Climate change effects on groundwater recharge and temperatures in Swiss alluvial aquifers

Climate change will have both quantitative and qualitative effects on groundwater resources. These impacts differ for aquifers in solid and unconsolidated rock, in urban or rural locations, and in the principal processes of groundwater recharge. Having knowledge about the intrinsic key parameters (aquifer geometries, storage properties, groundwater renewal rates, residence times, etc.), the principal groundwater recharge processes, and the temperature imprinting makes it possible to compare and forecast the sensitivity of individual aquifers to climate change. The sensitivity of future groundwater temperature development for selected climate projections was qualitatively investigated for representative Swiss unconsolidated rock groundwater resources in the Central Plateau as well as the Jura and Alpine region. For non-urban and rural areas, climate change is expected to have a strong overall impact on groundwater temperatures. In urban areas, however, direct anthropogenic influences are likely to dominate. Increased thermal subsurface use and waste heat from underground structures, as well as adaptation strategies to mitigate global warming, increase groundwater temperatures. Likewise, measurements for the city of Basel show that groundwater temperatures increased by an average of 3.0 ± 0.7 °C in the period from 1993 to 2016, and that they can exceed 18 °C, especially in densely urbanized areas. Similarly, regarding shallow aquifers with low groundwater saturated zone thicknesses, such as in Davos (Canton Grisons), groundwater temperatures will strongly be influenced by changes in groundwater recharge regimes. In contrast, groundwater temperature changes within deep aquifers with large groundwater saturated zone thicknesses, such as in Biel/Bienne (Canton Bern), or in some cases in aquifers with large distances from the land surface to the groundwater table and extended unsaturated zones, such as in Winterthur (Canton Zurich), are strongly attenuated and can only be expected over long time periods. In the context of the presented research we hypothesized that quantitative groundwater recharge and the associated temperature imprinting of aquifers is primarily determined by infiltrating surface waters (i.e. “river-fed aquifers”). We show that seasonal shifts in groundwater recharge processes could be an important factor affecting future groundwater temperatures. Moreover, the interaction with surface waters and increased groundwater recharge during high runoff periods are likely to strongly influence groundwater temperatures. Accordingly, for the “business as usual” climate change scenario and for the end of the century, a shift in precipitation and river flood events from summer to winter months could be accompanied by an increase in groundwater recharge in comparatively cool seasons, which would be accompanied by a tendency to “cool down” groundwater resources.