CHEMICAL AND ENVIRONMENTAL ISOTOPE INVESTIGATION ON HYDRODYNAMICS OF A MONOMICTIC LAKE: A CASE STUDY ON QARAOUN DAM, LEBANON

Hydrogeochemical and isotopic investigations have been carried out in a small basin in the Bekaa plain in Lebanon, to determine the exchange between Qaraoun Lake and the surrounding groundwater. Qaraoun lake is a warm monomictic lake. The poor vertical mixing of the lake is due to the temperature-induced seasonal stratification. There are few groundwater inflow points along fractures. The mineralization in the lake is higher than the surrounding groundwater, due to evaporation and anthropogenic inputs. The concentration of most chemical species decreases downgradient in groundwater with distance from the dam. Groundwater is recharged by a source other than the lake in this region. The isotopic composition of groundwater from observation wells, which are not influenced by the lake, lies on the local meteoric water line. The isotopic composition of the lake and of the wells that contain water from Qaraoun lake fall on the evaporation water line. The position of the points indicates the extent of mixing between locally infiltrated groundwater and lake water. The contribution of lake water diminishes with increasing distance from the lake. The mixing zone, delineated by combining the stable isotope data and hydrochemical parameters, is oriented to