Hydrogeochemistry, rare earth elements and stable isotopes geochemistry of surface and ground-waters in the Ravenna coastal area

The geochemical and isotopic budgets associated with groundwater-seawater mixing and reaction processes in the coastal aquifers and surface waters bordering the Adriatic Sea shoreline of Ravenna were investigated. The area encompasses the site of European Community Importance of the Pineta San Vitale, the Pialassa Baiona lagoon and the inland network of canals and ditches used for irrigation purposes. The Pineta San Vitale is submitted to deterioration due to subsiding phenomenon, yielding groundwater salinization, and the whole area suffers for anthropogenic pressure related to an industrial site and to intensive agricultural practice. The water samples considered in this study consist of rain water, surface water, soil water and groundwater collected at station transects perpendicular to the coast line. In these waters, physical-chemical parameters, a large set of major, minor and trace components, and the O, H, Sr and B isotopic composition were determined. The soil and groundwater samples are near neutral to slightly alkaline (pH: 6.9 ÷ 8.2), likely reflecting the buffering effect of carbonate minerals widespread in sediments. pH values as high as 9.3 were determined in some of the ditches. The waters show variable redox potential (Eh: -0.12 ÷ 0.49 V), reflecting different hydrodynamic conditions and the occurrence of reductants such as dissolved organic matter. Some major (Na+, Mg2+, Cl-, SO42-) and minor (Sr, B, Li) components in soil water and groundwater generally show increasing concentrations from the West to the East, that is towards the Adriatic Sea, and appear mostly derived from a marine source. Isotopic relations between δ18O (-10.9 ‰ ÷ -0.69 ‰ vs. V-SMOW) and δD (-67.85 ‰ ÷ -5.84 ‰ vs. V-SMOW) in soil water and groundwater reveal the conservative mixing between a marine component and freshwater. Such mixing is also highlighted by the isotopic composition of Sr (87Sr/86Sr in the range 0.7086 ÷ 0.7094) and B (δ11B in the range +3‰ ÷ +48‰). However, deviations from conservative mixing are observed for both the isotopic systematic. Total rare earth element (ΣREE) concentrations vary from <0.1 to 190 μg/l with the highest values being observed in soil waters. The ΣREE does not correlate with salinity or marine derived components. ΣREE values increase with increasing Al and Fe concentrations in the 0.4 μm filtered fraction. REE patterns in the soil waters show an enrichment in the middle REE (MREE), likely due to preferential adsorption onto <0.4 μm particles, such as colloidal clay, iron and organic matter. Geochemical and biological processes at the interface water-soil appear to enhance the mobility of REE in the soil waters. Overall results in the Pineta San Vitale aquatic system indicate salinization and additional processes such as isotopic and ion exchanges between water, clay minerals and organic compounds. Minor shifts due to anthropogenic input are suggested by Sr-B isotopes and REE.