Study of 2H and 18O in the Kouris catchment (Cyprus) for the description of the regional groundwater balance

The Kouris catchment covers 300 km2 on the southern side of the Troodos Massif of Cyprus and bounded on the South by the Mediterranean Sea. Elevations range from 2000 m to sea level within a distance of 30 km. The basin consists of an ophiolitic complex in the North (ultramafic rocks, gabbros, sheeted dykes and pillow lavas) and an overlying sedimentary complex in the South (chalks, marls, calcarenites and limestones). The ophiolites highly heterogeneous and contain the major groundwater resources in Cyprus. While the geology of the Troodos Massif is very well documented, the hydrogeology of the area is much less known. The aim of this study is to apply 144 new and 72 published analysis of δD and δ18 in groundwater as well as as new and reported in IAEA database analysis of rainfall samples for better understanding of main mechanisms of ground water flow and for checking the assumptions of the groundwater model, developed at the previous stage. 2H and 18O in Precipitation. 2H and 18O contents in rainfall over the Kouris catchment samples are linearly related by the equation of the local meteoric water line: δD = 6.5*δ18 + 10.6 that differs from the equation of the Global Meteoric Water Line by a smaller slope, indicating partial evaporation of rain drops prior to infiltration. Seasonal variations might be extremely high (-36 per mille for δD and -5.5 per mille for δ18 in 2000/2001). Averaged monthly mixed rainfall samples for October-February 2000/2001 at the two meteostations and some data from springs at the highest altitudes suggest the regression equation for δD(per mille) and the altitude(m): δD = -0.012H(m) - 24,86 (1), where H(m) - the altitude of the rainfall 2H and 18O in groundwater. Almost all points at the 2H versus 18O diagram (except some in the ophiolites) are displaced from the local meteoric water line due to evaporation. Samples taken from the sedimentary rocks seem to be more affected by evaporation than those in ophiolites. Seasonal changes due to evaporation in ground water over the whole catchment are clearly illustrated by the stable isotopes contents. In order to analyze the stable isotopes altitude effect in the aquifer, we eliminated samples with the clear evidence of evaporation; 108 remaining samples were used for further analysis. For every sample of groundwater the altitude of the rainfall was calculated as a function of δD(Eq.1). The scatter diagram between altitudes of sampling points, obtained from the Digital Elevation Model and altitudes of the recharge areas and calculated from the 2H content, are shown. Three groups of groundwater can be clearly distinguished: sedimentary complex, ophiolitic complex and alluvial aquifer. Sedimentary and ophiolitic complexes seem to contain the groundwater from the local recharges at the low and high altitudes respectively, although uncertainty due to evaporation effect might be high, especially at the low altitudes. On the contrary, alluvium complex in the lower part of the catchment contains water from considerably higher altitudes, than the altitudes of the sampling points. The analysis proved the result of the groundwater model, carried out at the previous stage, that only negligible amount of water flows from the ophiolites to the sediments. Additionally the δD data were input in the model for the calibration of the recharge rates. The model calibration only by piezometric heads resulted in several reasonable variants with different input recharges. Using stable isotopes for calibration allowed to choose the optimal variant