Geochemistry of waters in the dismantled mine area of Baccu Locci (Sardinia, Italy): the arsenic contamination

The water chemistry of the Baccu Locci river, which drains the homonymous mine area, changes several times over a distance of about 6 km as a consequence of various, often concomitant, processes acting downward the river course: 1) mixing with metal-rich sulfate waters; 2) dissolution/precipitation of metal-bearing sulfates; 3) mixing with HCO3-dominated lake waters; 4) gypsum dissolution coupled with calcite precipitation; 5) mixing with dilute ground waters. Differently from other elements (e.g. Zn, Cu, Pb, Cd, Ni) whose dissolved concentrations rapidly decrease downstream of the upper mine area, the As concentrations (hundreds of mug/l) tend to generally increase along the river course as far as the alluvial plain. However, the marked variations of dissolved As concentrations observed locally are not exclusively referable to the above-indicated processes, but sorption/desorption onto/from Fe-Mn hydroxides is also believed to influence the As distribution in the Baccu Locci river.