İNŞaat mühendiSLİĞİ anabiLİm dali

Investıgatıon Of Saltwater Intrusıon In Terkos Basın Usıng Transıent Electromagnetıc And Dırect Current Resıstıvıty Methods
Terkos Lake is the biggest and the most important freshwater source which supports drinking water supply to the European side of Istanbul. The formation of the lake is related to the development of a dune barrier that separates the lake from the sea. This barrier is formed from the sands accumulated by Black Sea at the place where several rivers were converging and draining into the sea. At the East region where the thickness of the sand between sea and lake is too thin, the thickness is 165 meters. Because of the importance of Terkos Lake as a drinking water supply, it is urgent to reveal the characteristics of the subsurface system of the narrow coast line between the lake and the sea. It is important to understand the hydraulic relation between the lake and the sea and to determine the location of the possible salt water wedge in terms of protecting the critical balance in the basin. Unfortunately, the subsurface structure of Terkos Lake barrier was not examined by any geophysical method until now.
In this study Direct Current Resistivity (DCR) and Transient Electromagnetic (TEM) measurements were applied to detect possible saltwater intrusion and to delineate the subsurface structure in the north of Terkos Lake. The results of inverted DCR and TEM data were examined and the optimum models were chosen. When the inversion of these data is interpreted along with the hydrogeology of the area, it is concluded that the salt water intrusion along the barrier is not common and it occurs at a special area where the distance between lake and sea is very close. Along a measurement profile where the lake and the sea is nearest, a water resistivity model was prepared by using borehole data nearby the profile and salt water intrusion was determined. By using the information of boreholes around the lake, it was verified that common conductive region on other profiles were made up of clay levels and clay lenses. This study revealed the place of clay lenses at the lake system and the influence clay lenses on the water cycle with geoelectrical images. And a model about Terkos Lake subsurface formation was proposed.