Traveltime-based modelling of nitrate reduction in a pyrite- and siderite-bearing fractured micritic limestone aquifer

Elena Petrova1, Evgenii Kortunov2, Miсhael Finkel1, K. Ulrich Mayer3, Peter Grathwohl1
1 Center for Applied Geosciences, University of Tübingen
2 Faculty of Geology, Department of Hydrogeology, Lomonosov Moscow State University
3 Department of Earth, Ocean and Atmospheric Sciences, University of British Columbia

12.5 in Reaktive Stofftransportmodellierung

28.03.2020, 10:30-10:45, Telemann-Saal

Reactive transport of nitrate and other solutes in the subsurface is a frequent and relevant problem for drinking water production in fractured aquifers. We consider a carbonate aquifer (Upper Muschelkalk), which was exposed to nitrate contamination for at least half a century. The fate of nitrate and its distribution in the micritic limestone aquifer is controlled by physical and chemical processes including redox-reactions and diffusion in the rock matrix. Microbial growth in the rock matrix is assumed to be limited due to small pore sizes. A travel-time based reactive transport model is developed to simulate long-term nitrate reduction and to investigate the role of biotic and abiotic iron oxidation as well as type and amount of iron bearing minerals as electron-donors. Results highlight the importance of considering the preconditioning of the aquifer, namely the oxygen ingress before the application of nitrate. The simulations reveal that pyrite and siderite (or often saddle dolomite), when considered together, provide enough denitrification potential to achieve the level of denitrification observed in the aquifer. Each of the minerals, when appearing alone in concentrations typical for the aquifer, wouldn’t be able to provide sufficient electron donor  as required for the observed denitrification. Taking into account estimated residence time distributions in each of the sampling points the model can be calibrated against measured values of nitrate concentration and mean age of sampled groundwater. The model shows a very good fit for pyrite and siderite concentrations in the rock matrix of approx. 0.1 Vol. %.

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