Climate-driven effects on regional groundwater dynamics and the thermal field

Among the most direct impacts of climate change on aquifers are changes in temperature and groundwater recharge. Rising near-surface air temperature increases potential evapotranspiration (PET), while also increasing shallow groundwater temperature. Groundwater recharge likewise exhibits an intrinsic climate sensitivity, being dependent on the balance between precipitation and PET. It affects groundwater storage and governs advective heat transport in the subsurface. This illustrates the strong interconnection between energy and water fluxes in the subsurface and their sensitivity to climate variability. Socio-economic impacts of climate change include groundwater depletion and declining groundwater quality in large parts of the world, but also present opportunities for managed aquifer recharge and shallow geothermal heat utilization. Methodological approaches to assess these impacts are diverse but not comprehensive. Hydrologic models do not capture the full complexity of the subsurface geology and typically neglect heat transport. Thermo-hydraulic groundwater models, in turn, rely on a simplified representation of the water table dynamics and recharge fluxes. In the current study, these limitations are bridged by conducting coupled simulations of groundwater flow and heat transport using boundary conditions derived from climate data and a hydrologic model. This enables evaluating the subsurface response to distributed climatic forcing across various timescales. The main objective of this thesis is to investigate the effects of climate change, e.g., rising air temperatures and recharge variability, on regional groundwater dynamics and the subsurface thermal field. This approach is applied to the region of Brandenburg in northeastern Germany, characterized by active shallow groundwater circulation, variations in water table depth, and a growing pressure on groundwater resources from droughts. For the selected region, the thesis investigates (1) geological and topographic controls on the variability in baseline subsurface conditions, (2) the propagation of periodic climatic signals with depth, and (3) potential impacts of rising surface temperatures and declining recharge trends. The results from the investigation highlight how regional groundwater flow evolves from hydraulic head differences between elevated and low-lying areas and is further shaped by the basin-wide structural and permeability configuration. Century-scale variability in recharge and surface temperature affects permeable fluvioglacial aquifers to depths of up to 500 m due to high groundwater fluxes. Periodic weather signals (e.g., temperature, precipitation, droughts) are damped and delayed as they propagate into the subsurface. The unsaturated zone thickness exerts the primary control on the degree of recharge damping at the water table, while soil texture and aquifer storativity contribute to increasing variability in damping with depth. In lowlands and glacial valleys, characterized by a shallow water table, the recharge damping is weak and groundwater level fluctuations are dominated by an annual periodicity. In contrast, on glacial plateaus with a thicker unsaturated zone, high-frequency atmospheric signals are damped, and groundwater levels exhibit an inter-annual periodicity, potentially linked to climatic teleconnections. The key effects of the sustained climate change, namely groundwater warming and declining heads, also showcase a spatial variability. Shallow downward groundwater flux enhances the climatic signal on glacial plateaus, while largely upward flux in valleys tends to mask it. When preferential downward advective flux is superimposed over erosional windows in the first regional aquitard, the climate signal has the ability to penetrate into the underlying aquifers. Magnitudes of projected groundwater temperature increase depend primarily on the surface warming rather than on the recharge scenario. Neither a projected late-century decrease in annual recharge of 10-20% nor an increase in the proportion of colder winter recharge has the capacity to reverse the ongoing groundwater warming trend. Climate-driven subsurface warming is projected to increase heat-in-place stored in aquifers by hundreds of MJ m-2, creating potential for shallow geothermal utilization, especially in urban centers. However, a rising demand for geothermal cooling may “disbalance” summer and winter loads, thereby reducing the efficiency of geothermal installations. This thesis demonstrates how variability in present-day relief, geology, and recharge scenarios should be taken into account when developing groundwater-climate adaptation strategies and planning shallow geothermal exploration. The presented modeling framework allows flexible quantification of distributed climatic effects on groundwater and is applicable to diverse hydrogeological and climatic conditions. Prospectively, it provides a robust basis for integrating heterogeneity in hydraulic and thermal properties of the unsaturated zone and shallow aquifers and for applying higher-frequency weather forcing to capture the effects of extreme precipitation events on preferential flow, heat transport, and aquifer-stream interactions.