The ancient Maya city of Ucanal once stood as a model of careful water management in a region with limited natural water sources, but it ultimately led to mercury poisoning. Built in what is now northern Guatemala, the city relied heavily on reservoirs that collected and stored seasonal rainwater. For centuries, this system helped residents maintain drinking water that was largely free from visible contamination.
However, new research has revealed a troubling contradiction. While the Maya engineered reservoirs that prevented biological pollution, cultural practices may have quietly introduced a toxic substance into the same water supply.
The study was led by doctoral researcher Jean Tremblay from the Université de Montréal under the supervision of archaeologist Christina Halperin.
Over six years, from 2018 to 2024, researchers examined sediment from several reservoirs at Ucanal to understand how the city managed its water. Their findings were published in the journals Archaeometry and Journal of Archaeological Science: Reports.
Ucanal was a large and active urban center that lacked nearby rivers or lakes. Like many Maya cities, it depended on stored rainwater. Scientists focused on three key reservoirs — Aguada 2, Aguada 3, and Piscina 2 — analyzing sediment layers for chemical signals and biological remains that reveal how clean the water was in the past.
Ancient engineers designed clean reservoirs
The results show that Ucanal’s water system worked remarkably well for nearly 1,500 years. Sediment evidence suggests the reservoirs avoided major outbreaks of cyanobacteria, commonly known as blue-green algae, which often make stagnant water unsafe to drink.
At many other Maya settlements, algae blooms contaminated stored water. At Ucanal, however, researchers found little evidence of such growth. Scientists believe Maya residents likely recognized the danger because algae are visible on the surface of the water.
A new study reveals the Mayan paradox at Ucanal.
— Tom Marvolo Riddle (@tom_riddle2025) March 4, 2026
Ancient engineers built reservoirs that kept water clean for 1,500 years — but hidden mercury from ritual cinnabar poisoned the system.
A sophisticated water system with a deadly secret.#Archaeology #Maya #AncientHistory pic.twitter.com/WkJsvwyxdc
The city’s infrastructure also included natural filtration. Aguada 2, located at a higher elevation, received water through channels filled with stones that filtered debris and sediment before the water entered the basin. Surrounding vegetation may have also shaded the reservoirs, keeping temperatures lower and reducing algae growth.
Sediment analysis showed that most organic material in the reservoirs came from land plants rather than aquatic algae. Researchers also found no signs of eutrophication, a process where excess nutrients trigger harmful algae blooms.
Chemical markers linked to human waste were also surprisingly low in the main drinking reservoir. The findings suggest residents likely used organized sanitation practices, possibly placing sealed waste pits away from rainwater collection areas.
Ritual practices created an invisible threat
Despite this careful engineering, researchers discovered high levels of mercury in the reservoirs. The likely source was cinnabar, a bright red mineral containing mercury sulfide that was widely used in Maya ceremonies and burials.
Cinnabar served as a ritual pigment across Maya culture, often associated with sacred objects and funerary practices. Over time, particles from the mineral appear to have washed into the reservoirs and settled in the sediment.
Unlike algae or visible waste, mercury contamination would have been impossible for ancient residents to detect. Natural filtration systems that removed debris and sediment could not eliminate dissolved toxic metals.
Researchers say the findings reveal a striking paradox. Ucanal’s residents developed sophisticated methods to keep their water biologically clean, yet cultural traditions may have unintentionally introduced a hidden chemical hazard, such as the Maya water mercury poisoning.
The discovery offers new insight into how engineering, religion, and environmental management intersected in ancient Maya cities.