Changes in Shallow Groundwater Recharge Due to Drought Impacting the Po River Basin

The Po River Basin in Northern Italy is facing its worst drought in the last decades. A fundamental aspect for water resources management is the understanding of drought effects on the hydrogeological water balance, that is addressed here for the shallow Po Plain aquifers in Lombardy region. The Po Plain alluvial aquifer system in Lombardy evolves from a mono-layer unconfined aquifer in the piedmont of the Alps (higher plain), made of sands and gravels, to a multi-layer system approaching the Po River (lower plain), where sands (confined aquifers) are intercalated with silts and clays (aquitards). The higher plain is the recharge area of the whole aquifer system. Land use is mainly agricultural. Water for irrigation is mainly provided by diversion of Alpine rivers and Subalpine lakes into an extensive network of irrigation channels. The traditional surface irrigation method is largely practiced. Analysis of tracers of groundwater recharge, i.e., δ2H-H2O, δ18O-H2O and Cl/Br, in 44 wells tapping the unconfined aquifer in the higher plain, distributed over the entire Lombardy and sampled in March 2022, revealed that surface water, and not local precipitation, is the main recharge of the shallow aquifer, with only a few exceptions. Binary mixing models calculated using δ2H-H2O and Cl/Br indicated that the contribution of surface waters to shallow aquifer recharge is generally between 50% and 75%, the remainder being from local precipitation. Analysis of groundwater level time series detailed that surface waters mainly recharge the aquifer through irrigation return flow. Due to drought, and surface water shortage during the growing season, two important changes are occurring: (1) the abandonment of surface-water-irrigation in favor of groundwater-fed irrigation and (2) the shifting from the inefficient surface irrigation method to more efficient sprinkler/micro/drip methods. The consequences on the hydrogeological water balance are: (1) the obliteration of the current first source of groundwater recharge and (2) the increase of groundwater abstraction, often uncontrolled, generated by the proliferation of irrigation wells. The effects are (1) the depletion of groundwater resources and (2) the drying of many typical low-land springs (fontanili). Mitigation actions are urgently needed. The managed aquifer recharge system named “forested infiltration area”, successfully experimented in Veneto region, could be a practicable strategy.