# Effects of Microbiota on the Soiling Process of PV Modules in Arid Zones of the Atacama Desert
## 3 Results and Discussion
### Mineralogical Characterization of Soiling
Regarding the PSDA, it was observed that the highest concentration of heavy metals corresponded to Pb, with an average value of 33.8 ppm, while the lowest recorded concentration corresponded to Hg, with an average of 3 \(\times\) 10\({}^{-3}\) ppm. The data obtained show that the UA had higher concentrations than those found at the PSDA for the following elements: Pb, Hg, Cu, Cd, and Fe, with ratios of 4.76, 4.53, 3.83, 2.83, and 2.63, respectively. The \(p\)-values (probability values) obtained from the ANOVA statistical test for these analytes indicate significant differences between the UA and PSDA (\(p<0.05\)). This can be explained by the fact that the coast is where anthropogenic activity is concentrated, which results in high levels of toxic metals. Conversely, the PSDA is located in the middle of the desert, away from anthropogenic activity. The results obtained, when compared with previous studies, demonstrate that the concentrations of heavy metals were within the range of previously recorded values for the coastal sector (UA) and the interior of the Atacama Desert (PSDA) ([PERSON] et al., 2019). Samples of dust deposited in the urban area of the city of Antofagasta showed average values of Pb and Zn of 164 ppm and 2155 ppm, respectively, which were similar to what was found in this study at the UA. Agricultural soils in the localities of Calama and Quillagua in the same region of Antofagasta recorded Fe values that ranged between 14,000.0 and 27,000.0 ppm, which were like those found at the PSDA ([PERSON] et al., 2019). The high concentrations of heavy metals in the soils of the Atacama Desert were related to the geological characteristics of northern Chile, which hosts the world's largest copper mineral deposits ([PERSON] et al., 2019). Furthermore, the determination of the TOC and inorganic elements that influenced the soil nutrition showed higher values at the UA compared with the PSDA (Figure 4). The TOC values measured at the UA were 0.9 ppm, whereas at the PSDA, they were 0.1 ppm. Similarly, the TP values at the UA measured 1490 ppm, while at the PSDA, they were 923.3 ppm. The \(p\)-values obtained from the ANOVA statistical test indicate that there were significant differences in the concentrations of the TOC and total phosphorus between the UA and PSDA (\(p<0.05\)). Despite the low concentrations of TOC obtained in our investigation, the difference between the values measured at the UA and PSDA indicates that the presence of organic matter was scarcer in the hyper-arid zone of the Atacama Desert. This condition was consistent with the results reported by [PERSON] et al. [30], who described that the presence of organic carbon in the Atacama Desert follows an altitudinal gradient pattern, with higher concentrations in the coastal zone that decrease toward the interior desert. When comparing our results with studies conducted in other deserts around the world, we observed some differences. For example, the TOC measurements taken from soil samples in the Mojave Desert (Eureka Valley, CA, USA) and the Sahara (near Abu Simbel, Egypt) showed concentrations of 7000 and 1700 ppm, respectively [31]. This represents a range that was between 70,000 and 17,000 times higher than the TOC concentrations measured at the PSDA and between 7800 and 1900 times higher than at the UA, which confirmed the extreme nature of the Atacama Desert. Figure 3: Average concentrations of six heavy metals (Cd, Cu, Fe, Hg, Pb, Zn) determined in the soiling deposited on PV modules (UA and PSDA). Previous analyses of the TP and TN in toposils from a locality near the PSDA showed TP values that ranged around 126 ppm [32]. This value was lower than those obtained in our study (923.3 ppm). Conversely, the same study determined a maximum of 1594 ppm of TN, which was higher than the values obtained in our study (96 ppm).