Travel time-based modelling of nitrate reduction in a fractured limestone aquifer by pyrite and iron carbonates under pore size limitation

Abstract

We investigate denitrification in a ferric iron-containing fractured micritic limestone aquifer (Triassic Upper Muschelkalk) in south-west Germany by numerical simulations. Low porosity values (average value of 1%), partly small pore sizes of the rock matrix (~ 0.1 μm), and thus potential absence of microbial activity in the rock matrix suggest that denitrification is taking place solely in the fracture. A key question is whether the nitrate reduction derived from groundwater observations at 25 locations in the study area can be explained by a model that restricts microbial denitrification to the fractures. A travel time-based reactive transport model is developed to efficiently simulate long-term nitrate reduction on the catchment scale. The model employs a 2-D numerical reaction model describing the fracture-rock matrix system and parametric travel time distributions. The role of (i) biotic and abiotic iron oxidation, (ii) the type and amount of iron bearing minerals, and (iii) mass transfer between matrix and fracture are investigated. The simulations show that pyrite and siderite (used as surrogate for iron carbonates) together as a source of electron donors provide enough reduction potential to decrease the nitrate concentrations as observed in the field. This confirms the hypothesis that diffusion-controlled mass transfer of electron donors from the matrix to the fracture is sufficient to establish considerable denitrification in the fracture. Uncertainty in modelled concentrations is demonstrated as a result of both the geochemical aquifer properties and the unknown shape of travel time distributions.