Implications of climate change on the groundwater flow regime and geochemistry of the Nile Delta, Egypt

Environmental isotope analyses in conjunction with the hydro-geochemical investigations and tentative reviewing of the paleoclimatic sea level changes are carried out to fingerprint the implications of climatic changes on the groundwater flow regime and geochemistry at the Nile Delta. Following up the footprints of groundwater flow history, it is observed that the Pleistocene, main groundwater aquifer of the Nile Delta was drained and refilled with Nile water several times due to the eustatic sea level propagations between dry and wet periods. Therefore, the present-day groundwater flow regime could be affected by the latest Holocene phase of climate changes during which no significant dramatic sea level changes were recorded. After the time slice of the Mediterranean humid phase, 8000-5500 BP, the sea level started to rise steeply from –15 m to the present-day level. Under this rising rate, the seawater invaded most of the northern delta lope and several 10ᵗʰˢ of kilometres inland via the mouths of the ancient Nile branches. During these arid conditions, deterioration of the Pleistocene unconfined aquifer could take place. This finding matches a famine and economic instabilities during the arid periods and flourishing and economic stabilities during the humid pluvial periods along the history of the ancient Egyptian civilization. In accordance to the latest active sea level rise stage in conjunction with the delta subsidence, a contagious groundwater level rise with a recent order of 3 cm/year is taking place leading to form several lake-like lagoons, water logging and soil salinization along the coastal plain and the eastern low lands. The Nile Delta is expected to suffer extreme soil salinization and gradual merging under the groundwater logging and seawater transgression especially, along the eastern coastal zone which suffers a high subsidence rate of about 5 mm/year. In contrast to previous studies, our findings show that, the present groundwater composition and salinity in the Nile Delta aquifers cannot be attributed to a recent seawater intrusion. The physico-chemical processes that explain this composition are combination of salt dissolution (mainly from Holocene fluvio-marine aquifer), flushing by recent Nile water, ion exchange and evaporation.