Ammonium natural attenuation in complex hydrogeological settings: insights from a multi-isotope approach

Abstract

A full methodology including a multi-isotopic approach coupled with depth-dependent groundwater sampling is here reported to investigate the occurrence of ammonium natural attenuation. Examples from three hydrogeological settings, representative of different environmental issues in relation to nitrate and ammonium pollution are here reported: a septic system plume (SSP), where the discharging effluent composition into a shallow aquifer is dominantly ammonium-based; a Groundwater Dependent Ecosystem (GDE), where river waters are impacted by upwelling of anthropogenic and natural ammonium negatively affecting the ecosystem’s functionality, and an alluvial coastal aquifer (ACA) affected by abnormal natural ammonium concentrations along with a high content of other undesired compounds. Several sampling methodologies were applied for depth-dependent groundwater sampling such as multi-level nested wells, straddle packers system, and common long screen wells. Such detailed sampling techniques were coupled with a multi-isotopic approach. Environmental isotopes (δ18O and δD in H2O) were used to gain insights into the hydrogeological flow system while other stable isotopes δ15NNH4, δ15NNO3, δ18ONO3 were purposefully applied for the identification of N compounds’ source(s), fate and processes. In addition, δ34SSO4, δ18OSO4, and δ13CDIC were proposed as tools for understanding the potential attenuation processes in relation to the other biogeochemical cycles. Tritium data was also applied for recharge rate and groundwater residence time estimation.