Nearly one-sixth of the land in China that could be used for farming is polluted by heavy metals. While modern mining and industrial operations share part of the blame, in some parts of the country, the problem has much older roots. Researchers have found that the environmental woes of the southwestern Yunnan province go all the way back to the invading Mongols, who now appear to be some of the region’s early large-scale metal workers and worst polluters. 

Yunnan has rich deposits of metals like copper, tin, lead, gold, silver, and iron, and a long history of mining and metallurgy. Exactly how long, though, has been unclear, since there’s little archaeological or chemical evidence to help put an age on the area’s early copper working sites. To get a better idea of the timing and scale of Yunnan’s pre-modern metalworking, Aubrey Hillman, a Ph.D. student at the University of Pittsburgh, went to Yunnan in 2009. Instead of looking at the metalworking sites for evidence of their age, she turned to nearby Lake Erhai. Modern day metal pollutants are well documented in the lake, and Hillman figured that older contaminants would have found their way into the waters, too. 

She and a team of researchers from the U.S. and China dug up cylindrical sections of the lake bed—called core samples—at three different spots in the lake that year, and another five in 2012. They measured the concentrations of a variety of metals—including copper, lead, silver, cadmium, zinc, aluminum, and magnesium—in the cores’ sediment layers and then determined the age of the metals by radiocarbon dating fossilized leaves and charcoal found in the same layers. 

They discovered that one of their cores spanned a period of 4500 years of sediment deposits, and the metals in the different layers tell a fascinating story. From 2500 BCE to 200 CE, the concentrations of lead, silver, cadmium, and zinc were low and stable, representing what the researchers think is the background level of metals that are naturally deposited in the lake by wind, forest fires and volcanic emissions. Beginning at 1500 BCE, copper levels increased while the other metals remained the same, suggesting the beginning of copper-based metalworking around the lake (an idea supported by what archeological evidence there is). 

From 200 to 450 CE, concentrations double for all of the metals, and increase even more for some of them. The researchers attribute the jump to increased sediment deposits from people tilling the land for agriculture and digging irrigation channels. 

Around 1100 CE, concentrations of lead, silver, zinc, and cadmium increased again and peaked at 1300 CE, before declining around 1420 CE. This coincides with reign of the Mongol Yuan Dynasty, which established a government-operated silver mine in Yunnan in 1290 CE and made nearly half of its tax revenue from taxes on silver production in the province.

The Mongol method for extracting silver was hardly clean, and relied on roasting silver-laden rocks in low-temperature furnaces. The concentration of pollutants that wound up in the lake during these years, the researchers say, was high enough to harm its plant and animal life, and was actually three to four times higher than what’s produced by industrialized mining activity today. The large-scale mining and smelting operations not only put metal contaminants into the atmosphere and the lake, but the furnaces required a lot of wood, suggesting the Mongols may have also deforested large swaths of land around the lake. 

The researchers conclude that Yunnan’s modern pollution issues aren’t entirely modern, but are part of a long history that goes back to the conquering Mongols and even before them. While we might think of the pre-industrial past as a simpler, cleaner time, Hillman says her team’s findings show for the first time that pollution was sometimes greater then than it is today and that “people may have been seriously impacting the environment for much longer than we thought.”