On Vulcano Island, Italy, the thermal water in the shallow water well W2 is a mixture of reservoir water and shallow steam-heated groundwater of meteoric origin. In the period 1986-1988, the composition of the W2 water changed just before two sequences of tectonic earthquakes, from the almost pure reservoir component to the almost pure steam-heated meteoric component. After the seismic activity, the W2 water returned to its pre-earthquake reservoir composition. These changes in composition could be explained by the dilatancy-fluid diffusion model. According to this model, the observed variations in W2. water composition could be the consequence of stress build-up prior to the seismic events, and stress reduction afterwards.