Three reservoirs with distinct functions were excavated and analyzed at the Ucanal site in northern Guatemala: Aguada 2, which served a wealthy part of town; Aguada 3, located in a humbler neighbourhood; and Piscina 2, which was connected to the city’s drainage system. Sediment records were analyzed for markers of biological pollution, such as cyanobacteria and fecal matter, and traces of chemical contamination.
The findings point to a Mayan paradox. For nearly 1,500 years, Ucanal’s residents enjoyed water free of biological contaminants—including during the Terminal Classic period, when Ucanal flourished while other Mayan centres declined. This achievement was the result of meticulous hydraulic planning and effective control of visible pollutants, reflecting sustained attention to drinking water quality.
However, despite this technical mastery, chemical contamination by mercury was widespread. Concentrations far exceeding toxic thresholds were found in all the reservoirs. The source was cinnabar, a mercury-based pigment that was central to Mayan rituals. This pollution was invisible and escaped filtration systems.
