Delayed nitrate dispersion within a coastal aquifer provides constraints on land-use evolution and nitrate contamination in the past
Résumé
Identifying sources of anthropogenic pollution, and assessing the fate and residence time of pollutants in aquifersis important for themanagement of groundwater resources, and the ecological health of groundwater dependentecosystems. 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, combininggeochemical and environmental isotopic data (δ18O-H2O, δ2H-H2O, 3H, δ15N-NO3−, δ18O-NO3−, δ11B), and groundwaterresidence-time tracers (3H and CFCs)was carried out in 2016, and integratedwith a study of land use evolutionin the catchment during the last century. Groundwater NO3− concentrations, ranged between 2 mg/L andup to 30 mg/L, displaying the degradation of groundwater quality induced by anthropogenic activities (agriculturalactivities). Comparatively high δ15N-NO3− values (up to 19.7‰) in combination with δ11B values thatwere 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 urbanizationdevelopment all over the alluvial plain,with numerous sewage leakages fromthe sanitation network and privatesewage systems. Integration of contaminant andwater-residence time data revealed a progressive accumulationof pollutants with timein the groundwater, particularly in areas with major anthropogenic pressure and slowdynamicgroundwater 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 NO3− source, from agricultural tosewage contamination. Yet, today's low groundwater quality is to large parts due to legacy pollution fromland-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.