A new study has uncovered a striking contradiction in the ancient Maya city of Ucanal. Residents built an advanced water management system that kept drinking water clean for centuries; however, the same system also exposed them to toxic mercury poisoning.
Researchers say the discovery highlights what they describe as a “Mayan paradox.” Careful engineering protected the city from visible pollution, but cultural practices introduced a hidden chemical threat.
The research was led by Jean Tremblay, a doctoral student at Université de Montréal, under the supervision of archaeologist Christina Halperin. The team studied the city’s water system over six years, from 2018 to 2024. Their findings were published in Archaeometry and Journal of Archaeological Science: Reports.
An urban city dependent on stored water
Ucanal was a densely populated Maya center in northern Guatemala. Like many Maya cities, it did not have permanent rivers or lakes nearby. Residents relied on rainwater stored in large reservoirs.
To understand how the system worked, researchers examined three reservoirs at the site: Aguada 2, Aguada 3, and Piscina 2. Sediment samples taken from the basins preserved chemical and biological traces that revealed the quality of the water over time.
Scientists looked for signs of biological pollution, such as cyanobacteria, also known as blue-green algae, as well as traces of fecal contamination and chemical pollutants.
Engineering kept the water biologically clean
The results show that the Maya managed to keep their reservoirs largely free of harmful biological contaminants for nearly 1,500 years.
At many other Maya sites, cyanobacteria grew in stagnant water and made reservoirs unsafe to drink. At Ucanal, however, researchers found no signs of large algae blooms.
Tremblay said the Maya likely recognized the danger because algae are visible on the water’s surface. “The Maya knew about cyanobacteria, and these algae are clearly visible,” Tremblay said. “The Maya could deal with bacteria they could see.”
A new study reveals the Mayan paradox at Ucanal.
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
— Tom Marvolo Riddle (@tom_riddle2025) March 4, 2026
The city’s water system also included natural filtration features. Aguada 2, located on higher ground, received water through rock-filled channels that filtered sediment and debris before it entered the reservoir.
Researchers also believe the reservoirs were surrounded by vegetation. Shade from plants kept the water cooler, which helped prevent algae growth.
Sediment analysis confirmed that organic matter in the basins came mostly from land plants rather than algae. Scientists also found no evidence of eutrophication, a process in which excess nutrients trigger harmful algae blooms.
Evidence of sanitation planning
The study also revealed surprisingly low levels of fecal contamination in the main drinking reservoirs.
Researchers examined chemical biomarkers such as coprostanol, a compound linked to human waste. Aguada 2 showed very little contamination even during periods of high population density.
This pattern suggests that residents used organized waste management systems. Researchers believe sealed waste pits may have been placed outside the areas that collected rainwater.
However, Aguada 3 told a different story. Located among modest homes, the reservoir showed high contamination levels. Sediment layers contained broken pottery, domestic waste, and evidence of a disturbed human burial. “This reservoir was used as a small waste pit,” Tremblay said.
Piscina 2 showed fewer signs of pollution because it was connected to a drainage canal. Water movement likely helped reduce contamination.
Ritual pigment introduced hidden poison
Despite their success in controlling visible pollution, the Maya could not detect a more dangerous threat.
Researchers found high levels of mercury in all three reservoirs. The source was likely cinnabar, a bright red mineral made from mercury sulfide that played an important role in Maya rituals and burials.
Cinnabar was used in ceremonies and decorations across Maya culture. Over time, particles of the pigment appear to have entered the water system and settled in reservoir sediments. Unlike algae or waste, mercury contamination would have been invisible to ancient residents and impossible to remove using natural filtration.
The findings show that Ucanal’s residents developed a sophisticated system to manage water and protect it from visible pollution. Yet cultural practices unintentionally introduced a toxic element that their engineering could not prevent.
Researchers say the discovery provides a rare glimpse into how environmental management, social organization, and religious traditions intersected in ancient Maya cities.
