A multi-isotopic study to identify processes affecting nitrogen and sulfur in an anthropogenic impacted alluvial coastal aquifer

The integrated use of hydrogeologic, geochemical and isotopic approaches has been adopted in a coastal aquifer (Comacchio, Fe, Italy) for fingerprinting NH4 +, NO3- and SO4 2- sources and processes that affect these compounds in groundwater. The study area, recently reclaimed, is characterized by a flat topography mainly below the sea level. A drainage system maintains the lowland dry, causing a vertical head gradient. The floodplain is interested by intensive agriculture, saltwater intrusion, land subsidence and soil salinization. Water isotopes highlight the mixing between freshwater and saltwater, confirming that the hyper salinity detected in the phreatic aquifer’s deep part (ADP) derives from evaporated palaeo-seawater. According to the dominant reducing environment and to the groundwater concentration profiles, the most abundant form of N is NH4 +. The highest NH4 + concentrations, which increase with depth, have been detected in the ADP. NO3- is present at low concentrations in the aquifer’s shallow part (ASP). Nitrogen isotopes signals identify natural and anthropogenic sources. The NH4 + natural source is identified within the mineralization of organic-rich fine sediments. Redox conditions lead both nitrification and ammonification. The vertical head gradient promotes the upward migration of NH4 + where the oxidised groundwater supports nitrification. In addition, an anthropogenic NO3- source is related to fertilizer leaching. The positive correlation between 15N and 18O- NO3-, in the ASP, agrees with the occurrence of heterotrophic denitrification. Water fluctuation and recharge regulate SO4 2- concentrations: pyrite oxidation in the ASP and SO4 2- reduction in the ADP take place. Moreover, pyrite oxidation within dissolved organic carbon enhances NO3- reduction by denitrification. Stable isotopes coupled with high resolution multilevel sampling, represent an additional tool that can provide unique insights into nitrogen and sulfur cycle in groundwater studies especially to untangle multiple processes co-occurring within a coastal aquifer.