Reactive and Mixing Processes Governing Ammonium and Nitrate Coexistence in a Polluted Coastal Aquifer

A comprehensive hydrochemical, stable isotope and microbial analyses characterisation has been performed to evaluate the sources of groundwater, nitrogen pollution and degradation processes occurring in an industrial polluted coastal aquifer in the framework of a complex hydrodynamic system. The coexistence of ammonium and nitrate has been observed in almost all the investigated monitoring wells, reaching maximum values of 100 and 200 mg/L for both species. Chloride and potassium concentration coupled with groundwater stable isotopes data show the influence of local and urban recharge and the occurrence of seawater intrusion in areas near the coastline. N-15-NH4+ values ranging between -4.9 and +14.9% suggest that different processes such as partial nitrification of ammonium, probably anammox activities and sorption, are occurring at the site. The isotope data for NH4+ also showed the existence of the remnant of an old fertilizer plume in the downgradient area. The nitrate isotope data ranging between +9 and +46% and +6 and +26% for N-15-NO3- and O-18-NO3-, respectively, suggest that nitrate content is attenuated by denitrification and probably annamox. The fast groundwater flow field is one of the reasons for the coexistence of NH4+ and NO3- in groundwater, since both compounds can penetrate the reducing zone of the aquifer. The influence of leakage of sewage pipelines on the aquifer cannot be discerned due to the complexities of the nitrogen attenuation processes, also influenced by pumping activities.