Is mercury from small-scale gold mining prevalent in the southeastern Peruvian Amazon?

Area of Science:

• Environmental Science
• Geochemistry
• Ecotoxicology

Background:

• Artisanal and small-scale gold mining (ASGM) is a significant source of mercury (Hg) pollution globally.
• The fate and transport of ASGM-Hg in tropical river systems, particularly the Peruvian Amazon, remain incompletely understood.
• Previous research indicates substantial Hg release from ASGM activities, but its environmental distribution and impact are debated.

Purpose of the Study:

• To investigate the role of suspended matter and hydrological factors in the fate of ASGM-Hg in the Malinowski-Tambopata river system.
• To assess Hg accumulation in fish and analyze sediment cores for atmospheric Hg input from ASGM.
• To determine if Hg released by ASGM accumulates in riverbank sediments and aquatic life.

Main Methods:

• Analysis of mercury concentrations in riverbank sediments and suspended matter along the Malinowski-Tambopata river.
• Examination of mercury accumulation in fish tissues, focusing on piscivorous species.
• Analysis of a sediment core from an oxbow lake to assess historical atmospheric Hg deposition.
• Comparison of Hg levels in ASGM-affected and unaffected river sections.

Main Results:

• Mercury concentrations were significantly higher in suspended matter (∼400–4000 ng g⁻¹) than in riverbank sediments (20–53 ng g⁻¹), linked to particle size.
• Elevated Hg in suspended matter from ASGM sites was primarily due to increased sediment load, not higher Hg concentration per particle.
• Oxbow lake sediment showed no significant ASGM-related increase in atmospheric Hg input, with Hg flux driven by sediment accumulation rates.
• Only 5% of analyzed fish exceeded WHO Hg consumption guidelines; no increased Hg accumulation was observed in ASGM-affected river sections.

Conclusions:

• The fate of ASGM-Hg in tropical rivers is largely controlled by transport via suspended matter, especially during high flows.
• Released Hg is likely retained near the source or transported downstream, rather than accumulating significantly in local sediments or biota.
• Findings suggest that Hg enrichment in aquatic systems near ASGM areas may be less pronounced than often assumed, with transport dynamics playing a key role.