Thermal Response of Shallow Aquifers to Recent Climate Change

Abstract:

The impacts of climatic variations on different components of the hydrological cycle have been extensively examined, while studies on groundwater responses to recent climate change are limited. In particular, the implications for the long term increases of shallow groundwater temperatures, as an important driver for water quality, and the adherent consequences for groundwater-dependent ecosystems are not comprehensively understood. In this study we therefore scrutinized the coupling of atmospheric and groundwater warming by employing stochastic and deterministic models. Measurements of groundwater temperatures were obtained in observations wells over four decades from two aquifers in Germany and linked to air temperature time series from local weather stations and to aggregated time series over different spatial scales. In order to examine the relationship between air and groundwater temperature time series, breakpoints in the long term means were identified by employing statistical analyses. The observed increases in shallow groundwater temperatures can be associated with preceding positive changes in regional surface air temperatures, which are in turn linked to global air temperature variations. Furthermore, we employed an analytical solution to the conduction-advection heat transfer equation to explore the thermal processes that control heat propagation in the subsurface. The predicted groundwater temperatures generally concur with the observed trends in the measured time series. The temperature increase in shallow aquifers is more gradual than the causal atmospheric warming, because the thermal signals from distinct changes in air temperature are diffused in the subsurface. These observations indicate that shallow groundwater temperatures respond rapidly to atmospheric changes. Thus, future climate change is likely to have a significant impact not only on economically important aquifers, but also on groundwater-dependent ecosystems.