Hydrogeological investigation of tahtali dam basin and assessment of groundwater vulnerability by using AHS - DRASTIC method

Investigation was carried out in an area of about 550 square kilometers located on the Tahtalı Dam catchments area. Tahtalı Dam Basin is an important water resource for Izmir because of thirty four 34 percent of Izmir?s water supplied. As a result of geological investigation, the foundation of basin is formed by Paleozoic Menderes Metamorphics. The Mesozoic Bornova Melange that lies with unconformity via a thrust fault. The Neogene formations lie with non-uniformity over the Bornova Melange. The Quaternary alluvial layers overlie all the formation in the region. According to the hydrogeological studies, Quaternary alluvium and Neogene limestones are unconfined aquifer and conglomerates is semi confined aquifer.Chemical composition of groundwater are calcium-magnesium-bicarbonate, calcium- bicarbonate and sodium-calcium-bicarbonate type groundwater in the study area. Water budget of area reveals that 35 million cubic meters groundwater overexploitation. The Oxygen-18 and deuterium environmental isotopes show that effectual recharge elevation of basin ranged between 176-376 meters. Groundwaters originated from meteoric water and have short circulation time.Regional groundwater potential vulnerability assessment on the Tahtali Basin performed by using GIS based DRASTIC method. AHP is used to determinate the DRASTIC weighting and ratio value. Seven hydrogeological maps are used to assess the groundwater vulnerability zone. In addition to aquifer quality maps are prepared using AHP with multiple quality parameters. The AHS rating value are used with the groundwater quality standards. According to the results of these maps, there is very strong similarity between groundwater vulnerability map and groundwater quality map.