From Soil Contamination to Internal Metal Burden: Drivers of Fe, Cu and Zn Bioaccumulation in Tropical Achatininae Across a Peri-Urban Contamination Gradient (Abidjan, Côte d’Ivoire)

Abstract

Peri-urban soils in rapidly expanding West African cities are subject to increasing metal contamination from industrial, urban, and agricultural activities, yet ecotoxicological risk assessment tools adapted to tropical contexts remain scarce. This study characterises the tissue-specific accumulation of iron (Fe), copper (Cu), and zinc (Zn) in the hepatopancreas and edible flesh of two tropical Achatininae land snails, Achatina achatina and Archachatina ventricosa, exposed to thirteen contrasted peri-urban soils across the Abidjan district (Côte d'Ivoire). A Toxic Pressure Index (TPI) approach was applied in parallel to soil and tissue compartments to compare external contamination pressure with internal metal burden. Bioconcentration factors (BCF) were calculated for each metal, tissue, and species combination, and the influence of soil physicochemical properties on metal transfer was assessed through univariate and multivariate regression modelling. Soil TPI was dominated by zinc at all sites, with pseudo-total Zn concentrations reaching up to 37,656 mg kg⁻¹. No significant relationship was detected between TPI soil and TPI tissue in any compartment-species combination, demonstrating a systematic decoupling between external soil pressure and internal burden attributable to homeostatic physiological regulation and the limited bioavailability of pseudo-total metal fractions. BCF analysis revealed universal biodilution of Fe and Zn across all tissues and both species, with log–log regression slopes approaching − 1 for Zn in the flesh of both species, providing quantitative evidence of a homeostatic ceiling operating independently of soil loading. Copper was the sole element consistently bioconcentrated above unity in the hepatopancreas, in 8 out of 13 sites for A. achatina and 11 out of 13 for A. ventricosa, reflecting physiological demand for Cu. BCF values were significantly higher in the hepatopancreas than in the flesh for all metals and both species (Wilcoxon, all p < 0.01), with hepatopancreas-to-flesh ratios ranging from 3- to 22-fold. A significant positive correlation between hepatopancreatic and flesh BCF of Cu in both species has direct implications for consumer exposure assessment given the traditional whole-body consumption of these snails. No significant interspecific differences in BCF were detected, suggesting broadly comparable metal regulation strategies between the two species. Multivariate regression modelling failed to identify ecologically coherent models combining soil metal concentrations and physicochemical soil properties as joint predictors of BCF.