POTENTIALLY TOXIC ELEMENT CYCLING AND CHARACTERIZATION OF MULTIPLE SOURCES IN THE IRRIGATION DITCHES FROM THE RAVENNA COASTAL PLAIN THROUGH TRACE ELEMENTS AND ISOTOPE GEOCHEMISTRY

Trace elements and O-H-Sr-B isotopic data have been obtained on superficial waters from a number of canals and ditches which serves the water demand for irrigation in the Ravenna coastal plain, in order to highlight the cycling of potentially toxic elements and the role of different sources to the solute. Two different surveys were conducted during March and July 2008, intended as representative of the water faces during winter and summer, respectively. In summer, the water mass balance in the network is mostly controlled by the ingression of freshwaters from the Canale Emiliano Romagnolo (CER). The O-H isotopic data indicate that, during winter, waters are primarily recharged from Apennine catchments and undergo evaporation at different extents. The boron isotopic signature indicates the important role played by the marine component. A major seawater contribution is evidenced in canals sampled in proximity to the coastline; however, the process controlling the origin of dissolved boron is not solely related to direct mixing with sea water but requires an additional source probably related to water-soil exchanges, leaching boron of marine origin due to the prolonged exposure to sea water of alluvial sediments. An additional boron contribution from the agricultural practice is also evidenced. Calculation based on the conservative behaviour of chloride indicates that in canals and ditches not directly connected with the sea up to the 80% of the Sr budget did not originated from seawater, indicating a source from Al-silicates mineral and supporting the hypothesis of significant soil-water interactions and chemical exchanges. The positive correlation between pH and dissolved oxygen in winter waters is likely to reflect CO2 consumption during algal photosynthesis, favoring the in-situ generation of colloidal particles due to the oxidative precipitation of ferric iron oxy-hydroxides and probably small carbonate particles, which are able to adsorb trace metals on the surface and contribute to the ecosystem dynamics facilitating the transport of metals and affecting their bio-availability and cycling. During summer, the input of freshwater from CER lowers the concentration of most ions and superimposes to the evaporation-precipitation-exchange cycle, possibly dispersing the colloidal particles which could act as pollutant carriers and enhancing flocculation and precipitation, scavenging trace metals into the sediments.