Salt migration and export via subsurface irrigation in a saline reclaimed landscape of the Po River lowland (Italy)

• Continuous monitoring and vertical profiling revealed upward seepage to ditches. • Continuous monitoring allows salt export calculation from reclaimed saline soils. • Frequency domain analysis linked salt spikes to daily plant transpiration. • Upward flux of saline groundwater drives surface water salinity. • Sub-irrigation triggers salt flush with +80 % respect to non-irrigated periods. Upward saline groundwater seepage is provoking surface water canals to be salinized in a large, reclaimed area of the Po River lowland in Italy. The salinization is often exacerbated in agricultural fields sub-irrigated via tile drains, which create shortcuts among surface water and shallow groundwater bodies. To identify the causes of salinization and quantify the key sources of dissolved salts, a continuous monitoring of the vadose zone, surface water, and groundwater seepage contributions was established in two adjacent agricultural fields: A1 (≈5 ha) mildly saline cultivated with maize and crossed by a paleochannel and A2 (≈2 ha) saline, left uncultivated and covered by a salt tolerant weeds consociation with the prevalence of Genus Erigeron (Asteraceae). The vadose zone continuous monitoring allowed to identify capillary rise as the main driver of soil salinity in A1, while in A2 the capillary rise of groundwater is complicated by additional salt released by root decomposition after mowing, producing a clear EC pw increases up to 20 mS/cm. This information was integrated with remote sensing data on vegetation health (SAVI) and water requirement (NDMI), accounting for the vegetation activity within the field. Piezometers and drainage ditch continuous monitoring allowed for the identification of the surface waters/groundwater relationships and the saline sources in the aquifer/aquitard lenses, revealing a general GW flow toward surface drains as the main source of salinization. EC abruptly increased to 6.5 mS/cm along the SW2 ditch draining A2, while it rose more gradually from 0.55 to 2 mS/cm in the SW1 ditch draining A1. Frequency domain analysis further highlighted internal salinity dynamics, such as increased porewater salinity after mowing in A2, which increased temporary the electrical conductivity up to 20 mS/cm also confirming strong correlations between VWC and EC pw at 60 cm below ground level. Finally, continuous monitoring of water discharge and solute concentrations in the drained ditch enabled the assessment of total salt export during the sub-irrigation period, which was 67.2 ± 6.5 tons (approximately 80 % of the monitoring period). These outcomes show how sub-irrigation can accelerate salinization in shallow groundwater systems under saline conditions.