Study region: The area of study is the northern part of Belgium (Flanders). The seven evaluated Aquifer Thermal Energy Storage (ATES) systems are positioned in key aquifers, which contain major groundwater resources for the region. Study focus: The increasing number of ATES systems leads to concerns by drinking water companies and environmental regulators about the long term impacts of ATES systems on the groundwater quality. This study assesses the influence of ATES on groundwater chemistry by means of a literature review, and a comparison of groundwater quality monitoring data at seven ATES systems with ambient groundwater quality values from 69 monitoring wells. New hydrological insights for the region: The results of the analysis of the hydrochemical data confirm that the small temperature differences (ΔT ≤ 10) at which the ATES systems are operating do not influence the concentrations of the main chemical constituents. Mixing of shallow with deeper groundwater during ATES operation, on the other hand, can alter groundwater quality. The results of this study, however, suggest that the groundwater quality changes are rather small, so that there is no immediate risk for the drinking water supply. However, the installation of ATES systems in the vicinity of public drinking water supply well fields should be handled with care, especially in phreatic aquifers.