Heavy Metals Accumulation, Ecological Risk and Seasonal Variation in Indigenous Plants from an Artisanal Gold Mining Area in Kebbi State, Nigeria

Abstract

Artisanal and small-scale gold mining (ASGM) can release potentially toxic metals into surrounding soils and vegetation, creating possible pathways of exposure for communities living and farming in mining-impacted areas. This study investigated seasonal variation in heavy-metal accumulation, bioaccumulation, ecological pollution, and potential human-health risks associated with six indigenous plant species around the Mararrabar Birnin-Yauri artisanal gold-mining area in Kebbi State, Nigeria. Stem-bark samples of Adansonia digitata, Piliostigma thonningii, Odina barteri, Vitellaria paradoxa, Parkia biglobosa, and Anogeissus leiocarpus were collected from six sampling locations during the dry season (December 2024) and rainy season (August 2025). Samples were washed, dried, pulverised, and homogenised before elemental analysis. X-ray fluorescence (XRF) was used for oxide screening, while quantitative concentrations of selected metals were determined by Microwave Plasma Atomic Emission Spectrometry (MP-AES). Seasonal differences were evaluated using non-parametric statistical tests. Bioaccumulation Factor (BAF), Contamination Factor (CF), Enrichment Factor (EF), Pollution Load Index (PLI), Potential Ecological Risk Index (PERI), and Metal Pollution Index (MPI) were used to assess metal accumulation and pollution, while Hazard Index (HI) and Cancer Risk (CR) were used to estimate potential health risks from plant consumption. Heavy-metal concentrations varied considerably among species and between seasons. The Wilcoxon Signed-Rank test indicated a statistically significant overall seasonal difference only for Parkia biglobosa (p = 0.005). Particularly high CF values were observed for selenium, cadmium, silver, molybdenum, and thallium. During the rainy season, silver CF reached 6661.286 in Anogeissus leiocarpus, while thallium CF reached 594.550 in Piliostigma thonningii. Dry-season PERI reached 1946.331 in Vitellaria paradoxa and 1738.529 in Adansonia digitata. Several BAF values exceeded one, indicating efficient uptake of selected metals by particular species. HI values exceeded 1 for all species in both seasons and for both adults and children, with Adansonia digitata recording the highest values. CR values were also above the USEPA acceptable range for all species. The findings indicate that vegetation growing in the mining-impacted area may constitute an important pathway for human exposure to toxic metals and may warrant continued environmental and health-risk monitoring.