Use of isotopes and geophysical techniques in studying dam leakage-case of the Afamia dams, Syria

Different techniques such as: hydrochemistry, environmental isotopes, radon and geophysical surveys have been applied all together in investigating the water leakages from the Afamia (A, B and C) dams during the period 2006-2008. Groundwater movement in this area shows the existence of two major discharge cones or sinks in the vicinity of B and C reservoirs. This movement suggests a probable vertical leaking and interconnection between the shallow Neogene aquifer and the deeper Karstified water bearing system of the Upper Cretaceous. Spatial distribution patterns of major ions in the groundwater provided comprehensive arguments for the existence of an active dissolution zone of calcareous rocks at a depth of about 45 m below the land surface, facilitating hence the vertical penetration of stored surface water towards the Cretaceous aquifer. The remarkable contrasts in stable isotope compositions between the depleted and highly enriched waters nearby the B reservoir provide another support for water leaks by vertical penetration beneath or very close to the B dam body. The high radon values in the soils, linearly distributed along some tectonic lineaments, may indicate the usefulness of radon method for tracing and mapping unknown faults. Application of some selected geophysical methods (electromagnetic survey, vertical geo electric sounding and tomography plates) revealed the presence of alternating lithological heterogeneity between permeable and impermeable layers, which may lead to sub horizontal infiltration through the geological formations of the basin. Vertical leakage which could take place in certain locations via existing faults and kart and fractures that hit the main valley may pass through the dam lake. This structure is most likely causing hydraulic connections between the superficial Neo gene deposits and the underlying Cretaceous fractured and permeable carbonates rocks. The presence of clayey and silty zones, mostly of high electrical conductivity values along the B reservoir valley reflects also a possible vertical penetration. Although the Afamia B dam was constructed in a site quite rich in archaeological ruins (old water canals, domestic wells and plenty of old graves from the Roman civilization) a number of disconnected tectonic lineaments were noticed in the main stream valley in form of short faults or fractures. Such lineaments have been practically confirmed by several vertical and horizontal trenches that were carried out in some sites. The gained experience in dam's leakage assessment proved the importance of the integral use of isotopes and geophysical techniques in this domain, mainly for the subsurface characterization of lithological and hydrogeological features. These methods, which represent a useful promising tool for solving a large number of geo engineering problems, offer further encouragement to continue similar activities in other dam sites for better managements of water resources in the country.(author)