ISOTOPE HYDROLOGY STUDY OF THE AXIOS RIVER PLAIN IN NORTHERN GREECE

The isotopic composition of surface water and groundwater was used to investigate the mechanism of recharge of groundwater in the Axios River plain in northern Greece. Sampling of springs, for which the mean elevation of recharge could be estimated, in the Paikon Mountains provided an estimate of -0.28 parts per thousand per 100 m for the altitude effect. The deltaO-18 estimated for the recharge of the Neogene formation between the river valley and the Paikon Mountains agreed with many of the sampled locations. However, in the southwestern part of the area there is an indication of a contribution from the limestone formation of the Paikon Mountains. The stable isotopic composition of shallow groundwater close to the river indicates contributions from the river. Deeper groundwater farther away from the river also contains a proportion of infiltrated river water. Some of the boreholes close to the river have chlorinities ranging up to about 6000 mg l-1. The stable isotopic and chemical data indicate that the salinity is due to varying proportions of sea water, infiltrated river water and local recharge. The isotopic composition of Aravissos Spring water suggests that the spring is recharged by the upper series of the semi-metamorphic Paikon zone (dolomitic limestones, spilites-keratophyres and Gropi series; the mean altitude of recharge computed for the area so defined is 867 m and its potential annual yield, i.e. the amount of annual precipitation effective in recharge in this area, is 43.6 +/- 10 x 10(6) m3). This suggests that the actual annually exploited quantity of the spring water for the water supply of Thessaloniki (about 44 x 10(6) m3) is at the limit of the corresponding potential yield.