An electric potential measurement system for detecting contaminants in the subsoil, comprising: a reference electrode (2); a vehicle (3); a measurement electrode (4), with: a contact electrode (7) formed by an electrically insulating container (13) including a metal electrode (14) surrounded by a non-metal filler material (15), and a retaining mesh (16) covering the base of the container (13); a non-metal contact appendage (9) establishing continuous galvanic contact with the soil as the vehicle (3) moves over the ground (6); a data acquisition and positioning system (5) which acquires and stores position data (Pi) and electric potential difference measurements (Δvi) between the reference electrode (2) and the measurement electrode (4) in a memory (29). The data stored in the memory can be used to obtain an electric potential map (30) and/or a map (32) with the probability of the presence of contaminants.