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 4 ng m− 3, as well as in the town area, were maximum values reach 31 ng m− 3 with an average of 5.5 ng 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 3000 ng 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.

Research Highlights
► We study mercury concentration in the atmosphere at Potosí (Bolivia), a world-class silver-tin deposit with a historic usage of mercury.
► We study the relationship of mercury contents in mining contaminated soils from the old mills (“Ingenios”) with local atmospheric mercury concentration.
► Results put forward that after 250 years of mercury usage cessation, presence of mercury in the atmosphere is only found when the topsoil is removed.
► Preliminary interpretation of these observations suggests the formation in the topsoil of mineral phases (cinnabar/metacinnabar and/or schuetteite) avoiding the release of mercury to the atmosphere.