Arsenic and lead in the soils of San Antonio–El Triunfo mining district, B.C.S., México: a human health risk assessment

The MD SA-ET in Baja California Sur, Mexico, had over 200 years of intermittent mining activities. Studies in the area have reported Pb, Cd, Zn, and As dispersion from waste to soils, sediments, and groundwater, suggesting that even after all this time, biogeochemical processes have not completely mitigated the risk. Furthermore, evaluating how the fine particles in soils could control the human health risk by geochemical and mineralogical mechanisms has not yet been initiated. This study discusses the geoavailability and bioaccessibility based on the environmental and human health risks in an arid environment where the population is currently exposed to abandoned mining waste. The results show that urban soils (US) and stream sediments (SS) have very high total concentrations of As and Pb, but with low geoavailability, suggesting low environmental risk. Despite this, bioaccessibilities up to 100% were observed for As and Pb in the US, suggesting high risks for the inhabitants of San Antonio and El Triunfo towns. The blood Pb levels and the carcinogenic risk for As exposure assessed here show maximum values of 115.77 µgPb dL⁻¹ and 85 × 10^–4, respectively, which exceed the recommended limits according to USEPA. Chemical identification of microparticles (< 250 μm) by SEM–EDS shows bright metal solids with high iron content, commonly interpreted as iron oxide. However, it was possible to identify microparticles (< 5 μm) rich in Fe associated with ultrafine particles (< 1 μm) with high Pb and As contents. We conclude that differences between the geoavailability and bioaccessibility for soils and sediments in this study indicate that the solid microphases (possible Fe-oxides and oxyhydroxides) present in the fine soil fraction from MD SA-ET may control the geoavailability and bioavailability of As and Pb, as well as the environmental and human health risks.