The strontium content and the SO ₄ ²⁻ /Cl ⁻ and Mg ²⁺ /Ca ²⁺ ratios were used as natural tracers of the residence time of seawater intrusion into the Castell de Ferro aquifer. Analysis of these parameters indicated the existence of two principal flowpaths in the aquifer. The first flows through the eastern part of the aquifer, through the karstified Castell de Ferro massif; it accommodates a larger and more rapid flow, so that the residence time is shorter, leading to lower SO ₄ ²⁺ /Cl ⁻ ratios, lower Sr ²⁺ content and higher Mg ²⁺ /Ca ²⁺ ratios. The second flowpath is in the western sector, and flows exclusively through alluvial deposits; the flow here is slower, particularly that flowing towards the sea. Thus the residence time of the water here will be longer and there is scant flushing of the intruded seawater; this is manifested in the high Sr ²⁺ content, high SO ₄ ²⁺ /Cl ⁻ and low Mg ²⁺ /Ca ²⁺ ratios.