Hydrogeology and hydrogeochemistry of As, Fe, Mn rich groundwater of the multi-layer aquifer in the lower Po Plain, Lombardy region (northern Italy)

The present study concerns the analysis of the hydrogeology and the hydrogeochemistry of the As, Fe, Mn rich groundwater of the alluvial multi-layer aquifer in the lower Po Plain (northern Italy), referring specifically to the territory of Cremona. The main aim is to understand the origins (natural or anthropic) and mechanisms of the high groundwater As, Fe and Mn concentrations. The specific study area is located near the confluence between Adda and Po rivers. It covers a 50 km2 wide area around the urban territory of Cremona. The considered aquifer depths are around 200-250 m. The applied methodology involves the (a) collection of historical data related to water quality, water levels and well logs; (b) storage of collected data in specific databases and geographical information systems; (c) design and execution of two field surveys of water levels and water quality, realized in July 2010 and July 2012; (d) construction of a 3D model of aquifer hydrogeological properties (deposits texture, hydraulic conductivity and effective porosity), built by means of ordinary kriging interpolation of numerical values derived from the coding of well logs; (e) analysis of the hydrodynamic properties of the system; (f) analysis of water quality data (both field and historical data) considering the hydrogeological and hydrodynamic properties of the aquifer system; (g) implementation of a 1D reactive transport model in order to better understand the hydrogeochemical mechanisms in the system; (h) elaboration of a general hydrogeochemical conceptual model concerning possible origins and chemical mechanisms for the high groundwater As, Fe, Mn and NH4 concentrations, considering also possible anthropogenic influences; (i) development of management tools, as natural background levels derivation, supporting groundwater resources protection by public authorities. The 3D aquifer modelling underlines the presence of an alternation of sandy layers (K = 10-3-10-5 m/s) and silty-clayey layers (K = 10-7-10-8 m/s) with significant peat deposits and leads to the identification of 5 aquifer units. In relation to the hydrodynamic properties analysis, these 5 units are classified as (1) phreatic (F), from 0 to 25 m b.s., (2) semi-confined (S) from 30 to 50 m b.s., (3) confined 1 (C1) from 65 to 85 m b.s., (4) confined 2 (C2) from 100 to 150 m b.s. and (5) confined 3 (C3) from 160 to 250 m b.s.. The aquifer F can locally have semi-confined characteristic due to the presence of superficial silty-clayey deposits, while in the other zones it remains phreatic. The analysis of field and historical data of water quality underlines the general presence of reduced hydrochemical facies, characterized by high concentration of As, Fe, Mn and NH4, with the exception of the zones with phreatic conditions in aquifer F, where oxidized facies are identified. In particular, the survey of July 2010 points out high As concentrations (ranging from 1 to 180 μg/L), especially in the 30-100 m depth range, corresponding to aquifers S and C1. High concentrations of Fe and Mn are also found, they range from 100 to 6000 μg/L and from 10 to 1200 μg/L, respectively. The higher concentrations are found in superficial aquifers: in aquifer F for Mn and in aquifers F and S for Fe. NH4 is also found with high concentrations (1-5 mg/L, up to 18.9 mg/L) from aquifer S to C3. The measurements of July 2010 can represent the natural background of As, Fe, Mn and NH4 because no direct sources or indirect anthropogenic influences are found. The survey of July 2012 generally confirms the hydrochemical characterization based on July 2010 data. In the analysis of the hydrochemical historical data, a separation between the data referred to the natural background and to anthropogenic influences is done. Pollutions by hydrocarbons and organic matter in general can be considered as indirect human influences on As, Fe, Mn and NH4 concentrations, as reported by previous studies. The analysis of historical chemical data referred to the natural background generally confirms the hydrochemical characterization emerged from the data of July 2010 survey. The analysis of historical data also points out a probable anthropogenic influence on As, Fe, Mn and NH4 concentrations in two sites located in the study area: an oil refinery, affected by hydrocarbons pollution, and a municipal solid waste landfill, with probable organic leachate spills. In order to understand the origin and the mechanisms of the high As, Fe, Mn and NH4 concentrations, a hydrogeochemical conceptual model is implemented. The conceptual model considers the process of natural organic matter degradation (i.e. peat) as primary control factor on high As, Fe, Mn and NH4 concentrations. Degradation of peat is associated with the consecutive reduction of O2, NO3-, Mn(IV), Fe(III), SO42-, CO2. The reductive dissolution of Mn and Fe oxides (contained in the aquifer sediments) leads to high concentrations of dissolved Fe and Mn, but also to high concentrations of dissolved As, which is generally sorbed on Mn and Fe oxides. Dissolved As concentration can be also lowered by different processes (co-precipitation of As in iron sulfides, precipitation of arsenic sulfides, sorption of As on the remaining Fe-oxides and Mn-oxides, etc.). NH4 is released from the degradation of organic nitrogen of peat. Therefore, a natural origin of As, Fe, Mn and NH4 can be assumed. In order to understand if the hydrogeochemical conceptual model, based on literature, could be reliable on the present case study, a 1D reactive transport model, using PHREEQC code, is implemented. The modelled concentrations result in good agreement with the measured concentrations (July 2010). This result could support the validity of the conceptual model. In addiction, isotope and microbiological analysis, executed in the survey of July 2012, confirms the natural origin of NH4 and the occurring of Fe-oxide and sulfate reduction in the studied system. In conclusion, the present work can contribute to understand origins and mechanisms of high groundwater As, Fe, Mn and NH4 concentrations in the lower Po Plain, supporting the management and protection of groundwater resources by public authorities.