We performed, over a 598 day sampling period, chemical analyses of commercially bottled spring water (namely Alet water) rising in the vicinity of Saint Paul de Fenouillet (French eastern Pyrenees) where a ML = 5.2 earthquake occurred. These data display a chloride ion anomaly starting 5 days prior to the quake and lasting 10–13 days. The anomaly is characterized by Cl− concentrations 36% above background values. This precursory chemical change is attributed to a pre-seismic strain change, which induced mixing of geochemically different aquifers. Chloride-rich groundwaters are known to rise close (10 km) to the Alet spring, and mixing calculations suggest that a limited inflow of similar waters within the Alet hydrologic system is a viable explanation for the chloride ion anomaly. This result confirms that mineral springs are promising sites for the search of earthquake geochemical precursors.