Widespread elevated concentrations of gaseous elemental mercury in Guanajuato, Mexico, centuries after historical silver refining by mercury amalgamation

Centuries after historical silver refining, gaseous elemental mercury (GEM) concentrations remain significantly elevated in Guanajuato, Mexico. Ground-level air GEM levels range from 8 to 454 ng/m³, surpassing Northern Hemispheric averages by up to two orders of magnitude, while interstitial air in reprocessed mineral wastes, sediment, and soil reaches up to 44,700 ng/m³. This persistent elevation is attributed to the disproportionation of legacy calomel (Hg₂Cl₂), a mercury compound formed during the patio process used for silver extraction. The study supports this mechanism by identifying HgI in reprocessed wastes and analyzing GEM release patterns from calomel. These findings challenge previous assumptions about historical mercury emissions, suggesting that calomel's disproportionation must be accounted for to accurately assess past atmospheric mercury contributions.

Loria, A., Ramos-Arroyo, Y. R., Rocha, D., Cruz-Jiménez, G., Razo-Soto, I., de la Torre, Ma. C. A., Armstrong, D., Guerrero, S., & Wang, F. (2022). Widespread elevated concentrations of gaseous elemental mercury in Guanajuato, Mexico, centuries after historical silver refining by mercury amalgamation. Science of the Total Environment, 843, 157093. https://doi.org/10.1016/j.scitotenv.2022.157093