Mercury vapor emissions from the Ingenios in Potosí (Bolivia)

Potosí (South West Bolivia) is a well known historical mining site in the world, with mining activity centered at the so-called Cerro Rico. It is an impressive mount formed by rhyolitic rocks affected by intensive hydrothermal alteration, and hosting a complex vein deposit including mainly Ag and Sn minerals. From the start of the mining activity, in the late 16th century, to 1850, the main ore was silver minerals, and from 1850 the silver ores exhausted, and mining activity centered on tin minerals. During the first stage, the silver minerals were treated by amalgamation, using the so-called "método de patio", which implied the usage of mercury and other compounds as metallurgical agents, and supposed the release of important quantities of mercury to the local environment. This work was carried out at the "ingenios", milling and mercury processing facilities located next to streams, in order to have the water and mechanical energy needed for the process, and nowadays in ruins. Our results put forward very low mercury vapor concentrations in the region, reaching only occasionally values over 4ng m ^-3, as well as in the town area, were maximum values reach 31ng m ^-3 with an average of 5.5ng m ^-3; detailed surveys at the "Ingenios" demonstrated that in these facilities mercury vapor concentrations were also low, but the excavation of the topsoil causes an important release of the elemental vapor, reaching concentrations over 3000ng m ^-3. Causes of this low emission of unmodified soil are here interpreted as caused by biological and physicochemical transformation of the metallic mercury accumulated in the soil, to mineral phases such as cinnabar/metacinnabar and/or schuetteite, in reactions mediated by the formation of methylmercury.