Lead Isotopic Composition of Overbank Sediments in Areas with Different Anthropogenic and Lithological Characteristics

Abstract

Metals in overbank deposits may originate from natural or anthropogenic sources. The objective of this work is to study the anthropogenic influence over these deposits through the study of Pb contents together with Pb isotopic ratios in a group of selected overbank profiles sampled in several basins across Spain. The 16 selected overbank profiles show specific characteristics, due to land use (anthropogenic activity developed in the area where a profile is located), as well as the lithologies that supply material to the sediments. These profiles are expected to present some kind of anthropogenic influence as a consequence of mining, industrial, urban, or agricultural activities, while two profiles were sampled in areas where population is scarce and no significant impact is likely. The elements Pb, U, and Th were analyzed as well as the isotopic ratios of 206Pb, 207Pb, and 208Pb. Lead ratios and Pb contents were used in this work as a tool to trace Pb origin (natural or anthropogenic) in the studied profiles. This study offers an overview of the ranges that can be reached by Pb contents and by Pb isotopic ratios (206Pb/207Pb and 208Pb/206Pb) under several contrasting conditions. Lead contents vary from 12.7 to > 5000 mg/kg and 206Pb/207Pb ratio from 1.162 to 1.407. It was proven that the anthropogenic activity is reflected in some of the overbank profiles, showing a variation in Pb contents that agrees with shifts in the isotopic ratios. These changes are more relevant in urban areas, while in mining areas, changes are weaker. It was also proven that the Pb contents and Pb ratios are heavily dominated by the geology too.