## 3 Results and Discussion
### Trace Elements and Inorganic Nitrogen
In the phreatic aquifers (Figure 8a), where oxidizing conditions should theoretically be dominant, 66% and 57% of the samples show Fe and Mn concentration <D.L., respectively. However, most of the remaining samples have values that well exceed the legislative threshold of 200 ug/L for Fe and 50 ug/L for Mn (D.Lgs 31/01 [84]), sometimes even 1 order of magnitude. Most of the samples with high Fe concentrations are located along the Po River, in the lower VEN plain and at the base of the Apennines. The high concentrations recorded on the border between PIE and LOM, showing an increase in the warm season compared to the cold period, could be due to the submersion of the paddy fields. This effect, however, has not been documented by local studies, but it is known in the literature [88, 89]. Paddy fields are reducing environments developing because of the long term presence of water flooding the land. Here, the Fe/Mn mineral-rich strata and soil with abundant organic matter act as sources of Fe and Mn to the groundwater and the reductive environment in the lower terrain and areas containing water bodies favor Fe and Mn dissolution in the groundwater. Moreover, NH4+ generated by the application of N fertilizers during agricultural activities can promote the reduction of Fe-Mn oxides and cause Fe and Mn to be released into the water [89]. Figure 6: NH\({}_{4}\)+ distribution in the (**a**) phreatic, and (**b**) semi-confined and confined aquifers. In confined and semi-confined aquifers (Figure 8b) more than 32% and 41% of samples have values higher than the Fe and Mn legal thresholds, respectively. Being the deep aquifer less affected by anthropogenic activities, these exceedances are due to natural background [90]. Figure 9 shows As distribution. The mobility of this element in water strongly depends on the oxidation state: the reduced form (As\({}^{3+}\)) has greater mobility, while the oxidized form (As\({}^{5+}\)), which is found as oxyanions, tends to be absorbed by Fe-, Al-, and Mn- hydroxides and, to a lesser extent, by clay minerals and organic matter, favoring precipitation and removal from solution. In alluvial plains, the reductive dissolution of Fe- and Mn- oxides and hydroxides, linked to the anaerobic degradation of peat layers, has been identified as the main As source from sediment to groundwater [91; 92; 93; 94]. The peat was formed in the meanders abandoned by the rivers and in the water stagnation areas of the main watercourses and were buried and incorporated into the stratigraphic sequence by the subsequent alluvial depositions of the rivers [95]. The As distribution in the aquifers of the Po Valley reflects the NH\({}_{4}\)+ distribution (Figure 6). Most of the water samples (75%) have concentrations <L.R., while the majority of values above legal threshold (10 \(\upmu\)g/L) are localized in the semi-confined aquifers, where the effect of organic matter degradation could be greater than in deeper confined aquifers. Moreover, [PERSON] et al. (2015) found the main anomalies at depths between 10 m and 80 m below the ground [93]. The anomalous samples are concentrated in the central area, in the lowlands of VEN, LOM and EMR. The presence of extreme values in these areas have been extensively documented by many studies [96; 93; 86]. It must be noted that some difference is found compared to our study. In the phreatic aquifers of the lower VEN plain, high concentrations (>100 \(\upmu\)g/L) are mainly located in the province of Rovigo; however, [PERSON] et al. (2013) [97] detected concentrations higher than 300 \(\upmu\)g/L in many wells around the Venetian area, which, according to the authors, represent natural hot-spots. In the EMR confined aquifers, several water samples exceed the background values of 170 \(\upmu\)g/L identified by ARPAE [98], with values around 275 \(\upmu\)g/L in the provinces of Bologna, Ferrara, and Ravenna. Figure 7: Correlation between HCO\({}_{3}\)\({}^{-}\) and NH\({}_{4}\)\({}^{+}\) among the confined and semi-confined aquifers.