Data-driven analysis for source apportionment and geochemical backgrounds establishment of toxic elements and REEs in the Tohoku region, Japan

Potentially toxic elements pose significant and long-term human health risks in river basin districts with abandoned gold mines

Gold (Au) mining area is known to be one of the major sources of toxic elements; however, the potential risks of toxic elements from abandoned Au mines to the surrounding river basin districts and human exposure pathways to toxic elements need to be clarified. In this study, the distribution and mobilities of nine toxic elements (As, Cd, Cu, Pb, Sb, Zn, Cr, Ni and V) in Kesennuma City, Tohoku Region in Japan, a typical Au-mining district with several river basins, were studied through a geochemical survey (including element total concentrations and water-/acid-leaching concentrations determinations, as well as GB calculations), and environmental assessment on these elements in soil, river sediment, and river water samples from the study area. The contamination evaluation by index of geo-accumulation (I_geo) and enrichment factor (EF) suggested that As, Cu, Ni and Sb enrichments were greatly observed in the vicinity of the abandoned Au mines; moreover, calculated GB upper values for Cu in the river sediment surpass that of Tohoku Region. It has been found in this study that each element has particular mobility, which eventually influences its exposure pathway to humans. For instance, As in soil and sediment poses adverse non-carcinogenic risks and unacceptable carcinogenic risks to especially children mainly through groundwater ingestion. To minimize the potential risks associated with exposure to toxic elements in Au-mining districts, effective risk management measures should be implemented around river system by Au-mining companies even after their long-time closures, based on the consideration of each element's mobility.

Assessment of heavy metal pollution and ecological risk in river water and sediments in a historically metal mined watershed, Northeast Japan

Mining legacies continue to impact the geochemical cycles in historically mined watersheds after mine closure. The Hokuroku District in Northeast Japan is a famous metal mining area with a long mining history; however, studies on the distribution mechanisms and pollution characteristics of heavy metals in these historically mined watersheds after the boom period of mining activities are lacking. This study aims to provide fundamental insights into the effects of the mining activities and hydrological conditions on heavy metal pollution in the Kosaka watershed, Hokuroku District. Sampling was performed in terms of watershed segmentation, and the outlet of the tributary within each sub-watershed was also sampled to capture the diffusional pollution status. The distributions of Zn, Cu, Cd, Pb and As in river water and sediments, as well as their pollution characteristics and ecological risks, were analysed under different hydrological conditions. Our findings provide evidence of the ecological risk in surface water induced by Zn, Cu and Pb pollution in the Kosaka River system. In a high proportion of the sub-watershed, there was moderate to strong enrichment in Cd, Cu and Zn in the river sediments. The sub-watersheds with high pollution levels and ecological risk were highly consistent with the sub-watersheds encompassing abandoned mine sites. Suspended particles carried large amounts of Pb and Cu, especially on rainy days. The heavy metal contents in river water were very sensitive to occasional rainfall events; rainy days posed the most risk to organisms in the Kosaka River, followed by the low-water-level season and the high-water-level season.

Using data-driven analysis of geochemical environmental information to infer the environmental impact of closed mines

River sediments have the effect of aggregating geochemical environmental information, such as that related to geological and artificial pollution resulting from mine closure. This information comprises high-dimensional data and is related to the distribution and quantities of elements in river sediments. However, accessing and interpreting this geochemical information can be difficult. This study employed a data-driven analysis that can be mathematically and statistically reduced in dimension. Using high-dimensional geochemical and environmental information on river sediments, this study evaluated the environmental impact of closed mines. Sample for analysis were collected from three rivers. There are differences in the existence of mines and mine wastewater treatment methods in this river. A total of 33 elements were measured in river sediments. Frequency distribution analysis and Principal component analysis revealed that the elements had unique distribution and frequency characteristics in each river catchment. Four environmental factors could be extracted from the relationship of elements due to lower dimension. PC1 was influenced by the land use in the river area. PC2 captured the geological background. PC3 captured the mixing-diluting effect that occurs in rivers. PC4 effectively captured the effects of domestic wastewater and the effects of closed mines. The effects of the closed mines could be confirmed using the PC4 score for the Okawa River and the Akagawa River. By examining the elemental relationships obtained using these mathematical methods, it is possible to infer the effect of geological features and mines on sediment physiochemistry using existing data on river sediments.