Surface-water-irrigation return flow dominates groundwater recharge, groundwater age and nitrate dynamics in an alluvial basin aquifer

In agricultural regions where crop water demand is met by surface-water irrigation, excess irrigation water can play a fundamental role in aquifer recharge and contaminant mobilization. Despite much research, the role of surface-water-irrigation return flow in groundwater recharge, flow processes and nitrate pollution is not yet fully understood. It is therefore addressed here using the case of the Po Plain in northern Italy. Tracers of groundwater recharge (stable isotopes in water and Cl/Br ratio), age tracers (CFCs, SF6, 3H-3He, noble gases) and long-term average nitrate concentrations measured/calculated from a 64-point monitoring network revealed the dominant role of surface-water-irrigation return flow in recharging the aquifer (median contribution of 64.4 % to total recharge in irrigated areas), with effects even on groundwater apparent ages and nitrate concentrations. Regional groundwater flow from north to south gradually mixes with a vertical inflow of younger water from excess irrigation, inducing a renewal effect that rejuvenates groundwater ages and even reverses the typical increasing age trend along a flowpath. The median apparent age of 39 years in areas with no or non-intensive irrigation (i.e., recharge by surface-water-irrigation return flow <60 %) decreases to 27 years where intensive irrigation (recharge by surface-water-irrigation return flow >60 %) is practiced. At the same time, groundwater nitrate concentration increases progressively with increasing recharge by surface-water-irrigation return flow up to 60 %. Then it decreases in intensively irrigated areas with a decrease rate of 7.2 mg/L of nitrate for a 10 % increase in recharge by surface-water-irrigation return flow. This decrease in nitrate is due to dilution and flushing promoted by the greater volume of water infiltrating the soil under intensive irrigation. However, under non-intensive irrigation, nitrate leaching from fertilized soils still dominates and causes an increase in groundwater nitrate concentration. These results show that groundwater nitrate concentration is mainly controlled by hydrogeological processes such as dilution and flushing on a short-time scale rather than by reduction of nitrate sources, which can certainly abate nitrate concentrations in groundwater but on a long-time scale. These findings could serve as a valuable reference for other surface-water-irrigated areas worldwide with similar hydrogeological settings, highlighting critical aspects for sustainable management of water resources.