Local thermal equilibrium or not? Reconciling the groundwater heat transport mechanisms in heterogeneous sediments at different scales

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Shallow sedimentary aquifers in Central Europe are important freshwater reservoirs while also gaining attention for their ability to store enormous amounts of thermal energy. Understanding and controlling the thermal conditions in such aquifers is vital for quantifying flow regimes, estimating the potential for geothermal use and maintaining groundwater quality. While the fundamental heat transport mechanisms (i.e., diffusion and advection) are well established, interpreting their combined effects under the influence of scale-dependent heterogeneity remains unresolved. Natural materials are inherently heterogeneous at scales from small (e.g., grain size mixtures) to large (e.g., hydraulic conductivity distribution). In hydrogeology, a volume-averaging approach assuming local thermal equilibrium (LTE) between grains and the surrounding liquid has traditionally been applied to describe heat transport, yet this approach has neither been verified nor have conditions for its validity been defined. Further, this state-of-the-art approach ignores the variable effects of heterogeneities on heat transport at different scales. Such effects include local thermal non-equilibrium (LTNE) and an apparent scaling of macroscopic thermal dispersion. This project aims to reconcile the heat transport mechanisms at scales from grain (millimetres) to geological heterogeneity (tens of metres) by combining laboratory, field, analytical and numerical methods. At the small scale, specific flow-through column experiments will be conducted to investigate the influences of natural grain size distributions on LTNE and thermal dispersion. At the large scale, real aquifer analogues will be tested in heat transport models to scrutinize the role of sedimentary structures. At an established test site with well-known subsurface heterogeneity, field experiments will reveal the occurrence and intensity of LTNE and quantify thermal dispersion as a function of geological scale heterogeneity. Both laboratory and field experiments will be used to validate detailed numerical heat transport simulations. These will subsequently be deployed to elucidate advanced aspects, such as the relationship between mixtures of grain sizes and heat transfer coefficient as well as the influence of geological heterogeneity on LTNE and thermal dispersion. Finally, the findings will be jointly interpreted to reconcile heat transport mechanisms at different scales. The outcome will provide a novel and universal framework for modelling heat transport in sediments with natural heterogeneities. It is anticipated that the developed knowledge will advance standards in theory and practice and support improved planning and management, for example of heat tracing in groundwater and shallow geothermal systems.