## 3 Results and Discussion  
### Exploratory Data Analysis  
Histograms for both catchments show that the distributions of As, Pb, Cu, and Zn are asymmetric to the right, with longer tails at higher concentrations due to a relatively small fraction of high values (Figures S1 and S2). Box plot analysis shows some outliers in the higher concentration tails, indicating that external sources affect the natural concentrations [4]. High dispersion reflects that several point sources contribute to these concentrations and diffuse contamination of these elements within the urban sector. The summary statistics for the elements As, Cu, Pb, and Zn are presented in Table 1 for the Aconcagua and Casablanca Rivers. The first thing to consider is the coefficient of variation: in the Aconcagua River basin, soil samples from the whole basin present extremely high CV values (CV > 100%) for As and Pb and are considered high (50% < CV < 100%) for Cu and Zn. The Casablanca River catchment has a low CV for Cu, Pb, and Zn and an extremely high value for As. Elements with a lower CV are expected to be dominated by natural sources. In comparison, those with a higher CV are more likely to be affected by anthropogenic sources [23]. Industrial activity is considered an anthropogenic source for the Aconcagua River basin, which would explain the high CV values observed. For the Casablanca River basin, it is concluded that the total As concentration in soil may be affected by applications of fungicides or pesticides containing As [24]. However, the contribution of As-containing fungicides to soil As concentrations is expected to be similar at all sampling sites in both river basins, because fungicide application is not attributed to a specific crop or crop type. Instead, fungicide application is common for several vegetable and fruit crops in Chile [25]. The big differences between the minimum and maximum values indicate spatial heterogeneity of potentially toxic elements in soils [26]. For both watersheds, a high heterogeneity is observed. This is because soil in the same sector does not have the same characteristics, and as the samples were taken from different areas of the watersheds, it is correct to state that they have a high variability. Regarding the average concentration \begin{table}
\begin{tabular}{c c c c c c c c c c c c c} \hline \hline  & \multicolumn{4}{c}{**Aconcagua River Basin**} & \multicolumn{4}{c}{**Aconcagua River Basin**} & \multicolumn{4}{c}{**Casablanca River Basin**} \\  & \multicolumn{4}{c}{**(Including Mining Activities)**} & \multicolumn{4}{c}{**(Without Mining Activities)**} & \multicolumn{4}{c}{**(Only Agricultural Activities)**} \\ \hline  & **As** & **Cu** & **Pb** & **Zn** & **As** & **Cu** & **Pb** & **Zn** & **As** & **Cu** & **Pb** & **Zn** \\ \hline Mean & 14.3 & 382.1 & 40.9 & 170.3 & 10.6 & 85.8 & 26.7 & 139.5 & 2.5 & 33.1 & 18.7 & 66.7 \\ SD a & 8.6 & 868.8 & 27.3 & 76.7 & 4.6 & 32.6 & 5.9 & 73.7 & 3.6 & 13.3 & 5.9 & 20.5 \\ CV (\%) b & 337.5 & 213 & 63.4 & 45.1 & 457.1 & 948.8 & 97.2 & 55.1 & 146.8 & 40.1 & 31.7 & 30.8 \\ Min. & 1 & 28 & 14 & 46 & 6 & 53 & 18 & 78 & 0 & 10 & 10 & 30 \\ Max. & 44 & 5581 & 309 & 579 & 24 & 160 & 36 & 395 & 14 & 66 & 41 & 139 \\ \hline  & 25 & 8 & 90 & 32 & 120 & 7 & 62 & 20 & 95 & 0.4 & 24 & 15 & 56 \\ Percentile & 50 & 13 & 128 & 36 & 161 & 9.5 & 69 & 28.5 & 113 & 1 & 31 & 17 & 69 \\  & 75 & 21 & 316 & 43 & 216 & 12.8 & 119 & 31 & 159.3 & 2 & 42 & 20 & 80 \\ \hline Skewness & 0.77 & 4.7 & 7.8 & 1.6 & 1.6 & 0.95 & \(-\)0.16 & 2.4 & 2.1 & 0.44 & 2.02 & 0.97 \\ \hline \hline \end{tabular}  
* Standard deviation, and b coefficient of variation.  
\end{table} 
Table 1: Statistical summary of potentially toxic elements for soil samples from the Aconcagua River basin with and without mining activity and the Casablanca River basin.