A toxic inheritance from the rush for gold
In the Sagaing Region of Myanmar, a toxic legacy from colonial-era gold mining persists deep in the soil. The Kyaukpahto gold deposit, one of the largest in the country, is a sedimentary rock-hosted deposit where gold is bound within sulfide minerals, primarily arsenopyrite (FeAsS). To extract the microscopic gold particles, miners in the past employed a process called roasting. This involved heating the arsenopyrite ore to high temperatures, around 400 to 800 degrees Celsius, in the presence of oxygen.
This process breaks down the arsenopyrite matrix and liberates the gold, but it also releases large quantities of toxic byproducts. The arsenic within the mineral vaporizes, often forming arsenic trioxide (As₂O₃), a highly mobile and poisonous compound. This arsenic-laden dust and waste, known as tailings, were often discarded near the mining sites, beginning a long, slow process of environmental contamination that continues today. Studies in the area have measured arsenic concentrations in stream sediments at 24-35 parts per million (ppm), far exceeding the regional background of 0.1 to 1.6 ppm.
Unexpected depths and a tropical challenge
Decades after the initial contamination, scientific studies reveal a concerning pattern. The arsenic has not remained stationary. Instead, it has been migrating vertically through the soil profile, reaching depths far greater than predicted by standard environmental models. These models are often based on data from temperate climates, where soil chemistry, water flow, and biological activity are different.
The tropical climate of the Sagaing Region, with its heavy monsoon season, contributes to this deep migration. The intense rainfall flushes water through the soil, carrying the more soluble forms of arsenic, like arsenite (As(III)), downward. The alternating wet and dry conditions influence the soil's redox potential—its tendency to acquire electrons—which can change arsenic's chemical form and increase its mobility. Research shows that the inorganic forms of arsenic found in water are highly toxic. This deeper-than-expected contamination presents a serious challenge for remediation efforts, as the poison is not a surface-level problem. The findings show that a region's specific climate and geology are paramount in understanding and managing long-term industrial pollution.