Methylmercury in tailings ponds of Amazonian gold mines (French Guiana): Field observations and an experimental flocculation method for in situ remediation

Sites of monomethylmercury (MMHg) production in Amazonian regions have been identified in hydraulic reservoirs, lake sediments and wetlands, but tailings ponds have not yet received sufficient attention for this purpose. This work evidenced high MMHg production within the water column and the interstitial water of two tailings ponds of French Guiana Au mines located; (i) in a small scale exploitation (Combat) where Hg was used for Au amalgamation, and (ii) in an industrial on-going Au mine (Yaoni) processing without Hg. The (MMHg)D maximum (2.5 ng L−1) occurred in the oxic water column above the sediment–water interface (SWI) of the most recent tailings pond (Combat), where the substrate was fresh, the redox transition was sharp and the pool of total Hg was large. In the Yaoni pond, the (MMHg)D maximum concentration (1.4 ng L−1) was located at the SWI where suboxic conditions prevailed. Using the (MMHg)D concentration as a proxy for Hg methylation rates, the present results show that Hg methylation may occur in various redox conditions in tailings ponds, and are favored in areas where the organic matter regeneration is more active.A 3-month long laboratory experiment was performed in oxic and anoxic boxes filled with high turbidity waters from the Combat Au mine to simulate tailings ponds. Slaked lime was added in an experimental set (2 mg L−1) and appeared to be very efficient for the reduction of suspended particulate matter (SPM) to environmentally acceptable concentrations. However, at the end of the experiment, large (MMHg)D concentrations were monitored under treated anoxic conditions with the (MMHg)D maximum located at the SWI above the Fe-reducing zones. No (MMHg)D was detected in oxic experiments. The use of slaked lime for SPM decantation appears to be an efficient and non-onerous process for Au miners to avoid Hg methylation in tailings ponds when it is combined with rapid drainage of the mine waters. A subsequent human intervention is however necessary for the recovery of soil structure through the cover of dried ponds with organic rich materials and reforestation to avoid the stagnation of rain waters and the occurrence of anoxia.

Research highlights
► Two tailings ponds of French Guiana Au mines were studied.
► High monomethylmercury (MMHg) production was found within water column and interstitial water.
► A laboratory experiment simulating tailings ponds was performed in oxic and anoxic conditions.
► Slaked lime addition decreased suspended particles but not MMHg concentrations.
► Remediation must consider decantation, ponds drainage and reforestation to limit Hg methylation.