## 4 Discussion
### 2D-ERT Technique As a Proxy of Cd Dynamics  
Table 2 shows several methods developed during the past two decades to analyze Cd content in agricultural soils. As illustrated, the 2D-ERT technique is accurate for the detection of Cd in both topsoil and subsoil in cacao farms. In comparison to other techniques, such as the dynamic-based diffusive gradients in thin-films (DGT) or the portable X-Ray diffraction methods, which are also in situ non-invasive techniques, the accuracy of the 2D-ERT is greater. With exception of the DGT method [38], the mentioned below techniques have not yet been applied in cacao farm soils to our knowledge.  
Figure 9: Principal component analysis (PCA) plot of six soil parameters compared to pseudo-total Cd determination. Resistivity is the parameter close to Cd in the order: resistivity \(>\) P \(>\) SOM \(>\) Fe \(>\) pH \(>\) Ca.  
Cd has an excellent electric conductivity [44]. Therefore, the study of Cd distribution in soils under cacao trees requires non-destructive and non-invasive methods to assess its presence and its relationship with mobile fractions of Cd in cacao beans. We highlighted the use of 2D-ERT to assess both the horizontal and vertical distributions of Cd in geomorphs and their locations underground. The accuracy of the 2D-ERT technique (R\({}^{2}\) = 0.84) to describe Cd distribution with Cd counts by spectrometry is also highlighted, even when no (phyto)available Cd was described [44].  
In cacao farms from Boyaca and Santander districts, resistivities related to Cd were greater than 10\({}^{3}\) Ohm-m (Figures 5 and 6). Predicting the existence of underground rock or parent material was successfully accomplished according to our hypothesis that reservoirs of solid-state phase soluble and non-soluble Cd aggregates, close to CdCO\({}_{3}\) and CdSO\({}_{4}\), when present at higher concentrations, can induce outcropings of carbonates with higher resistivity values [45]. Therefore, solid-state phase Cd compounds can occur during soil formation in an agricultural soil system such as cacao [46; 47; 48], where geological and anthropogenic Cd interchange might occur frequently.  
Moreover, the biology of the system plays a key role in Cd dynamics. Since bacterial carbonate-genesis relies on the relation with soil pH and is highly correlated with the distribution of SOM, it is possible that microbial activity influences the secondary formation of otavite in neo-tropical acidic soils like those assessed; however, this needs to be studied in more detail. At the farms surveyed in this work, acidic pH, and higher SOM content, even at a 68 cm soil depth, might lead to an ideal scenario for bioweathering activity mediated by Cd-tolerant bacteria [1].  
Furthermore, the 2D-ERT profiling was an accurate technique in monitoring in situ and in real-time parent material and rock aggregates related to soil Cd, addressing the site-specific sampling in the trial pits. However, it is not yet clear if Cd selectivity is addressed by the bioweathering of a calcite system. Therefore, further research is necessary to examine Cd dynamics through resistivity when both otavite, and hydroxy- and fluorapatite, are present, and the bioweathering process occurs due to Cd-tolerant bacterial activity in interaction with ligands disposed in clays. Regardless of the specific mechanisms of Cd speciation in soils, an estimation of Cd levels was possible using the 2D-ERT technique.