Upward saline groundwater (GW) seepage is provoking surface water (SW) canals to be salinized in a large, reclaimed area of Po River lowland (Italy), particularly in tile drained agricultural fields embodying shortcuts among SW and shallow GW bodies. To identify salinization causes, a continuous monitoring network of SW channels, saturated and vadose zone (VZ) was established in two adjacent agricultural fields: the A1 field plot, mildly saline cultivated with maize and crossed by a paleochannel; and the A2 field plot, saline, uncultivated and covered by salt tolerant weeds.
The VZ continuous monitoring allowed to identify capillary rise as major driver of soil salinity in A1 field; in A2 additional salt is released by roots decomposition after the mow. These findings were integrated with remote sensing data on vegetation health (SAVI) and water requirement (NDMI).
Piezometers and drainage ditch continuous monitoring allowed to identify SW-GW relationships and saline sources in the aquifer/aquitard lenses.
Frequency domain analysis highlighted internal salinity dynamics, such as increased porewater salinity after mowing salt tolerant vegetation that increased temporary the EC up to 20 mS/cm.
Finally, measurements of water discharge and salt concentration at the outlet enabled a reliable estimation of salt fluxes from tile drained agricultural fields, showing that 70% of the total annual salt export (21±1.9 t/ha/yr) occurred during sub irrigation periods.
The upward saline flux from GW, together with the presence of halophilic vegetation fragments that slowly release salts into porewater and help to maintain elevated concentrations in GW and SW, pose a serious threat for the SW resources that are used for irrigation in these reclaimed lands, especially considering the ongoing climatic change that are already stressing the Po River lowland.