Lanthanum anomaly as monitor of the burning of fossil fuels pollution in surface soil surrounding the industrial complex in Tula (Mexico City megalopolis)

Power plants and oil refineries emit a variety of harmful pollutants that pose a significant health risk to the surrounding population. Therefore, it is important to understand the source of pollution to develop indicators for effective environmental management and control the risk of exposure. Lanthanum (La) is a chemical element normally enriched in atmospheric aerosols from oil-burning residues that can be used as pollution tracer but their role as a proxy is rarely documented. This study evaluates the La enrichment in topsoils from the major industrial emission hotspot in the northern part of the Mexico City megalopolis. Twenty-seven topsoil samples and three rocks were collected. Initial characterization of the soils included the evaluation of its physical and chemical properties, such as granulometry, pH H2O (6.83–8.33), pH KCl (6.80–7.80), organic carbon (0.58–12.02 %), cation exchange capacity (17.40–80.23 cmolc/kg), and magnetic susceptibility (36.25–153.61 10 −8 m 3 kg −1 ). Soils ranged from sandy loam to silty clay. Elements analyzed by ICP-MS included potentially toxic elements (PTEs: V, Cr, Co, Cu, Ni, Zn, As, Mo, Sn, Sb, Pb), lanthanides (La → Lu), and geogenic elements (Al, Ti, Rb, Zr, Ba, Hf, Th). Both natural and anthropogenic sources influenced the geochemistry of soils in the study area. Based on statistical analyses and ternary diagrams, the enrichment of V Ni is associated with atmospheric residues derived from heavy oil combustion. Magnetic susceptibility data, which are dominated by residual ferromagnetic minerals, correlate with V, Ni, Mo, and La. PAAS-normalized REE patterns mainly reflect local volcanic rock sources, but positive anomalies of La (La/La* > 1) in several samples are related to anthropogenic inputs from the industrial complex. Comparisons between La enrichment and other geochemical and geophysical pollution proxies, combined with spatial interpolation maps, indicate that La/La* is a suitable tracer to monitor soil pollution through the deposition of oil combustion residues on a local scale. • The industrial complex in Tula is the largest source of pollution in the northern part of the Mexico City megalopolis. • The magnetic susceptibility of the soil is influenced by residues from oil combustion. • The enrichment of V Ni in soils is also linked to atmospheric deposition of particles from heavy oil combustion. • La/La* helps to identify soil pollution by oil combustion residues. • Spatial variations of several proxies identify the Tula industrial complex as the main source of soil pollution.