Hydrogeochemical Processes in a Small Eastern Mediterranean Karst Watershed (Nahr Ibrahim, Lebanon)

Watersheds located in semiarid areas such as the eastern Mediterranean are particularly sensitive to the impact of climate change. To gain knowledge on the hydrogeochemical processes occurring in the Nahr Ibrahim watershed, a Critical Zone Observatory in Lebanon, we analyze the isotopic composition of the river water as well as the concentrations of the major ions exported (Ca2+, Mg2+, HCO3−, Na+, Cl−, K+, SO42−). Sampling campaigns were conducted from March 2014 to August 2016 to capture contrasting hydrological conditions. The results indicate that the carbonate lithology of the watershed is the predominant source of Ca2+, Mg2+ and HCO3−, whereas the low contents of Na+, Cl−, K+, SO42− mainly originate from sea spray. Except in the headwaters, the Nahr Ibrahim River is oversaturated with respect to calcite and dolomite. During wet seasons, calcite weathering and dolomite weathering contribute in an equivalent manner to the solute budget, whereas during dry seasons, calcite precipitates in the river. The isotopic composition of the river water reveals little seasonal dependency, the groundwater recharge by snowmelt infiltration leading to spring waters depleted in heavier isotopes during the dry seasons. A carbonate weathering rate of about 176 t/km2/year was determined at the outlet of the Nahr Ibrahim watershed. The calculated values of CO2 partial pressure, on average twice the atmospheric pressure, suggest that the river is a significant source of CO2 to the atmosphere (111 t/year).