Canal use purposes impact the water quality: a case study within the Life Green4Blue project floodplain area

Most of the floodplain ecosystems in the world have been reclaimed for allowing urbanization and agriculture. In reclaimed floodplains, water is addressed in artificial canals which could have several purposes such as irrigation, soil draining, hydraulic safety of the floodplain and source of biodiversity. In this context, the main aim of the present study was to evaluate the influence of artificial canal use (irrigation and receiving canals) crossing the Life Green4Blue project floodplain area on water quality. The study area is located within the Po plain (Italy) characterized by heavy reclamation activities for agricultural purposes in the last century. The irrigation canals, used for agricultural purposes, are fed during summer season (from April to September) by the Emiliano Romagnolo Canal which carries water from the Po River. The receiving canals, larger than irrigation canals, are mainly used as discharging canals for both irrigation and draining canal and to lesser extent for irrigation purposes. During the autumn and winter seasons (from October to March), both type of canals is used for hydraulic safety of the investigated floodplain area by keeping the water level of them low. The water survey was monthly conducted from the beginning of 2020 till December 2023. The cluster analysis (CA) showed a clear distinction between water of receiving canals and that from irrigation canals. According to the principal component analysis (PCA), the differences were mainly related to the amounts of nutrients and salts. In fact, water of receiving canals was characterized by higher amount of nutrients (e.g., N–NH₄, Ca, K, Mg, P and S) and higher values of electrical conductivity (EC). The poorer water quality of receiving canals can be attributed both to the water origin, namely soil leachates and water of irrigation canals that already flowed for several kilometres the agricultural land, and the absence of freshwater inflow. Therefore, the water quality index (WQI) showed higher value for the irrigation canals (67) compared to the receiving ones (61). For both canals’ type the PCA highlighted the worsening of water quality during the autumn and winter (AW) seasons. Indeed, during AW seasons a greater loading of nutrients and EC were observed compared to spring and summer (SS) seasons. The higher load of nutrients in AW compared to SS might be due to the higher nutrient leaching from soils resulting from the higher rainfalls occurring in AW seasons. In addition, the lower water flow during AW seasons prevented a ‘dilution effect’ and allowed a greater exchange of both cations and anions from the bed sediments. However, it was interesting to observe that the water quality worsening during the AW seasons was marked for irrigation canals compared to receiving ones suggesting the major role of freshwater input on water quality of such type of canals. The present study highlighted the importance of canal use on water quality. Specifically, in a view of a sustainable conservation of floodplain ecosystem services, this study showed the needing to ensure the input of freshwater in all canals’ type and throughout the year.