Abstract Hydropeaking indicates the sudden and frequent fluctuation of river stage caused by the operation of hydropower plants. This river alteration affects numerous physical, chemical, biological, and ecohydrological processes in surface water bodies. The impact of hydropeaking on groundwater hydraulics has been investigated only recently through modeling and experimental studies at multiple spatial scales. This study presents new hydrological and geochemical surface water and groundwater data providing the first evidence of the impact of hydropeaking on groundwater hydrogeochemistry. To interpret these observations, we propose a conceptual model where the rapid vertical oscillation of the groundwater table acts as a pressure‐driven ‘geochemical pump’ impacting redox conditions in the aquifer. This work has important implications for better understanding of groundwater biogeochemistry as well as for nutrient and carbon cycles.