Tracing salinity sources of S'Almadrava karstic brackish spring (Mallorca, Spain) with sulfur and oxygen isotopes

S'Almadrava spring (Mallorca Island) is a good example of a brackish spring in a highly karstified coastal carbonate formation. It is located at 6 m.a.s.l. and discharges water of variable salinity, depending on the discharge regime. Its average flow rate is 12.3 hm(3)/y, and the electric conductivity can reach 40 mS/cm. The phenomenon has been explained as the result of seawater intrusion into the karst system, but discussions still remain on whether this is the unique source of salinity. The objective of this study is to elucidate the role of other possible salinity sources, such as dissolution of Keuper evaporites or fertilizers. The methodology is based on the analysis of concentration, and delta S-34 and delta O-18 of dissolved sulfate in a series of samples collected during a high-frequency sampling campaign. Analyses of isotope data show an excellent agreement with a conceptual model of pure mixing of seawater intruding a freshwater aquifer. The influence of other potential sources of salinity, if any, has been estimated to be very limited at any stage of the spring hydrological cycle.