Environmental magnetism and geochemical screening of anthropogenic contamination in suburban university campus topsoil: lithogenic controls and localized ferrimagnetic enrichment in Querétaro, Mexico

Magnetic susceptibility (χ) is widely used as a rapid indicator of anthropogenic influence in urban soils; however, its interpretation can be complicated by heterogeneous lithogenic backgrounds. To this end, this study examines topsoil magnetic variability across the UNAM Campus Juriquilla (Querétaro, central Mexico), a suburban educational infrastructure underlain by basaltic andesite and affected by secondary carbonate inputs. Topsoil samples were collected and analyzed via integrated mineralogical, geochemical, and environmental magnetism approaches. To complement bulk magnetic susceptibility measurements, we conducted thermomagnetic analyses, hysteresis loops, and Day-plot domain-state characterization, together with X-ray diffraction (XRD), scanning electron microscopy (SEM–EDS), portable X-ray fluorescence (pXRF), X-ray fluorescence (XRF), and ICP–MS analyses. Mineralogical and geochemical results demonstrate that the soils are compositionally immature and derived primarily from mafic volcanic parent material. Variable contributions of secondary calcite locally dilute mass-specific magnetic signals. Furthermore, diffractograms, PAAS-normalized REE patterns, and other geochemical proxies support strong lithogenic inheritance modulated by weathering and organic matter accumulation. Magnetic data indicate that ferrimagnetic carriers are dominated by magnetite and low-Ti titanomagnetite, illustrating pseudo-single-domain behavior and mixed assemblages. Mass-specific susceptibility shows a positively skewed distribution, with discrete spatial clusters that exceed background variability. While spatial interpolation of χ and geochemical proxies underscores localized enrichments of χ, Fe, and Mn near buildings and internal circulation corridors, low-χ sectors align with green areas and carbonate-rich soils. The analytical results demonstrate dual control on the campus magnetic signal: (i) a heterogeneous volcanic–carbonate lithogenic baseline and (ii) localized ferrimagnetic enrichment consistent with near-surface anthropogenic inputs. This study develops a robust campus-scale baseline and demonstrates the value of combining environmental magnetism, geochemistry, and geospatial modeling as a screening framework for determining priority areas for targeted urban soil contamination assessments.