Abstract: We investigated the release characteristics of thallium (Tl) in soils under natural rainfall conditions within a granite geological high-background area, as well as the influences by agricultural activities and rainfall acidity. Two Tl-rich soil types with distinct land-use patterns (natural forest soil and pomelo orchard soil) in southern Fujian were selected. Dynamic soil column leaching experiments were conducted to simulate Tl release behaviors under varying pH conditions (pH 3.0, 4.5, and 5.4). Results showed that the initial Tl concentrations in the two soils were 2.21 and 2.59 mg·kg^-1 respectively, significantly exceeding the background value of Fujian Province (0.81 mg·kg^-1), indicating pronounced Tl enrichment due to geological high-background conditions. The leached Tl concentrations exhibited a consistent upward trend with increasing rainfall acidity. Notably, Tl concentrations in leachates from pomelo orchard soils (1.73±1.20 μg·L^-1) were significantly higher than those from natural forest soils (1.33±0.82 μg·L^-1) and exceeded the measured Tl levels in local surface water (0.58±0.11 μg·L^-1). These findings suggest that Tl in soils poses a risk of migration into surface water via rainfall leaching and further vertical infiltration into groundwater. Hydrogeochemical correlation analysis revealed significantly positive correlations between Tl and cations such as K, Ca, Mg, and Mn in leachates (P<0.05), indicating co-regulation of Tl release by multiple cations. Overall, this study demonstrates that Tl release and migration in soils under natural rainfall conditions are jointly controlled by rainfall pH and agricultural activities, with agricultural practices significantly intensifying Tl release rates and mobility. This study highlights potential risks of Tl contamination in groundwater and surface water within high geological background regions.

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