## 2 Materials and Methods  
### Calibration of 2D-ERT In Vitro  
The calibration was performed in the Laboratory of Physics at the University of Narino, in Pasto, Colombia. The resistivity measurements of cocoa farm soil samples were adjusted to small scale processes [14]. Before calibration the first measurements were done in soil samples with no Cd amendment. The aim of performing a calibration was to correlate the resistivity values with the Cd content of soil samples amended with pure reagents Cd. These values would be expected to be found in Cd enriched or contaminated soils. Two controls of Cd sources were used. The first soluble source was cadmium sulphate powder (\(\geq\)99.99% _w/w_ purity, Sigma-Aldrich, St. Louis, MO, USA). The second non-soluble source corresponds to cadmium carbonate powder (otavite \(\geq\)99.99% _w/w_ purity, Sigma-Aldrich, St. Louis, MO, USA). Whereas CdSO4 is soluble in water at 25 \({}^{\circ}\)C (76.7 g \(\times\) 100 mL H\({}_{2}\)O), CdCO\({}_{3}\) is not [15]. However, HCl can be used to solubilize CdCO\({}_{3}\)[16]. Therefore, 2.5 mL of HCl 36% (_w/v_) was added to 1 g CdCO\({}_{3}\) 99.99% (_w/w_) and gently mixed for 10 min at room temperature using a vortex. In both cases, the Cd sources were amended to soils collected from non-Cd contaminated cacao farm (\(\leq\)0.02 mg\(\cdot\)kg\({}^{-1}\) Cd). These farms were located in the same districts as the contaminated farms.  
Two hundred and fifty grams of soil samples were mixed for 10 min with four different concentrations of each Cd source. The treatments consisted of an amendment of 0, 1, 2, and 5 g of the Cd source. The mixture was completed by adding 70 mL of nanopure sterile water and depositing the samples in three blocks of 125 cm\({}^{3}\) plastic cube molds (triple mold buckets of cement and mortar, ref. INV-E 323, Pinzuar Ltd.a. Bogota, Colombia) for physical analysis. After drying the samples at room temperature (20 \({}^{\circ}\)C \(\pm\) 0.2 \({}^{\circ}\)C on average) for 96 h, four resistivity testing electrodes (Stanley steel iron based alloy electrodes, ref. AISI 316 L Hilo 5 mm diameter FF215140, Goodfellow SARL, Lille, France) were displaced at an \"_a_\" distance of 1 cm, each located equidistantly within the cube (Figure 1A), to produce an average measurement of Cd disposition in the sample. Additionally, a \"_b_\" distance was used between the external electrodes. The electrical resistivity was measured and quantified as the mean values of a Wenner four electrode array, according to the American Section of the International Association for Testing Materials (ASTM) protocol G 57 [17].  
The measurement setup was carried out according to a previous study [18]. In summary, the electrodes were connected to a high voltage power supply (SF-9585A: PASCO, Tecnodidacticas Ltd.a. Bogota, Colombia). The electrodes were also pinned to a digital multimeter (UNI-T UT71D, UNI-T, Hong Kong, China) to register both the current and voltage parameters (see Figure 1A). Fifty replicated measurements were recorded per treatment; therefore, the results were expressed as the mean of the treatments.  
Figure 1: Calibration of the two-dimensional electrical resistivity tomography (2D-ERT) technique in vitro. (**A**) The equipment was set for two pure Cd reagents (CdCO\({}_{3}\) and CdSO\({}_{4}\)), measured in four concentrations and in triplicate using a simple electronic device, consisting of the voltage current power, a multimeter, and electrodes; (**B**) the correlation between resistivities and Cd concentrations using cadmium sulphate (red line) and cadmium carbonate (black line). The correlation was high according to the coefficient of determination, as shown in the plot. When no Cd was added the resistivity remained lower than 800 Ohm\(\cdot\)m.  
A linear regression was made to analyze the correlation between resistivity values with the Cd concentrations of the Cd treatments (see Figure 1B).