Modeling nitrate remediation in groundwater using CH₄/H₂ gas injections

Groundwater pollution from decades-old NO₃ inputs is still a problem throughout Germany and the rest of the EU, with concentrations in many aquifers exceeding the 50 mg/l threshold mandated by the EU groundwater and drinking water directives. This situation has remained largely unchanged since the early 2000’s and quality targets for groundwater set by the European Environment Agency until 2030 are projected to not be met (EEA, 2025). In the light of these nitrate pollution legacies in situ remediation schemes have some appeal. In the NitratLURCH project funded by the German Federal Ministry of Research, Technology and Space (BMFTR, FONA-LURCH) as part of a funding scheme on sustainable groundwater management, we investigate the in situ remediation of nitrate pollution in groundwater via stimulated denitrification using CH₄/H₂ gas injections. In a pilot study at a former drinking water well contaminated with nitrate, nested numerical groundwater flow and transport models, in conjunction with intensive geologic site characterization, are used to support the setup of a gas injection system and to evaluate its ability to reduce nitrate concentrations in the groundwater flowing to the well. Regional geologic and hydrogeologic data were compiled to build a MODFLOW subcatchment scale groundwater model surrounding the drinking water well. The local aquifer system consists of about 150m thick glacio-fluvial Quaternary and Tertiary deposits with high transmissivity shallow sands and gravel beds significantly affecting groundwater flux dynamics in uppermost 10 – 20 m. A complex hydrofacies architecture, revealed during site characterization, was implemented into evolving versions of the flow model and further refined for a local, inset reactive transport model (codes Min3P and PHT3D) for a 1600 m² area and 20 m deep section of the local aquifer. Conservative and reactive transport simulations and dozens of scenarios were realized to plan and operate an in situ CH₄/H₂ gas injection system, controlling and mitigating explosivity risks, optimizing reactant quantities and budgets, and evaluating reactions and turnover (e.g. incomplete denitrification leading to NO₂ generation, unreacted CH₄ etc.) in order to ensure legal compliance with the local water agency. Modeled optimal system performance under the local physical constraints and the assumption of maximum denitrification rates predicted a 30% reduction in nitrate concentrations in the pumped water (from about 50 to 35 mg/l) considering dilution from untreated water from parts of the complex groundwater flow system. Overall our modeling results suggest the viability of the tested remediation concept at the site.

EEA (2025). Nitrate in groundwater in Europe. Published 10 Nov 2025.