Southern Mediterranean lagoons are considered among the systems most vulnerable to anthropogenic activities. In its present condition, the Mar Menor lagoon in south-eastern Spain must be considered as degraded as a result of the pressures to which its shoreline is exposed. These include the transport of arsenic from nearby mining areas, linked to soluble or particulate run-off following rainfall events and flooding. This work studies the solubility and bioavailability of arsenic in the soils of watercourses that drain into the Mar Menor, using nine single chemical extractions with different reagents. The values obtained for As, as well as for Fe and Mn, throw light on the interactions of these metals with As and predict future behaviour patterns for different scenarios. The mineral composition is seen to significantly influence the mobility of the studied elements and shows the relation between the soils and materials of mining origin. It has been suggested that the sediments deposited in the in the watercourses or "ramblas" that transport these materials may act as foci of dispersion.