Temporal changes in groundwater chemistry can reveal information about the evolution of flow path connectivity during crustal deformation. Here, we report transient helium and argon concentration anomalies monitored during a series of hydraulic reservoir stimulation experiments measured with an in situ gas equilibrium membrane inlet mass spectrometer. Geodetic and seismic analyses revealed that the applied stimulation treatments led to the formation of new fractures (hydraulic fracturing) and the reactivation of natural fractures (hydraulic shearing), both of which remobilized (He, Ar)-enriched fluids trapped in the rock mass. Our results demonstrate that integrating geochemical information with geodetic and seismic data provides critical insights to understanding dynamic changes in fracture network connectivity during reservoir stimulation. The results of this study also shed light on the linkages between fluid migration, rock deformation and seismicity at the decameter scale. Changes in the crustal stress state caused by natural or human-induced subsurface fluid overpressures can lead to brittle rock mass damage, from the formation of grain-scale microcracks to the failure of kilometric-scale faults 1. Characterizing the timing and spatial extent of crustal deformation is critical for both industrial applications, including geothermal, oil and gas production, and improving our mechanistic understanding of earthquakes. From this perspective, seismic and geodetic monitoring systems generally provide the vast majority of data collected. Several authors have also proposed that geochemical anomalies may be used as proxies for rock deformation 2-5. Fluids in the Earth's crust display highly variable geochemical traits due to the spatial and temporal variability in fluid recharge composition, fluid-rock interactions and residence times. Previous studies have shown extremely variable distributions of residence times, ranging from decades 6 to several hundred or even millions of years for fluids stored in low-permeability rocks 7,8. In this context, the fluid composition may evolve from diluted waters with modern signatures in recharge areas to saline fluids in the deeper crust, where the fluid composition tends to equilibrate with the host rock mineralogy through dissolution/precipitation processes. By analyzing specific dissolved chemical tracers, one can reconstruct the origin of fluids and gain insights into their recharge, percolation and storage conditions 9,10. From this perspective, noble gases such as radon (Rn), helium (He) and argon (Ar) have received particular attention in recent decades because of their widespread occurrence in the subsurface and their low chemical reactivity, proving to be ideal tracers to track the origin of fluids, analyze fluid-rock interactions and determine residence times 7,8,11-14. Concentrations of dissolved noble gases in subsurface fluids and their isotopic compositions are driven by two main mechanisms: 1) water/air partitioning during recharge, which is sensitive to atmospheric