Arsenic and mercury mass loads released by the Fosso della Chiusa creek waters (Mt. Amiata, central Italy)

The Mt. Amiata Hg-district (southern Tuscany, Italy) is dominated by the homonymous volcanic system (0.3-0.2 Ma) and it is well-known as a ‘’World class deposits’’ from where liquid mercury was produced by roasting processes of cinnabar (HgS). At least sixteen sites of HgS-rich ore deposits in a radius of about 20 km (i.e. Piancastagnaio, Pietrineri, Carpineto, Le Bagnore, Monte Labbro) were cultivated up to the early eighties. Among these mining sites, that of Abbadia San Salvatore was by far the most important, with galleries reaching 400 m depth and with a production of more 100,000 tons of liquid mercury. The underground water system is almost exclusively drained by the Galleria Italia circumneutral mine drainage, characterized by a Ca(Mg)-SO4 composition and high concentrations of Fe, Mn and Al (18714, 493 and 487 μg/L, respectively) and dissolved CO2. This mine drainage is connected through the Fosso della Chiusa creek to the riverine network of Pagliola, Paglia and Tiber rivers, the latter crossing Rome before entering the Tyrrhenian Sera. The Fosso della Chiusa is a 2.1 km long creek with an average flow rate of 40 L/s. This work is aimed at: (1) characterizing the As and Hg concentrations in the different environmental compartments (waters, suspended material and sediments); (2) studying the factors that affect their mobility in the aqueous medium; (3) estimating the yearly loads of As and Hg discharged by the Fosso della Chiusa . Thirteen sampling sites, located at about 150-200 m each other, all along the Fosso della Chiusa were collected. At each site, the water physicochemical parameters were measured, stream sediments (with the exception of one) and water samples for the determination of the main cationic and anionic species and dissolved As and Hg were collected. At the input point and exit point of the creek, 2 L of water were filtered with the aim to collect the suspended particulate on cellulose filters and to measure the turbidity. The results confirmed the Ca(Mg)-SO4 water composition, with the content of dissolved As varying between 12 and 0.3 μg/L, with a decreasing pattern along the creek, while the Hg values are swinging (between 0.1 and 2.8 mg/L) with higher values in central points of the creek. In the particulate fraction, Hg and As showed concentrations at the inflow and outflow sites of 52 and 4.7 mg/kg and 77 and 0.27 mg/kg, respectively. Sediments showed a linear decrease for As (from 336 to 7 mg/kg), while for the Hg is showed from 7.5 to 153 mg/kg without a specific trend. The mass load, computed by applying a 40 L/sec flow rate, released from the Fosso della Chiusa creek was of about 1.3 kg yr-1 for Hg and 0.7 kg yr-1 for As, these values being comparable with those calculated for the Bonanza mine (western Oregon, Gray et alii, 2012) and the San Carlos creek which drains the New Idrija Hg mine (California, Ganguli et alii, 2000).