Unravelling pore-scale mechanisms of the nitrogen cycle in porous aquifers

• Grant number: 25/06742-6
• Support Opportunities: Scholarships in Brazil - Post-Doctoral
• Start date: July 01, 2025
• End date: June 30, 2027
• Field of knowledge: Physical Sciences and Mathematics - Geosciences - Geology
• Principal Investigator: Ricardo César Aoki Hirata
• Grantee: Daphne Silva Pino
• Host Institution: Instituto de Geociências (IGC). Universidade de São Paulo (USP). São Paulo , SP, Brazil
• Associated research grant: 20/15434-0 - SACRE: integrated water solutions for resilient cities, AP.TEM

Abstract

Nitrate contamination is almost ubiquitous in urban unconfined aquifers in the state of Sao Paulo, Brazil, representing a difficult situation for a state where more than 80% of its municipalities are supplied entirely or partially by groundwater. One of the current critical scientific challenges is establishing the conditions under which nitrogen species degrade in aquifers and the effective role of anthropogenic and biogeochemical mechanisms that regulate the extent of contamination plumes in the hydrological cycle. Reported cases vary, with nitrate contamination reaching deeper portions of the aquifer (>100m bgs), and in others, with well-established plumes at superficial depths or no deeper than 60 m bgs.Recently, advanced research groups have suggested integrating traditional hydrogeochemical techniques with new tools, such as multi-tracers (CFCs, SF6, 39Ar, 14C, 11B, 15N, 18ONO3, 87Sr/86Sr, and artificial sweeteners), microbiology (RNA 16S, narG, napA, nirS, nirK, norB, and nosZ genes) and synchrotron technology (X-ray microtomography resolved in time). The focus on the hydrobiogeochemical mechanisms happening at a pore scale shall yield valuable data on nitrogen contamination and hence support environmental management at field scale of the extensive contaminated urban aquifers. Although X-ray microtomography (µCT) has long been used in the oil industry for reservoir characterization, the application of synchrotron technology focused on groundwater is recent, particularly in the remediation process with metal nanoparticles in porous media. Regarding the nitrogen cycle, there are studies focused on its characterization in soils, however, only at a laboratory scale. The proposed study, therefore, would be one of the first to analyse such reactions in deep aquifer systems (30-60 m), where the gaps in knowledge about the geochemistry of nitrogen species reside.The goal of this project is to identify the potential hydrobiogeochemical characteristics that control the nitrification and/or denitrification process and determine the parameters controlling the production and emission of N2O concerning the nitrification and denitrification reactions in the Bauru Aquifer System at a pore scale. The study area encompasses a region that chronically endures water insecurity: the Batalha and Bauru river watersheds, part of the Tietê-Batalha and Tietê-Jacaré watersheds, thus constituting an exceptional "field laboratory". This study is an important and unprecedented opportunity for methodological implementation related to the investigation of aquifers at high resolution, enabled by synchrotron technology. Its specific objectives are: (i) the characterization of porous media of the aquifer system, at pore scale, by means of synchrotron source µCT; and (ii) the evaluation of groundwater flow dynamics by means of microscale 4D injection experiments. The results are expected to allow characterizing the pore structure of different hydrogeological units; identifying evidence on the presence of denitrifying and nitrifying microorganisms; providing insights into the hydrobiogeochemical processes of the nitrogen cycle; and defining procedures for hydrobiogeochemical studies at synchrotron facilities. (AU)