Spatiotemporal geochemical evolution of groundwater through an open-loop geothermal system – a field pilot study

To investigate the operational challenges faced by groundwater heat pump systems (GWHPs), a field pilot study (two heat injection tests, HITs) was conducted to mimic operational processes involving pumping, heating and reinjection into the aquifer to simulate the cooling mode. This study aimed to acquire an understanding of the spatiotemporal evolution of groundwater chemistry as it circulates throughout different parts of the system (pumping well, heater inlet/outlet, injection well, monitoring well downgradient) and during different test phases (pre-test, HITs, post-test recovery). Hierarchical clustering analyses identified six clusters corresponding to different water types through space and time, which were categorised into two main conditions: natural (Na-Ca-Cl water) and perturbed (Ca-mixed-(HCO 3 -Cl-SO 4 ) water). Pumping-induced mixing between shallow and deeper water, along with draw-in of dissolved oxygen (DO), induced oxidising conditions and precipitation of iron oxides in the injection wells, subsequently leading to well blockage and premature termination of both HITs. The chemical signatures of the injected warm water reached the monitoring well faster than the thermal plume, suggesting different heat and solute transport mechanisms at play. While principal component analyses showed that water mixing was the main driver of differences between natural and perturbed geochemical conditions, geochemical modelling confirmed that DO intrusion was the main driver of iron oxide precipitation, surpassing the effect of water mixing and increased temperatures. Results from this study underline the importance of conducting a comprehensive spatiotemporal geochemical assessment of the hydrogeological system prior to and throughout the lifecycle of a GWHP system to predict and manage operational risks. • Two heat injection tests simulated operational processes of GWHP. • Pumping-induced oxidation caused well blockage by iron oxide precipitates. • Mixing drove a compositional continuum between natural and perturbed conditions. • The chemical plume of the injected water travelled faster than the thermal plume.