Geochemical behavior and fate of trace elements in naturally contaminated soils under projected land-use changes

Purpose This paper focuses on determining the geochemical fractionation pattern of trace elements (As, Cd, Cu, Pb, Tl, and Zn) naturally occurring at elevated levels in chestnut grove soils of SW Spain. The goal was to explore how environmental changes triggered by land use and management decisions might affect the resilience and adaptive capacity of soil to retain geogenic trace elements. Materials and methods Two plausible scenarios were considered: conversion of forestland to cropland (scenario I) and mining area (scenario II). The potential for trace element removal under the assumed scenarios was assessed by chemical extraction procedures designed to simulate the combined effects of experimentally induced pH and redox changes. Trace elements were partitioned into residual and labile fractions using a five-step sequential extraction scheme optimized for soils enriched in well-crystallized Fe oxides, and their concentrations in the soil extract solutions were measured by inductively coupled plasma mass spectrometry. Results and discussion Most metals are tightly bonded to residual and reducible phases, indicating that silicate minerals and Fe oxy-hydroxides, respectively, played a remarkable role in the metal geo-accumulation. Limited mobilization and dispersion of exchangeable and acid-soluble contaminants would be expected to occur through releases or accidental spills from hazardous wastes. An increase in the oxidation state of the soil environment would affect the stability of the organic matter involving the release of the associated trace elements, particularly Cu. Upon reducing conditions induced by land-degradation processes, reductive dissolution of Fe oxy-hydroxides could release large proportions (45-60%) of adsorbed and occluded potentially harmful elements, notably As, Pb, and Cd. Conclusions The increasing abandonment of the chestnut groves constitutes a driving force for environmental changes that might affect the geochemical status of the trace elements stored in the soil. Soil could shift from a sink to a source of harmful contaminants over time. This fact should be considered by local stakeholders engaged in planning and decision-making on future land uses.