Karst aquifers have distinct characteristics that make them complex compared to non-karstic aquifers. The complexity increases in coastal areas, due to the complicated mixing dynamics between groundwater and seawater. The combined use of hydrochemical and isotopic data is proved to be a useful tool to better understand the circulation of groundwater at coastal karst aquifer in southwest Turkey. The "Azmak Streamflow Karst System" is composed of about 150 spring outlets whose electrical conductivity values range between 1300 and 15,000 mu S/cm on the coast of the Aegean Sea. The total average flow is about 11 m(3)/s and the average total dissolved solids at the complete mixing point is about 3000 mg/l. This slightly brackish water is regarded to be strategic owing to its possible availability for future use. Major ions and stable isotope analyses as well as in situ measurements of physical parameters were carried out during a period of 2 years and evaluated to better understand the hydrogeological setting. Spatial and temporal variations of hydrochemical and isotopic constituents of sampled waters also provided a significant tool to elucidate the salinization mechanism in the study area. The results revealed that the Azmak streamflow-opposed to the common assumption-is the outflow of a multi-component hydrogeological system whose components are karst aquifer, alluvial aquifer, alluvial fan, seawater, and surface runoff. In addition, the results showed that sources other than seawater mixing contribute to the salinization of groundwater in the system.