Perchlorate (ClO−4) is an environmental contaminant of growing concern due to its potential human health effects and widespread occurrence in surface water and groundwater. Analyses carried out in France have highlighted the presence of ClO−4 in drinking water of Champagne-Ardenne (NW of France), with two potential sources suspected: a military source related to the First World War and an agricultural source related to the past use of Chilean nitrates. To determine the sources of ClO−4 in groundwater, major and trace elements, 2H and 18O, ClO−3 and ClO−4 ions and a list of 39 explosives were analyzed from 35 surface water and groundwater sampling points in the east of the city of Reims. ClO−4 ions were found in almost all sampling points (32 out of 35) with a max value of 33 µg L−1. ClO−4 concentrations were highest in groundwater ranging from 0.7 to 33 µg L−1 (average value of about 6.2 µg L−1) against from < 0.5 to 10.2 µg L−1 in surface water (average value of about 2.7 µg L−1). Most of the water samples showing high ClO−4 levels (> 4 µg L−1) were collected near a military camp, where huge quantities of ammunitions have been used, stored and destroyed during and after the First World War.