Adding elemental mercury and cyanide to gold-rich ores is a common extraction technology in artisanal and small-scale gold mining. In 2013, Ecuador banned mercury use in gold processing while continuing to permit cyanide-based processing . We surveyed a major gold-mining impacted river in Ecuador, the Puyango-Tumbes, 10 years following the ban to assess the effect of the legislation on water quality. We analyzed total and methylmercury, cyanide, and other heavily metals in surface waters and sediments over a 160 km reach spanning from the foothills of the Andes to the Pacific Ocean.
Downstream of processing plants, mercury remained elevated in surface waters and sediments. Unfiltered surface waters peaked at 4000 ng L-1 and declined but remained elevated at concentrations over 250 ng L-1 throughout the surveyed length. We hypothesized that cyanide concentrations would influence methylmercury accumulation in the river by suppressing methylator activity and/or enhancing mercury bioavailability but saw limited evidence of this effect. Instead, methylmercury accumulation in sediment did not exceed 0.3% of total or 2 ng g-1 at any point downstream of processing, irrespective of cyanide. In sediment, mercury concentrations decreased slightly relative to those in 2011-2012 (median of 2.1 vs 1.6 mg kg-1), but concentrations of lead, cadmium, zinc, and arsenic increased dramatically and remained highly elevated (e.g., max of 3860 mg kg-1 As in river sediments and 440 mg kg-1 in irrigation canals 160 km downstream).
Heavy metals naturally co-occur with gold in hard rock deposits and are readily solubilized when exposed to cyanide. While restricting mercury use in artisanal and small-scale gold mining is an important step, such single-contaminant management can allow for continued water quality degradation from unregulated co-contaminants. Holistic contaminant management is necessary to mitigate health risk for the people of Tumbes (population 110,000) and similar communities downstream of ASGM activity worldwide.