Legacy and Ongoing Mercury Contamination in Soils from ASGM Sites in the Tapajós Gold Province, Pará, Brazil

Mercury (Hg) from artisanal and small-scale gold mining (ASGM) remains a source of long-term soil contamination in the Brazilian Amazon, yet its long-term persistence and controlling factors in deactivated sites are still poorly understood. This study evaluates total mercury (THg) concentrations in surface soils from three ASGM sites in the Tapajós region (active, deactivated in 2024, and deactivated for approximately 30 years), with a focus on the role of soil properties in Hg retention. Results show that the site deactivated for ~30 years still exhibits elevated THg concentrations (27 to 8,862 ng g−1; median = 324 ng g−1) and ecological risk levels above regulatory thresholds, highlighting the long-term environmental legacy of ASGM. In contrast, recently deactivated and active sites presented average THg concentrations near the background value of 178 ng g−1 (134 ± 44 ng g−1 and 77 ± 93 ng g−1, respectively), with localized increases near operational structures. In the long-term deactivated site, THg concentrations were strongly associated with the proportion of fine particles (R2 = 0.73) and positively correlated with water‑holding capacity (WHC, ρ = 0.54, p <.01), indicating higher THg concentrations in finer-textured soils. These findings suggest that Hg persistence in this site is primarily associated with geoenvironmental controls rather than natural attenuation processes. Soil heterogeneity likely promotes the formation of microenvironments that are favorable to Hg retention and may limit its remobilization, even under high rainfall conditions. Overall, this study advances the understanding of Hg behavior in tropical post-ASGM landscapes by demonstrating that contamination persistence can be closely linked to substrate properties and small-scale variability. Our findings have direct implications for environmental monitoring and risk assessment, highlighting the need for site-specific approaches to address the long-term impacts of ASGM in the Brazilian Amazon.