AMMONIUM NATURAL ATTENUATION IN COMPLEX HYDROGEOLOGICAL SETTINGS: INSIGHTS FROM A MULTI-ISOTOPE APPROACH

The main objective of this thesis is to investigate the natural attenuation processes of nitrogen compounds at different hydrogeological and environmental settings, with special regards to dissolved ammonium. To achieve this purpose, a multi-isotopic approach coupled with depth-dependent groundwater sampling procedures is adopted. Three field sites were selected representative of three hydrogeological settings same as different environmental issues in relation to nitrate and ammonium pollution: the Po River floodplain, Italy (PR), an alluvial coastal aquifer affected by abnormal natural ammonium concentrations along with a high content of other undesired compounds; a septic system in the Killarney Provincial Park, Canada (KP), where the discharging effluent composition into a shallow aquifer is dominantly ammonium-based; and the Sagittario River Basin, Italy (SR), a Groundwater Dependent Ecosystem (GDE) where river waters are impacted by upwelling of anthropogenic and natural ammonium which negatively affect the ecosystem's functionality. In the PR site a validation of the aquifer salinization processes results useful to gain insight into recharge rates and groundwater flow patterns. Regarding ammonium, major pool derives from the lowlying aquitard as a biogenic source while anthropogenic minor inputs (NH4+ and NO3-) from the shallow circulation in the aquifer were also identified. NH4 + resulted attenuated by transport processes and partial nitrification processes. Stable isotopes gave insights into attenuation processes as well as into other compounds present at high contents such as SO4 2- and methane. Unexpectedly, strong evidence of sulphate reduction coupled with methane anaerobic oxidation were shown. In this case study, it was postulated how the reminiscent effect of the paleo-marine environment still has a pivotal role in nitrogen (N), sulphur (S), and carbon (C) biogeochemical cycle’s evolution. In KP field site, a detailed characterization of the septic system plume showed 60-80% of the total N removal at distal areas from the source. Measured concentrations and isotopic data confirmed the reactive loss of both ammonium and nitrate along the flowpath. Several attenuation processes are involved: in the uppermost zone ammonium is oxidized to nitrate, whereas in the anoxic main body of the plume processes such as anaerobic oxidation (anammox) play a major role. Clear isotopic evidence of anammox reaction together with denitrification and nitrification processes were provided. In this case study it was demonstrated that anammox is naturally active in groundwater, even at low pH and in rich organic carbon environments. Hydrogeological investigations in the SR constrained elevated connectivity between the River and the multilayer aquifer within the basin. Chemical and isotopic data showed that both ammonium and nitrate, coming mainly from agricultural practices, affect the deep and the shallow flow systems, respectively. Ammonium is locally attenuated by dilution and nitrification, whereas denitrification is negligible in nitrate attenuation. Additionally, in correspondence with the gaining stretches of the Sagittario River, it was demonstrated how the hyporheic zone can receive ammonium-enriched water upwelling from the deep aquifer through the high permeability layers. This is causing a severe decline on the biological diversity, as demonstrated by the microbiological indicators. Results from this study highlighted the impact of the multiple stressors such as nitrogen pollution within the basin. This indicates severe risks for the GDE functionality. The isotope and geochemical patterns at the three investigated sites showed the complexity of the biogeochemical processes involved in the attenuation of nitrogen in the subsurface, which can only be untangled using a complete suite of hydrogeochemical analyses coupled with high resolution multilevel sampling techniques. This study showed the advantage of stable isotopes as a tool for tracing origin and attenuation processes in the N cycle, especially in complex hydrogeological setting where geochemical reactions tend to overlap.