## 3 Results and Discussion
### Exploratory Data Analysis
of potentially toxic elements, we are guided by the \"world soil average\" [29], which tells us that the world average concentration per soil of the elements As, Cu, Pb, and Zn is 6.83 (mg kg\({}^{-1}\)), 38.9 (mg kg\({}^{-1}\)), 27 (mg kg\({}^{-1}\)), and 70 (mg kg\({}^{-1}\)), respectively. Thus, the concentrations of potentially toxic elements in all the soil samples of the Aconcagua River basin are well above the world average for soils (in order of highest to lowest are Cu, Zn, Pb, and As), which indicates that these elements contaminate the area. On the other hand, the average concentrations of the Casablanca River basin are below the world average, which suggests that the area is not contaminated. The comparison is made with the world average because Chile still has no regulation or reference values for potentially toxic soil elements. This is a problem, since soils do not have geochemical similarities even in the same country. In addition, it should be considered that soils in Chile are enriched in Cu and Zn, so it is urgent to pass laws that can provide optimal reference values for potentially toxic elements in Chilean soils. Finally, from the perspective of asymmetry, the values of these elements vary in the order Zn > As > Cu > Pb for soil samples without mining activity in the Aconcagua basin, Pb > Cu > Zn > As for random soil samples from the Aconcagua basin, and As > Pb > Zn > Cu for soil samples from the Casablanca basin. This is consistent with the coefficient of variation of trace elements, indicating that human activity may affect the elements, resulting in a significant positive skewness. [PERSON] et al. [30] proposed toxicity thresholds for humans in residential areas with As, Cu, Pb, and Zn, which are 16 mg/kg, 270 mg/kg, 400 mg/kg, and 2200 mg/kg, respectively. For As, the maximum concentration in samples from the Aconcagua River basin without mining activity and in samples including mining activity from the same basin is 24 mg/kg and 44 mg/kg, respectively, indicating toxic levels for humans. On the other hand, in the Casablanca River basin, the maximum concentration of this element is 14 mg/kg, slightly below the threshold. The concentration of Cu in the soil samples, including mining activity, reached a value of 5581 mg/kg, far above the allowed limit. The maximum concentrations in soil samples from the Aconcagua River basin without mining activity and from the Casablanca River basin are 160 mg/kg and 66 mg/kg, respectively. Regarding Zn and Pb, the risk of human exposure in the Aconcagua and Casablanca River basins is of minor importance, as their total maximum concentration in the soil of the Aconcagua River basin without mining activity is 395 mg/kg for Zn and 36 mg/kg for Pb. For soil samples from the same basin, including mining activity, the concentrations are 579 mg/kg for Zn and 309 mg/kg for Pb. In samples from the Casablanca River basin, the values are 139 mg/kg for Zn and 41 mg/kg for Pb, all well below the proposed human toxicity threshold. It should be noted that children playing outdoors can ingest soil contaminated with metals [31], which becomes dangerous and harmful if consumed in large quantities.