Mobilization of mercury, thallium and arsenic in soil hydrological processes in karst catchment, southwest China.

Soils are natural pools of potentially toxic elements (PTEs), and their hydrological processes, such as runoff, infiltration, and groundwater migration, profoundly influence the migration and transformation of PTEs through biogeochemical cycling. These hydrological processes were controlled by multiple factors, including vegetation cover and land uses. This study investigates sources, spatial patterns, and migration mechanisms of mercury (Hg), thallium (Tl), and arsenic (As) in rainfall-runoff and groundwater migration processes in Lanmuchang catchment, a typical karst region in southwest China. Our results showed soil profile averaged concentrations ranging from 16.68 to 3228.92 mg kg for Hg, from 23.72 to 319.63 mg kg for Tl, and from 109.77 to 2150.27 mg kg for As in different land uses. Three main source factors to soil Hg, Tl, and As were identified, including weathering and oxidation of rocks (68-80.1%), atmospheric deposition (1.4-19.2%), and agricultural production (12.8-18.5%). Compared to open field precipitation (OP) and throughfall (TF), the concentrations of Hg, Tl, and As in surface runoff (SR) increased, which was caused by erosion of the topsoil by rainfall that brings a large amount of the elements into runoff, while those in groundwater (GW) decreased, which was caused by interception of the elements by soils before reaching to GW. The Partial Least Squares-Path Modeling (PLS-PM) approach was used to quantify the hydrological pathways among rainfall, TF, SR, and GW, which demonstrated land-use mediated variations in infiltration capacity and overland flow velocity as pivotal factors modulating the contaminant pathways. This study provided key scientific evidence for the optimization of land uses in the protection of sensitive ecosystems in karst areas with a high geological background.