Application of lead isotopic methods to the study of the anthropogenic lead provenance in Spanish overbank floodplain deposits.

Changes in the principal sources of Pb in overbank sediment profiles have been documented for two Spanish areas by using Pb isotopes and Pb concentrations. These locations (Madrid and Tinto-Odiel basin) represent two of the most contaminated regions in Spain. The Community of Madrid is characterized by heavy industrial and urban activity, focused mainly in Madrid City. The Tinto-Odiel basin drains the Iberian Pyrite Belt, which hosts many polymetallic massive sulphides and is heavily affected by mining activities in their headwaters. It has been proven that the influence of anthropogenic activity is reflected in these overbank deposits by variations in Pb concentrations that, in general, correlate with shifts in the (206)Pb/(207)Pb ratio. Rivas profile (downstream of Madrid) was found to be the most anthropogenically influenced site. The sediments within this profile which were recently deposited (170 ± 40 years BP) have the least radiogenic signatures. (206)Pb/(207)Pb ratios ranged between 1.1763 and 1.1876 indicating significant contributions of anthropogenic Pb. In contrast, profiles upstream of Madrid possess an average (206)Pb/(207)Pb ratio of 1.2272. It is difficult to clearly identify the most prominent source as the sediments appear to be characterized by an input from several sources. The floodplain profiles in the Tinto-Odiel basin exhibit uniform (206)Pb/(207)Pb ratios ranging from 1.1627 (Odiel river) to 1.1665 (Tinto river). These ratios are similar to the ones possessed by sulphide ores in the area and differ from the ratios of other nonmineralized formations in the basin, indicating that mining activities are the primary, if not sole, source of Pb to the sediments.

A geochemical study of overbank sediments in an urban area (Madrid, Spain).

Overbank and stream sediments have been studied in the Community of Madrid. Four vertical profiles have been sampled corresponding to Guadarrama, Jarama, and finally Manzanares River, where two profiles have been studied upstream (El Pardo profile) and downstream (Rivas profile) Madrid city. Sieved samples (<63 µm) were subjected to total (ICP-MS/ICP-OES and INAA) and partial analysis (ICP-MS). AMS radiocarbon dating techniques revealed a young age (170 ± 40 years BP) at 2.40-2.65 m depth for Rivas profile. It has not been possible to detect pristine or pre-industrial overbank sediments, since in the rest of the studied profiles, anthropogenic wastes were found even in the lower levels. Three main sources of sediment have been detected in the area. Granitic and arkosic geology, located in the northern part of Madrid (U, Th, W, K, Na, or rare earth elements); clays located in the southeast of Madrid (Ni, Cr, or V), and finally an anthropogenic source has been identified (Au, Ag, Pb, Zn, or Sb) in Rivas profile. The influence of the anthropogenic activity has been established based on the contents distribution of contaminant elements in the profile. This can be associated to the growth of industrial activities and population in the city of Madrid during the last decades. The comparison of the profiles by a k-means cluster analysis showed some similarities for these samples that could have analogous sources (anthropogenic and geological).

Statistical evaluation of the geochemical variability in overbank sediments in Spain.

The overall objective of this study is to estimate, detect and specify the main sources of variance which affect the contents of the different elements in overbank sediments across Spain. These sources of variance were assessed and compared by means of a series of analyses of variance (ANOVAs), by regarding two parameters: their significance and their contribution to the total variance. Overbank sediments, sampled in erosion banks, were studied in several locations, in basins which drain different types of geological backgrounds and land uses (urban, mining, agricultural or pristine) across the Iberian Peninsula. Forty-eight elements (mostly in the < 63 µm fraction) were analysed by ICP-OES, ICP-MS and INAA. After an isometric log ratio (ilr) transformation of the data, three ANOVA analyses were performed considering three perspectives: (1) local scale, (2) regional scale: within-profile perspective and (3) regional scale: inter-profile perspective. On a local scale, it was observed that the variability of rare earth elements (REE) depends mostly on the grain size and that heavy metals are also influenced by depth. In the analysis carried out on a regional scale, from a within-profile perspective, depth and duplicates do not influence significantly the variability of the element contents. Finally, from an inter-profile perspective, the selected sources of variance were land use and provenance, whose significance is the highest. While grain size and the selection of depth are of crucial importance in the final results, on local studies, land use and provenance are the ones that influence the most the composition of sediments in regional studies.