Repeated Subsurface Thermal Profiling to Reveal Temporal Variability in Deep Groundwater Flow Conditions

Reliably quantifying groundwater fluxes to and from confined aquifers in sedimentary basins is increasingly recognized as a critical challenge that impedes sustainable groundwater management. One approach to quantify such fluxes is through the analysis of deep (e.g., >50 m) borehole thermal profiles penetrating through aquifer‐aquitard systems. Recently developed methods to interpret such data exploit the relationship between vertical groundwater flow and the downward propagation of surface temperature disturbances resulting from climate warming. In this note, we advance beyond prior studies that assumed steady‐state groundwater flow by demonstrating how hydrogeological regime shifts on decadal time scales can be quantitatively inferred from temperature‐depth profiles (TDPs). We use a set of repeated temperature‐depth profiles from one site in an unconsolidated sedimentary aquifer system, recorded in 1980 and 2016/2018 to tentatively infer a minimum of a threefold increase in groundwater downwelling to deeper aquifers (i.e., from 100 to 350 mm/year). The enhanced flux likely results from intensified, deep groundwater ion in the vicinity since the mid‐1980s. We reach this conclusion through analyzing the occurrence and downward propagation of the minimum temperature in the profiles as well as the temporal trend in deeper groundwater temperatures. We conclude that repeated temperature‐depth profiles can be suitable to archive hydrogeological changes. Our results provide the impetus for more systematic collection of present‐day TDPs to provide a historical benchmark from which to assess future groundwater flow alterations, especially in areas that lack traditional aquifer monitoring via hydraulic head measurements. Key Points Changes to long‐term groundwater recharge or discharge fluxes are challenging to record Repeated deep temperature profiles archive groundwater flow information over time Changes in temperature profiles at depth can reveal hydrogeological regime shifts