*2.4. Pollution Assessment*

Two indices were used to assess pollution in the studied soils [13]. The enrichment factor (EF) compares the concentration of an element in the studied soil with the background concentration, both concentrations standardized against a reference element (Al, Fe, or Ti). We chose Fe as the reference element, so that EF was calculated as:

$$\text{EF}\_{\text{X}} = (\text{C}\_{\text{X}} / \text{C}\_{\text{Fe}})\_{\text{soil}} / (\text{C}\_{\text{X}} / \text{C}\_{\text{Fe}})\_{\text{banks/ground}}$$

where X represents the element of interest; EFX, the EF of element X; CX, the total concentration of element X; CFe, the total concentration of iron (a lithogenic reference element); "soil" refers to the assessed soil, and "background" to the situation of natural unpolluted soils in the study area, taken as a reference. EF values below 2 indicate minimum enrichment; values between 2 and 5 denote moderate enrichment; significant enrichment is attributed to values between 5 and 20; values above 20 show very high enrichment; while values above 40 reveal extremely high enrichment [13].

The geoaccumulation index (Igeo) for a given element is calculated as:

$$\mathbf{I}\_{\text{geo}} = \log\_2 \left( \mathbf{C}\_{\text{X}} / 1.5 \,\mathrm{B}\_{\text{x}} \right)$$

where CX is the total concentration of element X; BX is the background value for element X; and the factor 1.5 is used to minimize the effect of possible variations in the background due to lithological variations. According to Igeo values, the soils are classified into seven categories: <0 = practically unpolluted, 0–1 = unpolluted to moderately polluted, 1–2 = moderately polluted, 2–3 = moderately to strongly polluted, 3–4 = strongly polluted, 4–5 = strongly to extremely polluted, and >5 = extremely polluted [13].

As background concentrations to calculate EF and Igeo, we used the quality reference values (QRVs) established by Rodríguez Alfaro et al. [26] for Cuban soils:

$$\begin{aligned} \text{EF}\_{\text{X}} &= (\text{C}\_{\text{X}} / \text{C}\_{\text{Fe}}) / (\text{QRV}\_{\text{X}} / \text{QRV}\_{\text{Fe}}) \\\\ \text{I}\_{\text{gco}} &= \log\_2 \left( \text{C}\_{\text{X}} / 1.5 \text{ QRV}\_{\text{x}} \right) \end{aligned}$$

QRVFe equals 54,055 mg kg−<sup>1</sup> [26].

The EF and Igeo for Ti were calculated using the average concentrations of Ti (4.4 g kg<sup>−</sup>1) and Fe (50 g kg<sup>−</sup>1) in the earth's crust, instead of the QRVs for Cuban soils, because there is no reference value for Ti in Cuban soils:

$$\rm EF\_{Ti} = (C\_{\rm Ti} \%\_{\rm Fe})\_{\rm soil} / (C\_{\rm Th} \%\_{\rm Fe})\_{\rm earth\ crust}$$

where CTi represents the Ti concentration and CFe represents the Fe concentration.
