends on GIS-based landslide susceptibility from 2001 to 2020.

To assess the status of hotspots and research trends on geographic information system (GIS)-based landslide susceptibility (LS), we analysed 1142 articles from the Thomas Reuters Web of Science Core Collection database published during 2001-2020 by combining bibliometric and content analysis. The paper number, authors, institutions, corporations, publication sources, citations, and keywords are noted as sub/categories for the bibliometric analysis. Thematic LS data, including the study site, landslide inventory, conditioning factors, mapping unit, susceptibility models, and mode fit/prediction performance evaluation, are presented in the content analysis. Then, we reveal the advantages and limitations of the common approaches used in thematic LS data and summarise the development trends. The results indicate that the distribution of articles shows clear clusters of authors, institutions, and countries with high academic activity. The application of remote sensing technology for interpreting landslides provides a more convenient and efficient landslide inventory. In the landslide inventory, most of the sample strategies representing the landslides are point and polygon, and the most frequently used sample subdividing strategy is random sampling. The scale effects, lack of geographic consistency, and no standard are key problems in landslide conditioning factors. Feature selection is used to choose the factors that can improve the model's accuracy. With advances in computing technology and artificial intelligence, LS models are changing from simple qualitative and statistical models to complex machine learning and hybrid models. Finally, five future research opportunities are revealed. This study will help investigators clarify the status of LS research and provide guidance for future research.

Heavy metal(loid) and Pb isotope compositions of black shale weathering profiles on the northern Yangtze Platform: insights into geochemical behavior, contamination assessment, and source apportionment.

Heavy metal(loid) (Cd, Co, Cr, Ni, Cu, Zn, Pb, V, Tl, and As) and Pb isotope compositions of two black shale weathering profiles were determined to investigate the geochemical behaviors of these toxic elements during black shale weathering and the heavy metal(loid) contamination and source apportionment of Pb in black shale-associated soils. Black shale has higher heavy metal(loid) concentrations than the upper continental crust and the worldwide average shale. In contrast, the surface soils have much higher heavy metal(loid) concentrations than the profile soils. The heavy metal(loid) concentrations in black shale-associated soils are higher than the Chinese and worldwide soil background values, except for Co and Pb. Black shale-associated soils, especially the surface soils, have higher average concentrations of As, Cd, Cr, Ni, Cu, and Zn than Chinese, Dutch, and Canadian soil guidelines. The enrichment factor (EF) and geoaccumulation index (Igeo) values indicate various degrees of heavy metal(loid) contamination in these soils, particularly for the heavy metals Cd, Tl, and V and metalloid As. Co and Pb contamination in these soils is not a current concern. According to the mass transfer coefficient (τTa,j) values, Cd, Co, Ni, and Zn show overall losses, and other metals (Cr, Cu, Pb, and V) exhibit different behavior in the studied black shale weathering profiles. Based on a simple binary Pb isotopic mixing model, black shale is the dominant contributor to the Pb in black shale-associated soils (70.5-91.1% to profile soils and 81.2-88.8% to surface soils), and vehicle exhaust contributes less (8.9-29.5%) to the Pb in profile soils. Vehicle exhaust can exert an impact on the Pb isotopic evolution at depth intervals of 60-80 cm below the soil surface.

Deterioration and Oxidation Characteristics of Black Shale under Immersion and Its Impact on the Strength of Concrete.

Black shale, which usually contains pyrite, is easily oxidized and generates acid discharge. This acidic environment is not favorable for concrete in engineering applications and is likely to affect the durability of engineering structures. This study investigated the effect of acid discharge from the weathering of black shale on the strength of concrete under partially immersed conditions. Black shale concrete immersion tests were conducted at different immersion depths to evaluate the oxidation conduction of black shale. Water chemistry and oxidation products were monitored during and after the immersion tests. The quality and strength of the black shale and concrete specimens were obtained before and after the immersion by testing the ultrasonic wave velocity and uniaxial compressive strength. The results indicated that a lower immersion depth of black shale reveals a higher degree of oxidation, and the capillary zone in black shale is critical for black shale oxidation in terms of mass transfer. The ultrasonic velocity of the concrete showed different change patterns in the immersed and non-immersed zones. Precipitation and additional hydration enhanced the quality and entirety of the concrete (increased ultrasonic velocity) at the non-immersed or partially-immersed zones, while the dissolution of concrete was dominant in the immersed zone (decreased ultrasonic velocity) and induced a reduction of concrete quality. The compressive strength of the concrete was enhanced after immersion. The concrete strength slightly increased by 5-15%. This phenomenon is attributed to the filling of the voids by the precipitations of minerals, such as goethite and anhydrite.

Geochemical mass balance and elemental transport during the weathering of the black shale of Shuijingtuo formation in Northeast Chongqing, China.

An understanding of the processes that control the behavior of major elements with respect to weathering profile is essential to calculate the mobility, redistribution, and mass fluxes of elements. Hence, this study aims to determine the geochemical mass balance, strain, elemental correlation, and transport in weathering profiles. We constructed three weathering profiles for the black shale of Shujingtuo formation. As per the principal component analysis of major elements, density, and pH values, the first component represents the "elemental factor" and the second denotes the "external factor." The "depletion" pattern is a mass transportation pattern, and Na, K, and Mg are depleted along transect relative to the composition of fresh rock. Fe is redeposited at the bottom half of the saprock zone, whereas Al is accumulated at the regolith zone. The Fe and Al patterns are attributed to the "depletion-addition" and "addition" patterns, respectively. The strain in profiles A and B demonstrates the expansion at the regolith zone and part of the saprock zone. In profile C, however, these zones collapsed at all depths. In chemical weathering, Na, K, Ca, Mg, and Si are depleted in the following order: valley (C) > near mountaintop (B) > ridge (A).