A baseline hydrogeochemical investigation was carried out as a contribution to the inventory of wetlands in Buyuk Menderes River (BMR) coastal wetland which is the most important wetland in Turkey that is protected as "Natural Park" by government. In addition to the wetland values, agricultural activities are also important at this delta. From inland to coastal zone, the groundwater type is changed from Ca-HCO3 to Na-Cl. Perfect correlation of Cl with Na and SO4 indicated the mixing process between saline and fresh water. The seawater intrusion and "winter-leaching" is the main causes of salinization. Minor ions as B, Br, and Li versus Cl relationship also showed the marine influence at the BMR coastal wetland. The delta O-18 and delta D composition of the waters are range between -8.11aEuro degrees and -1.65aEuro degrees and -49.2aEuro degrees and 8.1aEuro degrees, respectively. Isotope values show the seawater mixing line and also dissolution of salts in the BMR coastal wetland. Weathering of aluminosilicates in the Paleozoic schist and clays resulted in the release of silica to the water. Due to the organic activity, iron concentrations reached to the tolerance limits. On the other hand, the use of phosphate fertilizers increases the phosphorus content in the water. The high value of As (from 0.6 to 134 mu g/l) can be resulted as an evaporation process in the study area.