Delayed nitrate dispersion within a coastal aquifer provides constraints on land-use evolution and nitrate contamination in the past.

Identifying sources of anthropogenic pollution, and assessing the fate and residence time of pollutants in aquifers is important for the management of groundwater resources, and the ecological health of groundwater dependent ecosystems. This study investigates anthropogenic contamination in the shallow alluvial aquifer of the Marana-Casinca, hydraulically connected to the Biguglia lagoon (Corsica, France). A multi-tracer approach, combining geochemical and environmental isotopic data (δO-HO, δH-HO, H, δN-NO, δO-NO, δB), and groundwater residence-time tracers (H and CFCs) was carried out in 2016, and integrated with a study of land use evolution in the catchment during the last century. Groundwater NO concentrations, ranged between 2 mg/L and up to 30 mg/L, displaying the degradation of groundwater quality induced by anthropogenic activities (agricultural activities). Comparatively high δN-NO values (up to 19.7‰) in combination with δB values that were significantly lower (between 23‰ and 26‰) than the seawater background are indicative of sewage contamination. The ongoing deterioration of groundwater quality can be attributed to the uncontrolled urbanization development all over the alluvial plain, with numerous sewage leakages from the sanitation network and private sewage systems. Integration of contaminant and water-residence time data revealed a progressive accumulation of pollutants with time in the groundwater, particularly in areas with major anthropogenic pressure and slow dynamic groundwater flow. Our approach provides time-dependent insight into nitrogen pollution in the studied aquifer over the past decades, revealing a systematic change in the dominant NO source, from agricultural to sewage contamination. Yet, today's low groundwater quality is to large parts due to legacy pollution from land-use practices several decades ago, underlining the poor self-remediating capacity of this hydrosystem. Our results can be taken as warning that groundwater pollution that happened in the recent past, or today, may have dire impacts on the quality of groundwater-dependent ecosystems in the future.