Gaseous elemental mercury and total and leached mercury from the Rezzaio treatment plant (NW Tuscany, central Italy)

Abstract Anthropic activities are the main source of Hg released to the environment, although it also has natural sources (e.g., weathering of rocks, mineral deposits, volcanic emissions and volatilization from the ocean). In the Apuan Alps range (NW Tuscany, central Italy), numerous ore deposits have discontinuously been cultivated for many centuries. In terms of extracted mass ores and activity duration, the most important mining works were those distributed along a 10 km-long NE-SW band in the southern portion of the Apuan Alps. In this context, the Rezzaio treatment plant (Valdicastello Carducci, Pietrasanta), dismissed since 1991, was the site where pyrite ± baryte ± iron oxide ore bodies hosted within the metamorphic rocks of the Apuane Unit, mainly from Monte Arsiccio, Pollone and Buca della Vena mines, were transported and processed to exploit BaSO4 . Extractive activities, even those currently discontinued, exert a significant impact on the ecosystem of a territory. This study is aimed at evaluating the degree of mercury contamination released by treatment plant and dump of Rezzaio from past-mining activity. The main goals are, as follows: (a) measuring gaseous elemental mercury (GEM, Hg0 gas) in air and interstitial soil inside and outside the plant, including the working areas and the edifices where the workers were operating (e.g., offices, laboratory, library rock storage) by a portable Lumex RA-915M; (b) determining the total amount of Hg in the top- and sub-soils, mostly developed on a small mining dump; (c) quantifying the release of Hg from leaching tests carried out with CO2 -saturated Milli-Q water. Real-time GEM air measurement surveys were carried out in 3 different ways: (1) express surveys, performed in the edifices and structures of the Rezzaio plant with GPS control for each measured position; (2) network surveys, with cumulative measurement based on an average of measured values at each 33 selected points inside the buildings; (3) one fixed point survey in the NE portion of the study area for ca. 15 hours. For (1) and (3), the average measured values corresponded to 33 and 22 ng/m3 , whilst the maximum values to 226 and 34 ng/m3 , respectively. A total of ~ 30 selected sites were sampled in August 2021 inside and outside the Rezzaio plant by means of a stainless-steel tube (inner diameter 0.4 cm) inserted at 20 to 40 cm depth to collect interstitial soil gases. Moreover, 30 top- and sub-soils (0-30 and 30-50 cm, respectively) samples were collected to analyse the total and leachable Hg on the <2 mm fraction. The mercury distribution showed a heterogeneous distribution in the Rezzaio area. However, the highest concentrations (up to 5.2 mg/ kg) were detected near the former plant. In the leachates, Hg showed exceedances >1 µg/L (up to 5 µg/L) on 7 samples. The low content of leachable Hg suggests that mercury is likely trapped within crystalline structures resistant to alteration processes.