The stable isotope of the carbon atom (13C) give information about the type of the mineralisation of the groundwater existing during the water seepage and about the recharge conditions of the groundwater. The concentration of the CO2(aq.) dissolved during the infiltration of the water through the soil's layers has an effect on the mineralisation of this water. The type of the photosynthesis's cycle (C-3 or C-4 carbon cycle) can have a very important role to determine the conditions (closed or open system) of the mineralisation of groundwater. The isotope 13C of the dissolved CO2 in water give us a certain information about the origin and the area of pollution of water. The proportion of the biogenic carbon and its percentage in the mineralisation of groundwater is determined by using the isotope 13C.
The stable isotope of the sulphur atom (34S) and the 18O of the sulphates (S18O4) give information about the type of the mineralisation of the groundwater existing during the water seepage. The decrease of the concentrations in dissolved SO42− (meq/L) versus the increase of δ18O (‰ vs. SMOW) of the sulphates (S18O42−) confirms a partial reduction of the dissolved sulphates in the water. The Under-saturated waters versus the gypsum do not cause the precipitations of the sulphates. The study of δ34S (‰ CD) vs. Cl− (mg/L) indicates high variations in δ34S (‰ CD) for weak difference in the Cl− (mg/L) content, this is due to the reduction of the dissolved sulphates. Concerning the Jurassic water in Lebanon, an oxidation of the sulphide can take place.
L'étude géochimique des eaux souterraines dans la Plaine de la Bekaa montre un faciès calco-magnésien, bicarbonaté, à saturation vis-à-vis de la calcite et de la dolomite, à faible minéralisation et de faible conductivité. Les formations carbonatées (calcite et dolomite) constituent les roches encaissantes des eaux souterraines dans la Plaine de la Bekaa. La recharge des nappes souterraines est à haute altitude et a comme origine les eaux des précipitations qui tombent sur les hautes montagnes qui encadrent la Bekaa à l'est et à l'ouest. Une dissolution des formations gypseuses existe. La minéralisation se fait en système ouvert. Par conséquent le CO2 d'origine biogénique joue un rôle important dans la minéralisation.
The recharging's water of the aquifers is old and recent, but regarding the chemical facies of the aquifers (carbonated formations), it is very difficult to estimate the real ages of these waters without the use of a correcting's model of the ages after an intervention of the carbonated matrix in the mineralization of waters and in consequence in the activity of the Carbon-14 of the water (dissolution of the carbonated matrix formation with A14C (Activity of the Carbon-14) = 0%). In the Anti-Lebanon, the Cenomanian's waters are older than the Jurassic's waters; in consequence, the rate of renewal of the Jurassic's groundwater is greater than the Cenomanian one.
The stable isotopes of the water molecule (18O, 2H) give informations about the paleoclimate existing during the water seepage and about the recharge conditions of the groundwater. The effects of the Orography, the Continentality, and the origin of the masses of air have an effect on the isotopic abundance of the precipitations (rain + snow) in Lebanon. An evaporation of the recharge water exists in the atmosphere, with a mixing between the deep water and the shallow ones. The sea water has an isotopic abundance at 0 ‰ (SMOW: Standard Mean Ocean Water) for δ2H et δ18O, the Mediterranean Sea is at the origin of the rains which fall on the studied area, and which do not keep the isotopic abundances of the sea because the high mountains that they cross during their movement. This high altitude causes an impoverishment of the water of the rain on heavy isotope (18O).