Heavy metals in alluvial gold mine spoils in the Peruvian Amazon

This activity has led to the release of mercury in the environment, posing direct human health risks and potentially causing cascading effects throughout local food chains. Research was conducted in an alluvial gold mine concession in the Madre de Dios region to compare the degree of soil-borne pollution of heavy metals in areas where mining operations were abandoned more and less recently with non-impacted old-growth forest areas. Heavy metal concentrations in impacted areas were found to be below permissible levels according to Peruvian and Canadian environmental quality standards. Mean concentrations of arsenic (As), barium (Ba), lead (Pb), copper (Cu), chromium (Cr), nickel (Ni), vanadium (V), and zinc (Zn) in impacted areas were 1.90 ± 1.51, 29.80 ± 22.87, 4.60 ± 2.55, 12.68 ± 8.13, 7.90 ± 3.98, 7.93 ± 3.89, 12.67 ± 6.62, and 26.65 ± 13.53 mg kg−1 dry matter (DM), respectively. Heavy metal concentrations were higher in non-impacted old-growth forest soils than in mining spoils, and tended to increase with time since abandonment of mining operations. Mercury (Hg) was not detected in any of the sites. The low heavy metal concentrations in mine spoils may be explained by intense volatilization, reduced metal retention capacity due to the low clay and organic matter content, and leaching processes related to soil rinsing, which is part of the mining operations combined with intense rainfall. The findings suggest that heavy metal concentrations in mining spoils should not be considered to constrain forest restoration efforts or the development of similar land uses as in comparable non-impacted high forest soils. The study area, Fortuna, is located in the Peruvian Amazon region of Madre de Dios, which is one of the oldest areas where artisanal alluvial gold mining has traditionally been practiced. The climate is warm and humid, with a climax vegetation of subtropical humid forests. The soil moisture content is not dry in any part for more than 90 cumulative days per year, classified as Udic soil moisture regime. The soil temperature regime is classified as hyperthermic with mean annual soil temperatures above 22 °C. Soil sampling was conducted through stratified random sampling across 13 ha, with 90 samples collected in gold mine spoils and 3 in non-impacted forest sites. The sampling design was adjusted to the nature of cover vegetation and time since the last impact. Each soil sample consisted of approximately 1 kg of topsoil, which was mixed, stored, air dried, and passed through a 2.0 mm sieve. Particle size distribution, actual soil acidity in water extract, organic matter content, cation exchange capacity, pH, and clay particle content were determined. Heavy metal content was analyzed using ICP-AES, and soil characterization analyses were conducted at the Soil Chemical Analysis Laboratory, Universidad Nacional Agraria La Molina.