Thermodynamic coupling between atmosphere and ground yields increasing aquifer temperatures as a consequence of global warming. While this is expected to manifest as a gradual warming in groundwater temperature time series, such continuous long-term recordings are scarce. As an alternative, the present work examines the use of repeated temperature-depth profiles of 32 wells in southern Germany, that were logged during campaigns in the early 1990s and in 2019. It is revealed that the temperatures have increased in nearly all cases. We find a moderate to good depth-dependent correlation to trends in air temperature, which however is strongly influenced by local hydrogeological and climate conditions. While during the last three decades, air temperatures have increased by a rate of 0.35 K (10a)-1 on average, the temperature increase in the subsurface is decreasing with depth, with median values of 0.28 K (10a)-1 in 20 m and only of 0.09 K (10a)-1 in 60 m depth. Still, the slow and damped warming of the groundwater bodies are remarkable, especially considering naturally very minor temperature changes in pristine groundwater bodies and predictions of atmospheric temperatures. This entails implications for temperature-dependent ecological and hydro-chemical processes, and also for the heat stored in the shallow ground. Moreover, it is demonstrated that the annual heat gain in the groundwater bodies below 15 m due to climate change is in the range of one third of the state's heat demand, which underlines the geothermal potential associated with the change in natural heat fluxes at the ground surface.
The characteristics of climate and hydrology in mountain areas remain poorly understood relative to lowland areas. Our mission is to assess the groundwater quality and seasonal storage dynamics above the alpine timberline (2000 m). This critical recharge zone covers 23% of Switzerlands land surface and is the source of the countrys most important resource: clean water from a pristine environment. The value of pristine nature and free ecosystem services are often taken for granted, as they come without any costs (Brauman et al. 2007). In Switzerland, the alpine zone above the timberline is an excellent example of such free ecosystem services that relate to hydrology. The alpine zone forms the headwaters for the majority of Swiss rivers as well as major European rivers like Rhine, Rhone, Po, and Danube. However, not much is known about alpine groundwater, its recharge and water quality variations as these remote reservoirs are rarely monitored. Glaciers and permafrost will continue to retreat forming large new sediment deposits and changing infiltration conditions in high alpine terrain. Climate change will impact hydro-chemical composition of alpine waters, accelerate weathering processes, and might trigger mobilization of pollutants. Accordingly, in this proposal we plan to monitor and quantify free ecosystem services of alpine terrain, particularly those related to water quality and quantity. This project will start a pilot study of alpine porous aquifer observations in the Swiss Alps in the vicinity of the Tiefenbach glacier. To translate hydrological science into an ecosystem service context as suggested by Brauman et al. (2007), we will focus on four key attributes: - I. Water quantity: observations of groundwater level fluctuations combined with analysis of contributing water sources based on stable isotope analysis give quantitative understanding of origin and amount of water, - II. Water quality: groundwater temperature and electrical conductivity will be used as proxies for sampling of hydro-chemical parameters with automated water samplers during primary groundwater recharge (snowmelt and rainfall events), - III. Location: Alpine terrain above the timberline, especially recharge into/out of a alpine porous aquifer at a pro-glacial floodplain and - IV. Timing of flow (snow- and icemelt from May to September) and groundwater recharge during the growing season.
The presented dataset forms the basis for investigating present and future coupled effects of rising surface temperatures and temporal trends in groundwater recharge on subsurface pressure and temperature (PT) conditions in the North German Basin beneath the Federal States of Brandenburg and Berlin (NE Germany), for the period 1955-2100.
The study relies on a stochastic weather generator, a distributed hydrologic model, and a 3D thermo-hydraulic groundwater model to evaluate spatio-temporal subsurface feedback to two shared socioeconomic pathways (SSP) for seven general circulation models (GCM).
The results demonstrate a regional variability in both the intensity and maximum depths of projected groundwater warming, driven by hydraulic gradients and the underlying geological structure. The magnitude of groundwater warming primarily depends on the surface temperature scenario. Projected changes in recharge are not sufficient to reverse this trend, although recharge is still a key factor controlling groundwater dynamics within aquifers lying above the Rupelian Clay aquitard.
The dataset can be further utilized for assessing shallow geothermal potential and groundwater storage availability in the Berlin-Brandenburg region under climate change.