Spatial patterns of thallium in soil of polymetallic mining area: Threshold and interaction effects of environment...

Polymetallic mining areas are a major source of thallium (Tl) pollution. In-depth analysis of the spatial distribution patterns, key controlling factors, and potential influence mechanisms of soil Tl in such regions is critical for Tl pollution prevention, risk management, and ecological sustainability in mining areas. However, the driving factors underlying Tl accumulation haven't been extensively studied, with research on the threshold and interaction effects of associated covariates being particularly limited. This study aimed to investigate the spatial patterns, threshold effects, and interaction effects of Tl accumulation in the Shizhuyuan mining area of Suxian District, Hunan Province, China, a typical polymetallic mining region via gradient boosting decision tree model. Results indicated that soil Tl enrichment was controlled by a dual-source mechanism involving natural geological background and anthropogenic inputs. All 14 environmental variables examined explained variations in Tl concentration to varying degrees, with the top three factors being heavy metal-related enterprise density (HE), total potassium (TK), and stream power index (SPI). All covariates had threshold effects on Tl, most with two identifiable thresholds, such as HE (0.07 and 0.12 ha⁻¹), TK (0.85 and 3 % thresholds), and SPI (2.0 and 3.5 thresholds), which clarify the key functional ranges of these variables. Variations in Tl content stem from complex interactions among multiple environmental factors: two-way and three-way interactions further explained 21.42 % and 5.33 % of Tl concentration variation, respectively, significantly enhancing the explanatory power for Tl spatial distribution patterns.