Modified approach for estimating geogenic Pb isotope ratios in soils for metal source apportionment

Lead (Pb) isotope ratios are commonly used to determine the relative contributions of anthropogenic and geogenic sources to metal contaminations in soils. For successful source apportionment, it is critical to obtain a reliable Pb isotope ratio value for each end member. However, the conventional practice for estimating the geogenic Pb isotope ratio can get affected by the anthropogenic Pb, which might produce misleading results. Therefore, this study proposes a modified approach that enhances the reliability of geogenic Pb isotope ratio estimations. To evaluate the feasibility of the proposed approach, laboratory experiments and field soil sample analyses were conducted. In the laboratory experiments, possible transformation of the metal-binding forms was monitored for a year using soils artificially contaminated with Pb and zinc (Zn). The results indicated that the anthropogenic metals in the soil barely transformed into the residual form, and thus, soil samples without less stable metal forms may better represent the original soil minerals. Field soil samples were collected from a metal-contaminated site near a zinc-smelter and were investigated using both the conventional and modified approaches. The modified method involves collecting multiple soil samples along consecutive depths at a location, chemically leaching out the mobile fractions of metals from the samples and determining the converging value of Pb isotope ratio by comparing subsequent values. The results revealed that the modified method gives the Pb isotope ratios which gradually decrease to a certain depth and then converge to a value. On the other hand, the conventional method shows the irregular vertical distributions of Pb isotope ratios possibly due to the infiltrations of anthropogenic Pb. Finally, the application of the modified approach to the zinc-smelter sites was found to yield more reasonable metal source apportionments.