Nitrate migration in porous media: A 1-D Reactive Transport Model for optimizing heterotrophic denitrification in groundwater remediation

Nitrate is a significant contaminant in many aquifers due to the massive use of fertilizers in agriculture and wastewater discharge into streams. Globally, elevated nitrate levels in aquifers pose health concerns due to nitrate ingestion linked to diseases such as methemoglobinemia and blue-baby syndrome in children under six years of age. Furthermore, nitrates can potentially form N-nitroso compounds, known to cause carcinogens, after being transformed into nitrites in the esophagus. Consequently, the European Union and the World Health Organization have set a limit of 50 mg/L for NO₃⁻. Despite these regulations, global nitrate exposure persists, notably in Germany, where 27.1% of groundwater bodies exceed the established limit, indicating an increasing trend. To mitigate this situation, we propose implementing Permeable Reactive Barriers (PRBs) at a pilot site at the Fredersdorfer Weg area in northeastern Germany, about 21 km east of Berlin. The Fredersdorfer Weg is located within the Fredersdorfer Mühlenfließ catchment area, adjacent to the Münchenhofe Wastewater Treatment Plant. To date, Permeable Reactive Barriers (PRBs) have been one of the leading technologies for addressing the in-situ remediation of migrating contaminant plumes in groundwater. However, only a limited number of models simulate both advective-dispersive and reactive transport related to PRBS and microbial growth and decay, which leads to a lack of integration in the growth of microorganisms and changes in permeability because of biomass growth. To address this problem, we present a one-dimensional reactive transport model in MIN3P. Our model replicates the processes of nitrate degradation in heterotrophic conditions, employing different carbon sources. Processes in the model include advective-dispersive and reactive transport, attachment, and detachment of Microbial from the sediment, as well as allowing the dependence of reaction rates on biomass, dissolved organic carbon (DOC), and NO₃⁻ concentrations. To validate our model, we propose highly controlled soil column experiments under heterotrophic denitrification. The proposed experiments involve conducting tests under anaerobic conditions using plexiglass columns of 30 cm in length and 5 cm in diameter. Each soil column contained different carbon sources, such as wood chips, mulch, and soil/sawdust combinations, to determine their effectiveness as reactive materials for nitrate degradation. The bacterium Pseudomonas sp. was selected based on its denitrification capabilities, and four sampling ports were distributed along the columns to obtain better data resolution for DOC and NO3- concentrations. Through these experiments, we aimed to get a detailed and comprehensive dataset for validating kinetic model parameters, such as microbial growth yield, rate constants, and Monod half-saturation constants. Our main objective is to establish the ideal kinetic parameters for heterotrophic denitrification to minimize the migration of nitrates into groundwater through in-situ remediation strategies.