New research on the ancient Maya city of Ucanal in northern Guatemala reveals that its engineers maintained biologically clean drinking water for nearly 1,500 years, an extraordinary achievement in a region with no permanent rivers or lakes, by designing sophisticated reservoir systems that filtered sediment and prevented algae growth.
Despite this remarkable success, every single reservoir at Ucanal was found to contain mercury at levels far exceeding modern toxic thresholds. The source was not industrial contamination but the Maya's own widespread ritual use of cinnabar, a sacred red pigment made from mercury sulfide, which silently leached into the water supply through rain and runoff.
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A pyramid at the ancient Maya city of Tikal, Guatemala. (Pixabay)
A new study, widely reported in the news, has uncovered one of the most striking paradoxes in Mesoamerican archaeology. The ancient Maya city of Ucanal, located in the Petén region of northern Guatemala, was home to a water management system so sophisticated that it kept its reservoirs free of biological pollutants for roughly 1,500 years. Yet the same city was saturated with mercury contamination so severe that it would trigger emergency intervention by modern environmental agencies. The culprit was not ignorance of hygiene; it was devotion.
The research was led by Jean D. Tremblay, a doctoral student at the Université de Montréal, under the supervision of archaeologist Professor Christina Halperin. Over six years of fieldwork, the team analyzed sediment cores from three distinct reservoirs at the Ucanal site, publishing their findings in the Journal of Archaeological Science: Reports and Archaeometry.
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Christina Halperin at the Aguada 2 excavation site. (Dept of Anthropology, University of Montreal)
A City Built on Rainwater
Ucanal depended entirely on rainwater collected and stored in large, man-made reservoirs known as aguadas. Three of these, Aguada 2, which served the elite; Aguada 3, in a modest neighborhood; and Piscina 2, connected to the main drainage, were the focus of the study.
The sediment analysis revealed that the Maya of Ucanal were remarkably effective at managing visible threats. While cyanobacteria, or blue-green algae, plagued many other cities, researchers found no evidence of significant blooms at Ucanal. Tremblay attributed this to direct observation: "The Maya knew about cyanobacteria, and these algae are clearly visible. The Maya could deal with bacteria they could see".
Small pyramid in Tikal, Guatemala. (Dezalb/Pixabay)
The engineering was equally impressive. Aguada 2 featured inlet channels packed with rocks that acted as a natural sediment filter, while surrounding vegetation provided shade to keep temperatures low. Fecal biomarkers were also remarkably low, suggesting organized sanitation practices. This level of urban hygiene is exceptionally rare in pre-industrial cities. This was not the first time the Maya demonstrated such ingenuity; researchers have previously noted that the 2,000-Year-Old Maya water purification system would still work today, utilizing zeolite and quartz to clarify their supply.
The Red Poison Hidden in Plain Sight
Yet beneath this cleanliness lay a chemical catastrophe. As reported by Archaeology Magazine, sediments from all three reservoirs were contaminated with mercury. Concentrations were above the toxic threshold of 1 microgram per gram, and during the Terminal Classic period, they surged dramatically. Aguada 3 reached a staggering average of 11.88, more than eleven times the toxic limit.
Pycnoporus cinnabarinus, Zinnober holzbewohnende, Orange. (Kostenlose Nutzung/Pixabay)
The source was cinnabar, a bright red mineral of mercury sulfide (HgS), central to Maya cosmology. "Its color was reminiscent of blood," Professor Halperin explained. "In Mayan cosmology, blood, life, and death are omnipresent". Cinnabar was used on stelae, temples, and the dead, such as in the Mystery of the Mayan Red Queen. A 2022 meta-study found that the Ancient Maya were at high risk of mercury poisoning across Mesoamerica, with concentrations reaching 17 times the limit. We now know that the Ancient Maya poisoned Tikal's drinking water in much the same way, turning a vital resource into a source of illness.
The Paradox That Could Not Be Seen
What makes the Ucanal case compelling is the precision of the contradiction. The Maya actively managed their water supply across generations. "They didn't live day by day," Tremblay noted. "That's why their civilization survived for 2,000 years". And yet, mercury was beyond their perception, colorless, odorless, and tasteless. Their advanced filters were powerless against it.
According to Ancient Pages, mercury levels surged during the Terminal Classic as trade increased. "It wasn't just the elites using it, everyone was exposed," Halperin noted. The contamination permeated every neighborhood and social stratum.
A New Dimension in the Story of Maya Decline
The implications extend well beyond Ucanal. The discovery of pervasive mercury poisoning adds a troubling variable to the Maya's mystifying collapse. Chronic exposure causes tremors, neurological deterioration, and reduced cognitive function. A population suffering these effects would have been significantly less capable of responding to the droughts and political crises that ended the Classic Maya world.
Top image: The ancient Maya temple complex at Chichen Itza, Mexico. Source: Pixabay
References
Tremblay, J.D., Halperin, C.T., et al., 2024. Mercury contamination in ancient water reservoirs at the Maya city of Ucanal, Guatemala, (Journal of Archaeological Science: Reports). Available at: https://www.sciencedirect.com/science/article/abs/pii/S2352409X24004176?via%3Dihub
Tremblay, J.D., et al., 2025. ‘Fecal Steroids as Tracers of Human Population and Waste Management at Ucanal’, Archaeometry. Université de Montréal, 2026. ‘The Maya engineering paradox: masters of water, prisoners of mercury’. Available at: https://onlinelibrary.wiley.com/doi/10.1111/arcm.70077
