We used data from an aquifer thermal energy storage (ATES) system located 570 m from a public water supply well field in the south of the Netherlands to investigate the relation between production of renewable energy with an ATES system and the production of drinking water. The data show that the groundwater circulation by the ATES system can impact chemical groundwater quality by introducing shallow groundwater with a different chemical composition at greater depth. However, the observed concentration changes are sufficiently small to keep groundwater suitable for drinking water production. Microbiological results showed that the ATES system introduced faecal bacteria in the groundwater and stimulated the growth of heterotrophic micro-organisms. At the studied site this forms no hygienic risk because of the long distance between the ATES wells and the public supply well field A further degradation of either chemical or microbiological groundwater quality however may necessitate additional water treatment which raises the energy requirements. The additional energy requirements for drinking water treatment may be up the same order of magnitude as the harvested energy by the ATES system. © IWA Publishing 2011.