Using groundwater temperature time-series to reveal subsurface thermal and hydraulic processes

Understanding subsurface heat transport processes is important for geothermal energy development and heat-fow modelling applications, and for resolving hydrogeological, biogeochemical and microbiological processes. Studies of subsurface thermal regimes have predominantly focussed on repeat temperature-depth profle analysis. The application of groundwater temperature time-series data to characterise thermal and hydraulic processes is relatively under-exploited. Here, an unusually rich set of half-hourly groundwater level and temperature time-series data from 48 boreholes in the Cardif Geo-observatory (UK) between 2014 and 2018 is used to explore the interrelationships between subsurface hydraulic and thermal processes. Characteristic time-series curve shape categories were identifed in annual-scale temperature changes and shown to be indicative of distinct fow and heat transport mechanisms. Sinusoidal curves are found in conduction-dominant settings, while ‘right-leaning’ time-series indicate faster cooling than warming and are associated with the infuence of advection of heat due to recharge. Short-lived temperature events found on the cooling limbs of right-leaning curves correlate with sharp groundwater level rises, indicating recharge. Temperatures rebound quickly following these events but do not return to pre-event levels, having the efect of cooling groundwater faster in winter than it is warmed in summer. More complex behaviours observed in boreholes located close to rivers indicate recharge responses coupled with the infuence of changes in stream–aquifer interactions which co-occur with heavy rainfall. The results demonstrate that groundwater temperature time-series interpretation may be a cost-efective way of providing new insights into the characteristics of subsurface hydraulic and thermal processes with implications for geothermal exploration and a range of other hydrogeological applications.