Potential Human Exposures to Hg in Staple Crops from Agricultural Areas Impacted by Artisanal Small-Scale Gold Mining Activities

Abstract

Artisanal small-scale gold mining (ASGM) is a rapidly expanding sector that manifests complex political, social, and environmental issues. Chief among these problems are the emissions (air) and releases (terrestrial/aquatic) of mercury (Hg) that is employed as the primary method of recovering gold from mined ores and sediments (Hg-gold amalgam burning). Recent work with stable Hg isotopes has demonstrated stomatal uptake of elemental Hg (Hg(0)) by higher plants to dominate Hg deposition in vegetated systems. ASGM activities are indiscriminate in where they occur and often impact highly productive forests, wetlands, and agricultural areas. Considering the importance of the latter for sustaining human populations and the extensive atmospheric emissions of Hg from ASGM activities, the stomatal uptake pathway of Hg by staple crops in agricultural areas impacted by ASGM is worthy of scientific investigation. In this study, we examine potential human exposure pathways in ASGM impacted agricultural areas in Nigeria using a total systems approach. The study works collaboratively with a local mining community in Nigeria to facilitate knowledge dissemination with impacted individuals and to ensure a more holistic study design. Samples of tubers/grains, foliage, roots, and stems of maize, cassava, and peanuts were collected from two farms along a transect away from an active ASGM site (Farm 1 closest; Farm 2 furthest). Air (passive samplers) and soils were sampled at the ASGM site and both farms. Results reveal contamination of farms and crops in the farm closest to the ASGM area, with the control farm at background levels, confirming Hg exposure gradient away from the mine. Stable isotope data also reveal the atmosphere as the dominant source for the uptake of Hg in staple crops for all species, with minimal translocations to other parts of the crops. In assessing potential human health impacts from consuming these crops, MeHg levels in crop tissues measured were consistently less than 1% of THg, suggesting minimal risk from MeHg exposure. However, elevated levels of inorganic Hg highlight a significant potential for harm.