Ras-el-Ain: The great karst spring of Mesopotamia: An hydrogeological study

Abstract

With an average annual discharge of 1,219 million cubic metres, or 38.666 cubic metres per second, Ras-el-Ain is amongst the largest of the great karst springs of the world. Ras-el-Ain lies almost midway between the Tigris and Euphrates Rivers, just south of the Syrian-Turkish frontier, in a region of open rolling plains. The waters issue from some thirteen springs and combine to form the effective head of the Khabour River, tributary of the Euphrates. Around Ras-el-Ain, precipitation is some 280 mm, decreasing to the south, but increasing in other directions. The springs issue above a local anticlinal saddle in a general synclinal structure of Tertiary formations, terminating on the north in the east-west Mardin swell and on the south in the anticline of the Djebel Aabd-el-Azaziz range. The aquifers feeding the spring are the Eocene limestones, with subsidiary supplies from the Miocene evaporites and limestones. The major infiltration area is located over basalt and Eocene limestone outcrops in the north and is estimated to extend over some 8,100 square kilometres; there is minor infiltration in the south. Recharge appears to be a direct function of surface runoff. Two east-west faults are invoked to account for the separation of the springs into a northern and southern group, whose waters are also somewhat different in chemical composition. Karst dissolution of the buried aquifers has resulted in the formation of collapse structures at surface which are occupied by small lakes and ponds, fed by the artesian springs. Discharge gaugings from December 1942 to November 1959, indicate that the average monthly discharge varies from a minimum of 36.000 cubic metres per second in June to a maximum of 41.894 in November. The coefficient of exhaustion has been calculated as 0.000483 and the stored reserves as about 7,420 million cubic metres. The study concludes with the possibilities of geotechnical development allowing still more efficient use of the water for irrigation, by reducing winter and increasing summer discharge from the springs. The danger of additional collapsing is noted.