## 3 Results and Discussion
### Trace Elements and Inorganic Nitrogen  
Among the selected elements in paragraph 2.2, we present maps of NH\({}_{4}\)\({}^{+}\), Fe, Mn, and As distributions in phreatic and confined aquifers. Nitrate contamination of Northern Italian aquifers is well documented [79; 80; 81; 82; 83], while less is documented about NH\({}_{4}\)\({}^{+}\) distribution. Nitrate pollution is widespread through much of the plain (in PIE, EMR, along the Apennine alluvial fans, and in the low LOM plain; cfr. Figure S1 in Supplementary Material). This is connected above all with the massive presence of intensive farming and extensive use of nitrate fertilizers, with the disposal of stockbreeding waste, and with seepage from the urban sewage systems [42; 79]. Nitrate pollution reaches its highest levels (also over 100 mg/L) in the areas bordering the Alps and the Apennines where the hydrogeological structures appear to be the most vulnerable and land utilization the most intense. The absence of continuous aquitards, especially in the areas of aquifer recharge, allows nitrate leaching from shallow to deep aquifers. Contamination of confined aquifers is extremely localized since, in anaerobic condition, nitrate is reduced to NH\({}_{4}\)\({}^{+}\). Figure 6 shows NH4+ concentration in the phreatic and confined aquifers. While in shallow aquifers, most of the water samples (77%) have concentrations <0.10 mg/L, 32% of the samples collected in semi-confined and confined aquifers show values above the legal limit of 0.5 mg/L, set for water intended for human consumption (D.Lgs. 31/01 [84]). Ammonium in phreatic aquifers is found mainly in lowland areas (Figure 6a). Ammonium is used as fertilizer, especially in areas of intensive agriculture, such as the lower VEN plain. However, the presence of NH4+ is also strongly linked to organic matter decomposition processes, especially in aquifers characterized by the presence of peat and humic layers, such as in VEN, lower LOM and EMR [85, 86]. [PERSON] et al. (2013) [87] has demonstrated that in the phreatic coastal aquifers of the EMR, NH4+ is the prevalent nitrogen inorganic species in groundwater, and its concentration increases with depth and salinity. Very high NH4+ concentrations are found in coincidence with peaty sediments of salinized anoxic aquifers and in the low-lying aquitard and are not related to anthropogenic sources. In particular, the elevated NH4+ concentration derives from mineralization of organic matter present in fine sediments deposited in paleo-marsh environments, as supported by the significant correlation observed between HCO3- and NH4+ in the confined aquifers (Figure 7). It is difficult to certainly establish if water samples exceeding the NH4+ threshold in Figure 6 are due exclusively to anthropic impacts; it is more correct to affirm that farming activities cause a local intensification of natural NH4+ background. In confined aquifers, NH4+ concentrations higher than 0.5 mg/L are found downstream of the spring line (_fontanili_), limited to the EMR plain and the lower VEN plain (Figure 6b); in both cases the natural origin is established [85, 86]. Moreover, Fe and Mn distributions significantly differ between phreatic and confined aquifers (Figure 8 and Figure S2 in the Supplementary Material). Dissolved Fe and Mn concentrations in water are strongly influenced by redox conditions; Fe2+ and Mn2+ are more soluble in acid or reducing conditions, so higher concentrations are found in reducing groundwater, typical of deep aquifer; while are precipitated rapidly with increasing pH and Eh, forming (hydro)oxides and decreasing Fe and Mn concentration in water. The amount of dissolved Fe also depends on the presence of complexing agents in solution, such as Cl-, F-, SO42-, PO43-, and organic matter; while concentration of carbonate, bicarbonate and sulphate ions affect dissolved Mn due to the formation of complexes.