The fate of iron in waters from a coastal environment impacted by metallurgical industry in Northern Italy: hydrochemistry and Fe-isotopes

• Coastal aquifers inland the northern Adriatic Sea (Italy) are characterized by high iron concentrations.
• Interactions between marine water and freshwater occur at different extent.
• Salinity gradients and redox zoning contribute in driving iron cycling.
• Iron isotopes traces Fe-oxide precipitation–dissolution reactions and possible sources.
• The iron cycle has implications for the fate of potentially harmful elements.

The source and cycling of Fe in groundwaters from the phreatic aquifer which characterizes a site impacted by metallurgical activity in a coastal area of northern Italy (Adriatic Sea) have been investigated by Fe-isotope analyses, H–O–Sr stable-isotope systematics and major and trace element chemistry. Waters are characterized by circum-neutral to alkaline pH and dissolved O2 zoning, and range from a low-salinity Ca–Mg–HCO3 type to a brackish Na–Cl type resulting from seawater intrusion. The O–H isotopic data indicate that the Ca–Mg–HCO3 waters originate from meteoric precipitation that infiltrated at elevated altitudes, and that a variable seawater fraction, in some cases exceeding 90%, characterizes the Na–Cl type waters. The Fe content ranges from 0.48 to 9.99 mg/L and from 2.50 to 43.8 mg/L in low-salinity and brackish waters, respectively. The δ56Fe value varies over the wide range from +0.87‰ to −5.29‰ in low-salinity waters and between +2.15‰ and −2.34‰ in brackish waters. The isotopically lighter compositions are interpreted as reflecting isotopic fractionation during repeated cycling of Fe precipitation. Positive δ56Fe values might indicate either a higher solubility of oxyhydroxides, which during diagenesis preferentially incorporated the isotopically heavier fraction of Fe, or the leaching of the foundry landfill disposal which characterizes the site.