Sea, agriculture and water quality: the case study of Vallevecchia (Caorle, Venice, Italy).

Recently, several droughts have negatively affected agricultural activities on the Padana plain (Italy), causing scarcity of irrigation water and both extended (up to tens of kilometres) and prolonged saltwater intrusion. The wetland system of Vallevecchia (Caorle, Venice) is based on a 700 hectares-wide drainage basin reclaimed during the sixties on the sand littoral zone of the North-Western Adriatic Sea.The system, by means of several hydraulic works allowing the inside recirculation of rainwater and drain water, has multiple pioneering goals: limiting saltwater intrusion, enhancing irrigation water availability during droughts, and reducing the diffuse pollution from agriculture (i.e. the nutrient loads discharged into the eutrophic Adriatic Sea) through phytoremediation. We have intensely monitored the water quality of Vallevecchia since 2003. Ever since, the system was sensibly altered by the construction (ended in 2005) of two hydraulic infrastructures in order to increase its efficiency: a check-gate to control the outflow from the system and a basin for water storage. The traditional functioning scheme of the system was based on pumping the water collected by the Sbregavalle channel into the Canadare channel, by the Vallevecchia pumping station. In order to allow water recirculation inside the basin, the abovementioned deep structural modifications were realized: the water storage basin, characterized by an internal volume of about 200000 m3, was created by excavation works and construction of surrounding levees. A check-gate was built in the Sbregavalle channel, to allow water retention inside the channel itself, limiting water expulsion through the Vallevecchia pumping station. In this way, water can be retained into the channel and be pumped inside the storage basin by a new pumping station which was appositely designed for this purpose. This pumping station contains three submersible pumps, and is well integrated in the local environment thanks to its limited size. The storage basin is linked to the Vallo channel, which flows along the sea border of the Vallevecchia basin, by a pipe line in which gravity flow realizes. A tank interposed between the pipe line and the Vallo channel allows flow regulation. Water can be distributed in the area between the Sbregavalle and the Vallo channels through some regulating gates which allow water flowing inside first order and second order channels. The following parameters have been monitored: ammonia nitrogen N-NH4, oxidate nitrogen NNOX, dissolved organic nitrogen D.O.N., particulate nitrogen P.N., total nitrogen T.N., orthophosphate phosphorus P-PO4, soluble unreactive phosphorus S.U.P., particulate phosphorus P.P., total phosphorus T.P. and total suspended solids T.S.S. The choice of these chemical parameters has been made because they are of maximum interest to control the pollution in drainage basins and in lagoons of the northern Adriatic area. The long-term monitoring dataset, including nutrient concentrations, water flows and environmental data, has consequently allowed the evaluation of the system performances both under varying environmental conditions and different system configurations. Monitoring data analysis shows that the two new hydraulic infrastructures have led to a markedly increased within-system abatement of nutrients from 2006 to 2009 as compared to the 2003-2005 period. Strikingly, the reduction of discharged nutrient loads has been achieved with only a minimal management effort, e.g. each year the water storage basin was filled once. Consequently, our results indicate that the check-gate plays a fundamental role in increasing the system efficiency, and suggest that starting a simple, routine management (e.g. of the check-gate position and of water recirculation, to achieve a better exploitation of the wetland) would make the system functioning remarkably more efficient.