Mercury in Quaternary sediments of the Paglia-Pagliola River system (Monte Amiata)

The Monte Amiata area (southern Tuscany, Italy) hosts one of the most important Hg districts in the world. Cinnabar ores have been exploited to produce metallic Hg until the last century, causing an extensive Hg contamination of stream sediments both of the Pagliola Creek, which drains the Abbadia San Salvatore mine, and of the Paglia River, which is the main water collector of the areas affected by mining activities. Therefore, this area is characterized by a geogenic geochemical anomaly, resulting from the natural leaching process of the mineralization associated with an anthropogenic one. The main purpose of this study is to characterize the composition of major elements and Hg within stream sediments of the Paglia-Pagliola River system that predate the onset of mining activity, trying to define a geochemical background value that allows to discriminate between the two coexisting anomalies. Sediments from Quaternary fluvial terraces along the Paglia River were sampled at three sites (total 20 samples) as part of this study; pre-, syn- and post-mining sediments (total 74 samples) from the Pagliola Creek were also analyzed as part of this work by integrating a database obtained from a previous sampling campaign. Compositional analysis of the data indicates a common source for sediments from the Pagliola-Paglia basin. However, differences are identified in the geochemistry of major elements and their relationship with Hg. For the pre-, syn-, and post-mining Pagliola sediments, CaO, resulting from mining processing waste enriched in that oxide, can be considered a proxy for Hg. CaO enrichments thus correlate to high Hg concentrations, especially in sediments coeval with mining activity, which report up to thousands of μg/g for Hg. Along the Paglia River, instead, the alteration of carbonate lithotypes outcropping in the basin causes an enrichment in CaO of Quaternary sediments, and also a dilution of Hg value. Nonetheless, the concentrations obtained from the pre-mining sediments (0.08-2.98 μg/g) are 1-2 orders of magnitude higher than the crustal Clarke value (0.056 μg/g). The Hg background value found is 2.3 μg/g for the Pagliola Creek and 0.3 μg/g for the Paglia River, 1.3 μg/g considering the whole investigated area (Pagliola + Paglia). As a result of this study, a new reliable regulation for Hg should be defined for this area, taking into account the high natural occurring Hg levels in the environment.