The simulation of reactive transport of nitrate and other solutes becomes complicated when fractured and karstified media are concerned due to the large heterogeneity and complexity of water-rock interactions. We consider a carbonate aquifer (Upper Muschelkalk) in the catchment of river Ammer close to Tübingen (Germany), which was exposed to nitrate contamination for at least half a century. The fate of nitrate and its distribution in the aquifer is controlled by physical and chemical processes including redox-reactions and diffusion processes in the rock matrix. Since the contribution of individual processes to denitrification is not known, we test different modeling scenarios for their appropriateness to describe measured nitrate concentrations. A reactive transport model based on residence time as a scaling factor was developed to simulate long-term redox evolution in the micritic limestone aquifer. Iron-bearing minerals (siderite and pyrite) serve as major electron donors. The role of biotic and abiotic iron oxidations and type and content of iron-bearing minerals as electron-donors are studied in a series of modeling scenarios.
