The Maya engineering paradox: masters of water, prisoners of mercury

Throughout Ucanal’s history, its reservoirs were free of cyanobacteria, commonly known as blue-green algae, which was a well-documented problem elsewhere in the Mayan world.

Ucanal’s success on this front was due to the fact that biological contaminants were visible to the naked eye. “The Maya knew about cyanobacteria and these algae are clearly visible,” Tremblay explained. “The Maya could deal with bacteria they could see.” 

Mayan water supply systems were carefully designed. The monumental Aguada 2 reservoir, perched on high ground, featured a natural filtration system: sediment and waste were filtered by rock-filled inlet channels. 

Preliminary analysis suggests that the basins were also surrounded by vegetation. “Shade keeps the water cooler,” said Halperin. “Warm water stimulates the production of cyanobacteria.”

The analysis confirms the effectiveness of these features. Carbon-to-nitrogen ratios indicate that the organic matter observed originated from terrestrial plants rather than algae. The limiting factor—i.e. the missing ingredient for algae growth—was the lack of phosphorus. No signs were detected of eutrophication, the ecosystem degradation caused by excessive phosphorus levels that leads to algae growth. 

Another surprising finding was the virtual absence of fecal contamination in the drinking water reservoirs. 

By analyzing biomarkers such as coprostanol, the researchers were able to reconstruct the city’s sanitation practices. Even during periods of high population density, Aguada 2 showed low fecal contamination levels, indicating effective human waste management. The researchers believe there were probably sealed waste pits located outside the catchment areas—a rarity in dense pre-industrial cities.

Aguada 3 was an exception. Nestled among modest residences, it exhibited contamination levels comparable to those of polluted lakes today.

“This reservoir was used as a small waste pit,” Tremblay said. Analyses reveal an accumulation of broken ceramics, domestic waste and even a disturbed human grave, suggesting that the reservoir was used for artisanal purposes or wastewater collection rather than for drinking water.

By contrast, Piscina 2 was connected to a large drainage canal and benefited from water movement and aeration, limiting contamination despite its urban setting.

But beneath the clean surface lay a darker reality. All the reservoirs, sediment layers and neighbourhoods in Ucanal showed massive mercury contamination. Almost all samples exceeded the thresholds for toxic effects on aquatic environments.

Researchers have identified the source: cinnabar, a bright red pigment based on mercury sulfide that was ubiquitous in the Mayan world. “Its color was reminiscent of blood,” said Halperin. “In Mayan cosmology, blood, life and death are omnipresent.”

Used on steles, buildings, prestige objects and corpses, cinnabar was washed away by rain and leached into the soil and water. By the Terminal Classic period, mercury levels in the large basins had skyrocketed by over 300 per cent.

The rise coincided with increased trade and wider access to ritual objects. “It wasn’t just the elites using it—everyone was exposed,” Halperin observed.

Unlike biological pollutants, mercury is undetectable by the senses. “They had no way of knowing it was toxic,” Tremblay said. “It didn’t cloud the water or turn it red.”

Once dissolved, the mercury passed through the most advanced filtration systems of the time.

This paradox—water that was biologically safe but chemically toxic—reflects the limits of ancient knowledge, even though the Maya of Ucanal planned, looked ahead and managed their resources carefully.

“They didn’t live day by day,” said Tremblay. “That’s why their civilization survived for 2,000 years.”