Soil Geochemical Mapping of the Sal Island (Cape Verde): Ecological and Human Health Risk Assessment

Geochemical mapping is the base of knowledge needed to determine the critical contents of potential toxic elements and the potentially hazardous regions on the planet. This work presents maps of baseline values of chemical elements in the soils of Sal Island (Cape Verde) and the assessment of their ecological and human health risks. According to the results, Ba, Co, Ni, and V baseline values are above the international guidelines for agricultural and residential proposed uses. Arsenic in the soil overlying the Ancient Eruptive Complex shows a high potential ecological risk factor. It is not clear if high As contents in soils have a geogenic or anthropogenic source. Hazard indexes (HI) were calculated for children and adults. For children, HI is higher than 1 for Co, Cr, and Mn, indicating potential non-carcinogenic risk. These elements are present in high content in soils covering Quaternary sediments, the Monte Grande-Pedra Lume Formation, and the Ancient Eruptive Complex, inducing belief in a geogenic source. For the other elements and for adults, there is no potential non-carcinogenic risk. Cancer risk (CR) was calculated for As, Cd, Cr, and Ni exposures for adults and children, and the results are mainly lower than the carcinogenic target risk value, indicating no cancer risk. Only in a few soil samples are CR results slightly higher than the carcinogenic target risk of 1 × 10⁻⁴ 2 × 10⁻⁶ for adults exposed to Cr by inhalation. It is important to emphasize that these results of the health risk associated with exposure are likely to overestimate the bioavailable fractions of the elements in the soil once it is used as aqua regia instead of physiological fluids to digest the soil. However, since measured concentrations of potential toxic elements in soil reveal that they can be harmful to both the environment and human health, regional activities such as agriculture or water exploitation must be controlled by competent authorities. These conclusions highlight the insights and the applicability of soil geochemistry surveys for future policy progress, which are particularly relevant in developing countries like the Cape Verde archipelago.