Data-driven analysis for source apportionment and geochemical backgrounds establishment of toxic elements and REEs in the Tohoku region, Japan
Introduction
The contamination of terrestrial environments by excessive heavy metals and rare-earth elements (REEs) (e.g., As, Pb, Hg and Gd) has attracted considerable attention worldwide in recent decades (Guan et al., 2018; Asad et al., 2019; Atinkpahoun et al., 2020). At high levels, these elements are hazardous to human health and ecosystems due to their ubiquity, toxicity and persistence. To perform objective contamination assessments and to develop effective environmental pollution control strategies, it is therefore critical to have an accurate and comprehensive understanding, not only of the origins these potential hazardous elements, but also of their distribution mechanisms and how they are associated with other elements in terrestrial environments.
Geochemical assessments of river sediments may be a promising method for elucidating the influence of natural and anthropogenic activities on the distribution of potential hazardous elements. River sediment geochemistry has been widely studied in the context of mineral exploration, geochemical baseline determination, contamination assessment, and risk assessment (Di Giuseppe et al., 2014; Islam et al., 2014 ; Kumari and Paul, 2017; Sindern et al., 2016). These broad fields of application are supported by the numerous roles of river sediments, which mainly function as components of the Earth’s crust, act as sinks for contaminants, and are secondary sources of trace elements. Furthermore, the particular geological setting of an area often affects the geochemistry of trace elements in river sediments (Paller and Knox, 2016; Salomons, 1995), and agricultural, urban, and industrial activities also contribute to the enrichment of river sediments by certain elements (Di Giuseppe et al., 2014; Liu et al., 2018; Wang et al., 2014).
The natural and anthropogenic origins of various elements in river sediments can be studied by numerous methods, and among these, statistical multivariate analysis has been applied to the analysis of high-dimensional data. Principal component analysis (PCA) is one such multivariate analytical tool that can be used to describe the similarities, differences, variations, and associations within a dataset. Previous studies have successfully used PCA to elucidate the origins of elements, and to distinguish the roles of geogenic and anthropogenic sources on the distribution of elements (Khorasanipour et al., 2012; Loska and Wiechuła, 2003; Yang et al., 2017). In the study by Gülşen-Rothmund et al., 2018, a data-driven approach coupled with PCA was also applied to elucidate the distribution mechanisms of elements in river sediments.
The geochemical background (GB) of elements in river sediments is another feature that can contribute to elucidating the enrichment of elements in the environment. In the context of environmental science, the GB has been defined by Gałuszka, 2006 as “a theoretical natural” concentration of a substance in a specific environmental sample (or medium). This means that the GB is a categorical variable with several possible values, not only in entire regions, but also in classifications of particular areas (e.g., anthropogenic-influenced areas, natural environments). Thus, numerous types of GBs can be established to differ the geochemical distribution characteristics among various areas, while the outlier GB data could be due to anthropogenic contamination or naturally occurring ore deposits (Runnells et al., 1992; Galuszka, 2007). To increase the identification accuracy of the distribution mechanisms and origins of elements, the combination of a data-driven approach with PCA and GB establishment may be a powerful tool. It would be able to delineate the particular enriched elements in various area types and observe extreme enrichments which eventually leads to recognize potential risk to human health.
The Tohoku region of Japan is geologically diverse, with widespread ore deposits and a long mining history. It is also the rice basket of Japan. The existence of Cenozoic island-arc volcanic rocks constitutes a marked change in the geological setting from that of a continental margin to an island arc. Ujiie-Mikoshiba et al. (2006) have observed the possible association between the distribution of elements (e.g., REEs) and geological settings. However, little information has been obtained in terms of the origins and distribution mechanisms of elements in this region, and the GB of river sediments has not yet been clarified in detail. In the fields of contamination control and resource exploration, it is also important to understand the influence of ore deposits and mining districts, as well as anthropogenic activities, on elemental abundance.
The present study was conducted to bridge these gaps in knowledge by statistically assessing the geochemical characteristics of river sediments in the Tohoku region and to establish the GB of those sediments. A geochemical database published by The National Institute of Advanced Industrial Science and Technology (AIST, Japan) in 2007 containing data for 53 elements from 485 river sediment samples collected in the Tohoku region was used in this study. Due to the mining history of the Tohoku region, GB for mineralised areas (MGB) was newly proposed in this study. The possible findings are considered to be important for identifying the origins and distribution mechanisms of toxic elements and REEs, and for elucidating the influence of natural and anthropogenic activities on the distribution of elements in the Tohoku region. Furthermore, it is considered that the findings will contribute to the development of the environment and resource geology in this region.
Section snippets
Study area
The Tohoku region, located in the northern part of Honshu (main island), Japan (36°54′N to 41°29′ N, 139° 33′E to 141°58′E), consists of six prefectures: Aomori, Akita, Iwate, Yamagata, Miyagi and Fukushima (Fig. 1). Four mountain ranges extend through the region along a north-south axis; these are the Yoneshiro, Ou, Asahi, and Kitakami ranges. The Yoneshiro and Asahi mountains extend along the western side of the region and act as a backbone. The Ou Mountains are located within the central
Geochemical composition
The elemental properties of the river sediments in the Tohoku region are summarized in Table 1. For both major elements and trace elements, except Al2O3 and MgO, the median concentrations were lower than their mean values. P2O5 exhibited the widest variability (49.69%) among the major elements. Compares to the other trace elements from siderophile groups (i.e., Mo, Co), Ni was the most abundant (average value of 22.00 wt%). Furthermore, as in the global survey of river sediments undertaken by
Associations between REEs and granitic igneous rocks
The abundance of REEs is typically associated with local parent rocks and minerals. For instance, the Abukuma area contains extensive Cretaceous plutonic rocks (AIST, 2003; Kubo, 1991) and has a high in PC1 score (Fig. 6a). In this study, high PC1 scores were mainly attributed to the numerous strong correlations among some of the REEs, such as La–Ce–Pr–Sm; Eu–Tb–Dy–Ho–Er–Tm–Yb–Lu; and Y with Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu (Fig. 3). Thus, the Cretaceous plutonic rocks in the Abukuma area may
Conclusions
In this study, the origins and distribution mechanisms of 53 elements in river sediments from the Tohoku region were successfully elucidated using a data-driven approach coupled with statistical multivariate analysis. Various GBs (i.e., GGB, NGB, AGB and MGB) in the Tohoku region were also newly established using the calculated distribution function, which was then used to clarify the influence of anthropogenic activities.
By combining Pearson’s correlations, PCA and GB establishment results, it
Credit author statement
Arie Pujiwati: Methodology, Validation, Formal analysis, Writing - original draft. Jiajie Wang: Writing - review & editing, Visualization. Kengo Nakamura: Data curation, Resources. Yoshishige Kawabe: Software. Noriaki Watanabe: Investigation. Takeshi Komai: Conceptualization, Project administration, Funding acquisition, Supervision.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors are grateful for financial support from Japan Society for the Promotion of Science (JSPS, 19K22150).
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