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Calculating natural background levels (NBLs) in groundwater is vital for supporting a sustainable use of groundwater resources. Although NBLs are often assessed through a unique concentration value per groundwater body, where hydrogeochemical features are highly variable, spatial heterogeneity needs to be accounted for, leading to the calculation of so-called “local” NBLs. Despite much research devoted to the identification of the best performing techniques for local NBLs spatialization, a deep understanding of the link between local NBL values and their generating hydrogeochemical processes is often lacking and so is addressed here for the redox-sensitive species As, NH4, Fe and Mn in the groundwater bodies of Lombardy region, N Italy. Local NBLs were calculated by a tired approach involving the hybridization of preselection and probability plot methods. Since the spatial variability of the target species depends mainly on redox conditions, a redox zonation was performed using multivariate statistical analysis. A conceptual model was developed and improved combing factor and cluster analysis. Results showed that NBLs for arsenic were up to 291 μg/L, reached in groundwaters under methanogenesis, a condition related to the prolonged degradation of peat buried in aquifer sediments. Ammonium NBLs up to 6.62 mg/L were generated by the upwelling of fluids from deep sediments hosting petroleum systems; ammonium NBLs up to 4.48 mg/L were generated as the accumulation of by-products of peat degradation. Iron and manganese NBLs up to, respectively, 6.0 and 1.51 mg/L were generated by the oxidation of younger and less stable Mn and Fe oxides within river valleys, mostly the Po River valley. The evaluation of local NBLs, and their association to generating natural hydrogeochemical processes/conditions, achieves a step forward from the commonly used approach of a single NBL per groundwater body, improving decision-support tools for sustainable groundwater management and protection.

Discretizing anthropogenic and natural contaminations represents a crucial step in groundwater management and regulation. Natural background levels (NBLs) have a huge impact on groundwater protections and remediation strategies, but it is still an issue on the ground in terms of reliability and accuracy, thus its derivation needs further scientific efforts.The derivation of local NBLs (LNBLs) is intended to overcome the limitation of considering a groundwater body (GWB) homogeneous, hence accounting hydrogeochemical heterogeneities within the aquifer system.This work presents a statistical approach assessing LNBLs for sensitive redox species (As, Fe, Mn, NH4) in 30 GWBs within the Lombardy Region. Under the monitoring network of the Regional Agency for Environmental Protection of Lombardy (ARPA), more than 500 wells were investigated, thus each GWBs were identified within 4 aquifer types: shallow, intermediate, deep Po Plain aquifers and Alpine valley aquifers. The initial dataset underwent preselection and multivariate analyses, appointing at each well a geogenic redox zonation. It leaded to discretize geochemically-homogeneous subgroups and characterize them as function of site-specific natural facies: oxidised (293 wells), reduced (199 wells) and saline (11 wells). Interquartile range criteria, validations’ tests (Mann-Kendall and Shapiro-Wilk), probability density histograms and probability plots inferred temporally and spatially the datasets, one for each target species, discretized for aquifer and natural facies appartenances. This resulted in the identification of the statistical distributions from redox-homogeneous sets of data from which the LNBLs were derived.Considering the Po Plain aquifer (shallow, intermediate and deep), NBLs derivation for As revealed three subgroups within the oxidised facies, for which the NBLs values are of 2, 3 and 7 μg/L, four subgroups ascribe to the reduced facies with NBLs of 13, 49, 71 and 291 μg/L, and two subgroups for the saline facies with NBLs of 3 and 12 μg/L. According Fe, two are the subgroups within the oxidised facies, with NBLs of 40 and 94 μg/L, four subgroups fall in the reduced facies with NBLs of 653, 1430, 3200 and 6000 μg/L; within the saline facies, two subgroups are identified with NBLs of 1647 and 6000 μg/L. Two subgroups characterize the oxidised facies for NBLs of Mn with values of 8 and 27 μg/L, and NBLs of 34, 216, 485, 912 and 1514 μg/L refer to five subgroups in reduced facies, while within the saline facies fall two subgroups with NBLs of 381 and 921 μg/L. With regards to NH4, NBLs reach values of 49, 110 and 190 μg/L for the three subgroups within the oxidised facies, whereas values of 834, 2600, 3090, 4480 μg/L are derived for the four subgroups in the reduced facies; the two subgroups ascribed to the saline facies reveal NBLs of 1860 and 6620 μg/L.Data demonstrate how an in depth understanding of aquifers’ redox-zonations turned out to be functional for assessing LNBLs. Regional Legislation (D.G.R. 23novembre2020 n.3903) has been amended on the basis of the outcomes of this work, revealing site redox-specific LNBLs of practical significance. Funding: this work was granted and carried out in collaboration with Lombardy Region.