Hydrogeological and hydrogeochemical investigation of hot and cold water systems of Alasehir (Manisa)

Alasehir Geothermal Fields, where is located in the southern part of Gediz Graben in western Turkey, are one of the most important fields in Turkey. Faults and fracture zones on the metamorphic rocks of Paleozoic Menderes Massif constitute reservoir rocks of the geothermal system. Impermeable clayey and silty levels of Gobekli and Acıdere Units, which are terrestrial sediments of Neogene, are cap rocks. That hot waters ascend from the geothermal confined aquifer to the surface and mix with cold waters in the alluvium aquifer that depends on an increase in porosity and permeability due to the step-over of geometric fault segments. Consequently, water use for irrigational and drinking purposes gives way to loss water quality of cold waters in the alluvium aquifer. Alasehir geothermal fields are geographically divided into five groups; 1-Alasehir Geothermal Field, 2. Horzumsazdere Geothermal Field, 3. Kavaklıdere-Kurudere Geothermal Field, 4. Gobekli Geothermal Field and 5. Acıdere Geothermal Field. Although discharge temperatures measured on spa and thermal water wells are no more than 57 celcius degrees, 182 celcius degrees and 213 celcius degrees were measured at the bottom of KG-1 (numbered 5) and AK-2 (numbered 38) thermal wells. According to the IAH (International Association of Hydrogeology) classification, most of hot waters are sodium-bicarbonate and magnesium-sodium-bicarbonate in type. Hot waters have a maximum value of boron concentration as 124 ppm. It is obtained that estimated reservoir rock temperature vary between 100 degrees of celcius and 250 degrees of celcius and estimated mixing ratio of cold waters vary between 75 percentage and 97 percentage using silica mixing model in both. The diagram of silica enthalpy shows the reservoir temperature as 231.5 celcius degrees. Quartz, chalcedony and amorphous silica tend to experience scaling during the quenching. Isotopic data indicate that regional waters are meteorical in origin. Tritium values often vary between 0 and 5 TU (tritium unit), and reflect waters genetically dominated by rain waters with an age of about 40 years ago. Any isotopic value does not demonstrate a magmatic origin. The most enrichments in oxygen-18 are observed at the wells of AK-2 ve KG-1. Although soil pollution is resulted from geothermal activity and agriculture, it may be also attributed to irrigational purposes at the wells on aquifers where they bear sulphate deposits.