Contamination and risk assessment of Thallium in the alluvial paddy rice soil system of Pakistan

Abstract

Thallium (Tl) is considered a highly toxic trace metal. This study not only examines Tl contamination in paddy soil and its accumulation in rice tissues within the Alluvial Indus Basin of Pakistan, but also evaluates its potential risk associated with the food chain. Tl concentrations in paddy soil ranged from 0.01 to 5.89 mg/kg, averaging 0.89 mg/kg, which exceeded the typical background level of 0.58 mg/kg in Chinese soils. Tl levels in rice grains ranged from 0.11 to 1.78 mg/kg, averaging 0.33 mg/kg, with 15.5 % of samples exceeding the Food Safety and Standards Authority of India's (FSSAI) limit of 0.2 mg/kg, confirming Tl accumulation and raising health concerns. Geographically weighted regression (GWR) modelling revealed spatially varying correlations between rice Tl and soil (pH, SOM, TOC, TIC, TC, and Tl) and plant tissue (shoot and root Tl) variables, with differential impacts on Tl accumulation. The GWR model determined the Critical Tl Threshold (CTT) for Tl concentrations in paddy soil, confirming significant Tl risk in areas exceeding the CTT. The GWR model also assessed Tl Loading Capacity (TLC), defined as the difference between the CTT and the actual Tl concentration in paddy soil. This analysis identified areas at risk for Tl contamination in rice, classified as low-risk (10 %), high-risk (59.2 %), and very high-risk (30.8 %). The study highlights that children face a significantly higher health risk from Tl exposure, with a Hazard Index (HI) of 29.6 %, exceeding the safety threshold. In comparison, adults and lifetime exposure have lower HI values of 19.6 % and 19 %, respectively. These findings underscore the need for effective soil management strategies to mitigate Tl contamination and associated health risks.