Soils are natural resources, formed at the Earth’s surface by the weathering of underlying rocks due to physical, chemical, and biologic factors. They support agriculture and the main carbon reservoir of the terrestrial carbon cycle, they act as a sink for pollutants, protecting groundwater from pollution, and they are also used as construction material and support. Therefore, they have very important social, environmental, and economic functions [1
However, if contaminated or polluted, they can transfer potentially toxic elements (PTEs) to groundwater, to seepage waters and rivers [3
], and to crops and vegetables that are used by humans and animals, and can consequently affect human health. The soil contamination may be natural due to rock composition [6
]. Some elements can accumulate in topsoil to concentrations that are toxic to the plant, to the animal feeding on it, and to humans. Air quality may also be affected by contaminated soils due to the generation of airborne particles and dust [7
]. In deeper soils, due to changes in pH and Eh, PTEs may be released into the groundwater, resulting in its contamination [8
The chemical composition of dust, soils, and groundwater may cause metabolic changes that may favor the occurrence of endemic diseases in humans [14
]. The role of F, I, Se, and As concentrations in the health of human populations is well documented in the scientific literature [21
The high concentrations of PTE on topsoil can threaten human health (a) via soil ingestion by geophagism, rare in adults but quite common in children by hand-to-mouth intake; (b) by the inhalation of dust particles; (c) by dermal contact [27
], especially by farmers and construction workers; and (d) indirectly by ingestion of contaminated groundwater.
The geochemistry of the major, trace, and rare earth elements (REEs) of soils of Santiago Island (Cape Verde) has been studied to characterize the soils developed on volcanic rocks and Quaternary sediments, contributing to the establishment of a geochemical atlas of the island [9
The Mapping of Estimated Background Values (EBVs), the agricultural and residential Environmental Risk Index (ERI) for each element, and the agricultural and residential Multi-Element ERI (ME-ERI), which is the average of the ERIs of harmful elements in the soils of Santiago Island, were presented by Cabral Pinto et al. [30
]. The present work follows the precedent study to better understand the relationships between environmental geochemistry and public health in a volcanic island that still preserves many pristine geochemical characteristics and where the anthropogenic action is not yet too strong. We present the hazard index (HI) and the carcinogenic risk due to the exposure of potentially toxic elements to the Santiago Island population, according to the Exposure Factors Handbook [31
]. We consider soil ingestion, inhalation, and dermal contact as exposure pathways, because most of the population of the Island is rural, and the island is affected by strong winds (defined as “bruma seca”), which causes the mobilization of significant amounts of dust particles from soil [32
2. Geographic, Geologic, and Climate Settings and Soil Types
Cape Verde is formed by 10 islands, located off the shore of Western African (Figure 1
). The country capital is located on Santiago Island, which is the largest island. It is a mountainous island with a maximum altitude of 1394 m. It has 215 km2
of arable area and estimated water resources of 56.6 × 100 m3
/year at the surface and 42.4 × 100 m3
/year underground [33
The climate is semi-arid, with strong winds during the dry season, and a mean annual precipitation of 321 mm, mainly due to torrential rains, in the wet season [34
]. In the 1900–2012 years period, the mean historical monthly rainfall attained 347.7 mm and the highest value (109.2 mm) was recorded in September [35
]. In the same period, the mean historical monthly temperature varied from 20.4 °C in February to 25.5 °C in September [35
The islands are volcanic, intraplate, located over a submarine plateau known as the Cape Verde Rise, and relatively stable within the African Plate. The volcanism is a result of the interaction of a mantle plume with the fractured lithosphere [36
]. Santiago Island is a shield volcano, with periods of intense volcanic activity, the emission of alkaline basaltic lava flows, and subaerial pyroclastic materials, separated by erosion and sedimentations periods [38
]. A brief description of the lithostratigraphic formations of Santiago is presented in Figure 2
a and Table 1
b presents the adapted soil cartography of Santiago Island, based on studies by [9
], according to the FAO/UNESCO classification. Table 2
shows a brief description of the soil cartography. The main soils are lithosols, regosols, xerosols, and cambisols. Kastanozems occur mainly in association with luvisols, which are the soil group typically used for agriculture in Cape Verde (Figure 2
b and Table 2
4. Results and Discussion
The topsoil of Santiago Island is enriched with Cd, Ni, Co, Cu, V, Mn, Cr, and Zn to upper crust values (UCCs) of [47
], considering both the Estimated Background Values (EBVs) or the 95th percentile value calculated from the soil distribution values (Table 3
The HQs for ingestion, dermal contact, and inhalation routes and HIs were calculated for the metals (Table 4
), which showed enrichment compared to the UCCs, and for As, which is a toxic and carcinogenic element.
The selected elements are potentially toxic elements, and some (As, Cd, Cr, and Ni) are also carcinogenic [46
]. The pathways chosen were ingestion, inhalation, and dermal contact, and the calculations were performed for children and adults. Major neurodegenerative disorders, including Alzheimer’s and Parkinson’s disease, are characterized by the elevation of tissue metals, such as Fe, Cu, Mn, and Zn [48
]. Environmental exposure to Mn can induce parkinsonism; although the long-term medical significance of this finding is unclear, the data are troubling and point to the need for further investigation of manganese’s health risk [49
The non-carcinogenic HIs for all nine elements are given in Table 4
. For adults, the HIs were always less than 1, whereas for children they were higher than 1 for Co, Cr, and Mn. The HI values of these elements are mainly controlled by the HQingestion
, which are also greater than 1 for these three elements. For all elements, the HQingestion
is always the highest, while the HQinhalation
is always the lowest (Figure 3
Therefore, there is an indication of potential non-carcinogenic risk for children, due to the high Co (HI = 2.995), Cr (HI = 1.329), and Mn (HI = 1.126) values in soils. For the other elements, there is no potential non-carcinogenic risk for adults. For both children and adults, the HI is Co > Cr > Mn. Compared to adults, the children’s health index is greater, and the cumulative effect of these indices is also of greater concern for children.
The evaluation of cancer risk was performed only with those elements, which are potentially carcinogenic [46
]. For As, ingestion, inhalation, and dermal contact exposure pathways were considered; however, for the other carcinogenic elements (Cd, Cr, and Ni), only the inhalation risk was computed because the Risk Assessment Information System [46
] does not present slope factors for the other exposures.
For children, the As cancer risk (Risktotal
) is 6.38 × 10−9
) and of this, the fraction due to Riskingestion
represents 51.6%, while Riskinhalation
represents 48.0%, and Riskdermalcontact
represents only 0.4% of the total risk. For adults, Risktotal
for As is 1.24 × 10−8
), whereas Riskinhalation
represents 81.3%, Riskingestion
represents 16.6% and Riskdermal contact
represents 2.1%. These results reflect the higher daily ingestion dose for children and the higher inhalation rate for adults. For adults, the cancer risk due to Cr, Ni, and Cd inhalation is always higher than it is for children, reflecting the higher inhalation rate of adults.
For adults, the results for cancer risk are higher than the carcinogenic target risk of 1 × 10−6
] for Cr only, but these results underestimate the risk. The other pathways were not considered, and they can be particularly important for Cr, which presents a cancer risk of 1.3 × 10−6
, very close to the target risk. The lack of RfD for some elements prevents a more complete evaluation of cancer risk (Table S1
Santiago Island still has an almost pristine surface environment, and the topsoil composition is mainly determined by the composition of the underlying basic rock. These rocks are rich in siderophile elements, promoting a natural contamination of soils in Co, Cr, Ni, Cu, V, Zn, Cd, and Mn. Of these, Co, Cr, and Mn present a potential non-carcinogenic risk for children, a vulnerable subset of the population. On the other hand, the soil composition affects groundwater composition, so there is a flux of natural contamination from soils to the groundwater, which deserves to be evaluated. The inhabitants of Santiago Island depend on groundwater for consumption and for agriculture, and the flux water-vegetables-men also deserves evaluation because endemic diseases can be controlled with proper measures if its cause is well constrained.
The topsoil of Santiago Island, Cape Verde, has a geochemical composition, mainly controlled by the type of underlying rock, as most of the elements in the topsoil have primarily a geogenic origin.
The environmental risk index (ERI) calculations showed that Santiago Island topsoil is naturally contaminated with Co, Cr, Cu, Ni, and V, because these elements have contents well above those allowed by Canadian and Dutch legislations for agricultural soils.
The non-carcinogenic His were calculated for nine potentially toxic elements, and they are always less than 1 for adults, considering that the soil contaminants enter the human body by soil ingestion, dermal contact, and inhalation of dust particles. For children, the non-carcinogenic HIs are 2.9952 for Co, 1.3293 for Cr, and 1.1111 for Mn. For the other elements, they are less than 1.
For adults, the cancer risk is greater than the carcinogenic target risk of 1 × 10−6 for Cr. However, these results may be underestimated, as only the inhalation risk was calculated for Cr, Ni, and Cd. Moreover, soil contaminants may be indirectly ingested by groundwater and by crop and vegetable consumption, increasing the hazard and cancer risks.
There is need for an evaluation of the risks associated with groundwater consumption and diet on Santiago Island.