Fe isotopic ratios in selected groundwater samples from the Ravenna and Friuli coastal plains, NE Italy and significance for the environment

High Fe concentrations (from 1.34 to 12.72 mg/L) were measured by Pezzetta et al.(2011) in groundwater samples from the Lower Friulan Plain near the north Adriatic seacoast. The research has unveiled a positive correlation between Fe concentrations and groundwater salinization (0.9–19679 mg/L Cl) and a particularly wide range of d56Fe (from –5.3 to +2.15‰ relative to IRMM-14 international standard, Castorina et al., 2013), compared with literature data. Furthermore, a positive correlation between d56Fe and Fe concentrations has been found. The present study has investigated the relationship of Fe isotopic ratios to Fe and chloride concentrations in groundwater samples from the Ravenna Plain near the north Adriatic seacoast aimed at comparing the results with those obtained from the Friulan Plain area. Therefore, the following characteristics have been recognized: a) Ravenna groundwater displays a smaller isotopic range (d56Fe from –2.6 to 0‰) than Friulan groundwater, b) unlike the positive correlation in Friulan groundwater, no correlation exists between d56 Fe and Fe concentrations (<0.02–3.13 mg/L) in Ravenna groundwater, c) there is a positive correlation between d56Fe and pH in Ravenna groundwater while there are negative or nil correlations in Friulian groundwater, and d) there is a negative correlation between d56Fe and chloride concentrations(55–16,800 mg/L) in Ravenna groundwater, while there are positive or nil correlations in Friulian groundwater. These different characteristics of groundwater from the two coastal Plains likely reflect different processes taking place in the two sites despite the broadly similar environments. The particularly isotopic lighter Fe compositions of most of groundwater samples from the Friulan Plain have been explained by isotopic fractionation during repeated cycling of Fe precipitation. This multi-staged process apparently does not significantly operate in the Ravenna Plain, where groundwater generally contains much lower Fe concentrations, suggesting that redox conditions arecomparatively less exasperated. The low Fe concentrations may also explain the lack of any correlation with d56Fe as well as the positive correlation between d56Fe and pH in groundwater from the Ravenna Plain. Lastly, the negative correlation between d56Fe and Cl in Ravenna groundwater indicates that with increasing salinization, there is likely precipitation of 56 Fe-rich ferrihydrite, thus leaving groundwater 56Fe-poor. In contrast, in lower-salinity waters, probably biologically-mediated reduction of Fe(III) to Fe(II) in the 56 Fe-rich ferrihydrite present in sediments, can release 56 Fe-rich Fe(II) to groundwater, that thus acquires the 56 Fe-rich signature. Lastly, as the Friulan Plain was significantly disseminated of Fe wastes from smelter plants in the past, the more positive d56Fe values measured in those groundwater samples were explained by waste leaching (Castorina et al., 2013). As Fe industrial pollution does not occur in the Ravenna Plain, where however, isotopic values nearing those positive of the Friulan Plain, have also been measured, this may render the above explanation not fully plausible, suggesting that the more positive d56Fe measured in the Friulan Plain can be due mainly to more extreme natural causes. References 1. E. Pezzetta, A. Lutman, I. Martinuzzi, C. Viola, G. Bernardis and V. Fuccaro, Iron concentrations in selected groundwater samples from the lower Friulian Plain, northeast Italy: Importance of salinity. Environ. Earth Sci., 62, pp. 377–391, 2011. 2. F. Castorina, R. Petrini, A. Galic, F.F. Slejko, U. Aviani, E. Pezzetta and G. Cavazzini, The fate of iron in waters from a coastal environment impacted by metallurgical industry in northern Italy: Hydrochemistry and Fe-isotopes. Applied Geochemistry, 2013 (in press).