For several hundred years, farming in the Po Plain of Italy (46,000 km2, 20 million inhabitants) has been supported by intensive surface irrigation with lake and river water. Despite the longevity of irrigation, its effects on the quality and quantity of groundwater is poorly known and so is investigated here through seasonal measurements of hydraulic heads and water quality in groundwaters, rivers, lake, springs and rainwaters. In the north of the study region, an unconfined coarse-grained alluvial aquifer, infiltration of surface irrigation water, sourced from the Oglio River and low in NO3, contributes much to aquifer recharge (up to 88%, as evidenced by a δ2H-Cl/Br mixing model) and has positive effects on groundwater quality by diluting high concentrations of NO3 (decrease by 17% between June and September). This recharge also helps to maintain numerous local springs that form important local micro-environments. Any increase in water-use efficiency in irrigation will reduce this recharge, imperil the spring environments, and lessen the dilution of NO3 leading to increasing NO3 concentrations in groundwater. These findings can be extended by analogy to the entire Po Plain region and other surface-water-irrigated systems worldwide where inefficient irrigation methods are used and similar hydrogeological features occur.