This work, funded by the Regional Authority of Sardinia and leaded by ARPAS, aims to evaluate the baseline values of groundwater at Sardinian former mines, according to the Italian guidelines [SNPA, 2018]. Tertiary andesitehosted epithermal gold deposits were exploited at Furtei (Sardinia) from 1997 to 2003. Propylitic, argillic, advanced argillic and silicification alterations with increasing sulphide contents were recognized [Ruggieri, 1997]. Weathering of abundant pyrite and enargite generated acid drainages, metal and metalloid contaminants in the water system. At present, the former mine is under environmental remediation. Monitoring of dissolved contaminants at 28 piezometer sites was established in 2016, resulting in a dataset comprising 376 samples. In the 2020 survey the dataset was implemented by the determination of major components, hydrogen and oxygen isotopic signatures. Water samples were distinguished into two subsets: WARP contains the waters interacting with the weakly altered rocks; DARP contains those interacting with deep altered rocks. Different approaches were used for the evaluation of baseline values. The dataset was first processed excluding the potentially contaminated waters; then, dataset processing included all waters and those outliers consistent with hydrogeochemical criteria. The latter approach ensured a greater reliability considering the geological and geochemical context, so it has been chosen for the calculation of baseline values. The 95%USL (upper limit of the maximum value) was adopted and a range of values proposed. Lower values correspond to WARP 95%USL and higher values to DARP 95%USL (e.g. As baseline was 46 μg/L, much higher than limits established for drinking water).
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