Source apportionment of Pb in a rice-soil system using field monitoring and isotope composition analysis

Abstract

Annual inputs and isotope compositions of Pb were investigated to assess Pb pollution and source apportionment in a paddy field around a mining and smelting area. The concentrations of Pb in surface soils (0–20 cm) were significantly higher than those in deep soils (20–100 cm) and the mean concentration of Pb in rice grains exceeded the maximum acceptable level of Pb in food (0.20 mg/kg). These observations indicated that soils and rice in the study area were polluted by Pb. Lead input was higher than Pb output. The annual net input flux of Pb was approximately 18.7 mg/m2/year for the study area. Atmospheric deposition was the most predominant input pathway for Pb, accounting for 95.3% of the total Pb input. However, contributions of fertilizer and irrigation water were merely 3.11% and 2.57%, respectively. Lead isotopic signatures in surface soils and rice grains were different from those in possible sources. This difference suggested that Pb in surface soils and rice grains came from multiple sources. The calculation results from IsoSource program showed that geological background, atmospheric deposition, fertilizer, and irrigation water contributed between 20–84%, 16–42%, 0–42% and 0–28% to soil Pb, respectively. As for rice grains, 76–98% of Pb was derived from atmospheric deposition, whereas source contribution of soil Pb was only 0–26%. These results indicated that mining and smelting activities posed a higher risk to rice grains than soils. Field monitoring (FM) combined with isotope composition analysis (ICA) can provide more information for source identification of Pb than the separate methods.