3D multiple point geostatistical simulation of joint subsurface redox and geological architectures

Abstract. Nitrate contamination of subsurface aquifers is an ongoing environmental challenge due to nitrogen (N) losses from intensive N fertilization and management on agricultural fields. The distribution and fate of nitrate in aquifers are primarily governed by geological, hydrological and geochemical conditions of the subsurface. Therefore, we propose a novel approach to model both geology and redox architectures simultaneously in high resolution 3D (25 m × 25 m × 2 m) using multiple point geostatistical simulation (MPS). Data consists of 1) mainly resistivities of the subsurface mapped with towed transient electromagnetic measurements (tTEM), and 2) information of lithology and redox conditions obtained from the borehole observations at point scale interpreted from the geological descriptions and colors of sediment samples, and chemistry analyses of water samples. The conceptual understandings of the geological and redox architectures of the study system were introduced to the simulation as training images. These data were combined with detailed soil maps and digital elevation models to identify main geological elements defined as volumes within the subsurface. From a geological perspective, these data are considered independent from each other in terms of formation. This approach became computationally attractive by simulating smaller geological elements individually instead of the entire catchment. The final realizations were stitched together using simulations of the individual geological elements, resulting in an ensemble of realizations representing a quantification of the uncertainty for the given setup. The joint simulation of geological and redox architectures, which is one of the strengths of the MPS simulations compared to other geostatistical methods, secures that the two architectures in general show coherent patterns. Despite the inherent subjectivity of interpretations of the training images and geological element boundaries, they enable an easy and intuitive incorporation of qualitative knowledge of geology and geochemistry in quantitative simulations of the subsurface architectures. Altogether, we conclude that our approach effectively simulates the coherent geological and redox architectures of the subsurface that can be used for hydrological modelling with N-transport, which may be fundamental to better understanding of the nitrogen (N) retention of the subsurface and to future more targeted regulation of agriculture.