Nitrate contamination of groundwater remains a pressing environmental and public-health concern in regions affected by intensive agriculture and urbanization. This study investigates how temporally and spatially variable boundary conditions and groundwater regimes influence reactive nitrate transport, using the “Auf dem Grind” bank filtration well field (near City of Düsseldorf) as a conceptual case study. A one-dimensional reactive-transport framework was developed by coupling Phreeqc with a Python-based workflow implemented in Jupyter Notebook; this approach captures the essential processes of advection, dispersion and decay while avoiding the complexity of a fully coupled 3-D hydrodynamic model.
Model scenarios represent urban, agricultural and natural boundary conditions and include three groundwater-regime conditions (baseline, dry-season and flood) defined by combinations of Rhine stage and pumping intensity. The 1-D flow path is discretized into monthly travel-time cells; for each cell a scenario-specific Phreeqc input is autogenerated by the Python script, enabling iterative simulation of transport and biogeochemical kinetics. Key model inputs (recharge volumes, nitrate and DOC concentrations, hydraulic parameters) were compiled from monitoring data, regional studies and guideline values; sensitivity analysis identified the hydraulic conductivity of pond bottom sediments and DOC input per cell as a highly influential parameter.
Results indicate that denitrification is most probable in urban and pond zones, where persistent anoxic conditions and abundant organic substrates favor nitrate reduction. In pond-affected cells, two complementary mechanisms explain observed nitrate attenuation: (i) denitrification in oxygen-depleted bottom sediments reduces nitrate in pond water prior to recharge, and (ii) infiltration of organic-rich pond water to the sediment–aquifer interface can deliver labile carbon and microbial biomass that sustain subsurface denitrification. By contrast, agricultural sections show a slow net accumulation of nitrate: the modelled increase equals approximately 0.15 mg L⁻¹ yr⁻¹ under the agricultural scenario, consistent with the range reported for intensively farmed lands.
From a groundwater-management perspective, the study highlights practical mitigation options. Redistributing abstraction across multiple lower-yield wells (thereby reducing individual drawdown and increasing groundwater residence time) can enhance the aquifer’s intrinsic capacity for denitrification. Passive remediation measures, such as permeable reactive barriers or managed infiltration employing existing ponds, can replicate conditions that sustain denitrifying
communities without continuous operational input. Finally, the developed scripting workflow provides a simple, transparent tool for preliminary scenario analysis of nitrate reactive transport where detailed 3-D flow models are not available.