The subsurface thermal regime alteration brought by groundwater pumping

Groundwater is widely exploited, leading to substantial drops in groundwater levels, which modify the recharge/discharge relationships between large-scale hydrogeological units. In addition, this anthropogenic hydraulic forcing is also responsible for changes in thermal regimes of groundwater systems. While the impact of climate and hydrologic forcings on the hydrogeological cycle has long been demonstrated, we still have insufficient knowledge on the influence of human activities on thermal regimes of groundwater systems and, as a consequence, on groundwater quality. The major scientific obstacle that prevents accurate understanding of the impact of these forcings on the critical zones thermal dynamics is a dire lack of field observations, i.e., repeated temperature-depth profiles collected over decades. In this paper, we analyse new temperature data illustrating the complex interplay of climate warming and enhanced groundwater flow on subsurface thermal regimes from two sites in different hydrogeological settings: (1) an aquifer in fractured crystalline bedrock (Ploemeur, France), and (2) a sedimentary aquifer (La Cabine, Netherlands). A simple numerical model is then used for the interpretation of the observed temperature anomalies. We first demonstrate that pumping may have a significant impact on the thermal regime of the critical zone, and that depending on hydrogeological conditions and the natural geothermal gradient, this impact might be even more important than that of the climate change.