STUDY OF ENVIRONMENTAL ISOTOPE DISTRIBUTION IN THE ASWAN-HIGH-DAM LAKE (EGYPT) FOR ESTIMATION OF EVAPORATION OF LAKE WATER AND ITS RECHARGETO ADJACENT GROUNDWATER

Oxygen-18 (O-18) and deuterium isotopes were used to estimate the evap oration from the Aswan High Dam Lake and to investigate the inter-rela tion between the lake water and adjacent groundwater. According to sta ble isotopic analysis of samples taken in 1988 and 1989, the lake can be divided into two sections. In the first section extending between A bu Simbel and a point between El-Alaki and Krosko, a remarkable vertic al gradient of O-18 and deuterium isotopic composition was observed. T he second northern sector extending to the High Dam is characterised b y a lower vertical isotopic gradient. In this sector in general, highe r values of O-18 and deuterium contents were found at the top and lowe r values at the bottom. Also a strong horizontal increase of the heavy isotope content was observed. Thus, in the northern section evaporati on is of dominating influence on the isotopic composition of the lake water. With the help of an evaporation pan experiment it was possible to calibrate the evaporative isotope enrichment in the lake and to fac ilitate a preliminary estimate of evaporative losses of lake water. Th e evaporation from the lake was estimated to be about 19% of the input water flow rate. The groundwater around the lake was investigated and samples from production wells and piezometers were subjected to isoto pic analysis. The results indicate that recent recharge to the groundw ater aquifer is limited to wells near to the lake and up to a maximum distance of about 10 km. The contribution of recent Nile water to the groundwater in these wells was estimated to range between 23 and 70%. Beyond this distance, palaeowater was observed with highly depleted de uterium and O-18 contents, which was also confirmed by 14c dating. The age of palaeo groundwater in this area can reach values of more than 26,000 years. Recommendations are given for efficient water management of the lake water.