Groundwater and surface water nitrate pollution in an intensively irrigated system: Sources, dynamics and adaptation to climate change

Freshwater pollution by nitrate is a major threat to human and ecosystem health. Basin-scale studies on nitrate pollution generally focus separately on surface water or on groundwater bodies, thus the role played by their interaction on nitrate concentrations, possibly including also agricultural irrigation, is often overlooked and so is addressed here in the intensively irrigated hydro-system of the Oglio River basin (Northern Italy). Tracers of groundwater recharge (stable water isotopes and Cl/Br ratio) together with nitrate and boron stable isotopes indicate that the main source of the diffuse nitrate pollution affecting groundwater resources in the area is related to agricultural activities and, locally, to untreated civil/industrial effluents. Moreover, these data reveal the strong control of irrigation return flow on groundwater nitrate concentrations, with contrasting effects: groundwater-fed irrigation promotes higher concentrations due to the recirculation of high-NO3 groundwater, whereas intensive surface-water-irrigation, fed by low-NO3 river water, generates lower concentrations due to dilution. The control of irrigation return flow on groundwater nitrate links nitrate pollution and climate change: if surface-water-irrigation will be abandoned, as a consequence of intensified summer droughts, in favor of groundwater-fed irrigation, an increase in groundwater nitrates is expected due to a basin-scale groundwater recirculation and the cessation of the dilution effect. In addition to the reduction of the N input to soils from fertilizers, i.e., the sole pollution mitigation strategy able to solve the problem, an adaptation strategy to climate change might be the implementation, during non-irrigation rainy periods, of managed aquifer recharge operations, such as the so-called “forested infiltration areas”, to take advantage of water abundancy from rivers under high-flow conditions, thus combining a supplement on groundwater recharge with the beneficial dilution effect on dissolved nitrate.