Composition and history of ground water in the western Nile Delta

Abstract

A hydrochemical and environmental isotope investigation was conducted in the Nile Delta and adjacent desert fringes within the framework of a routine hydrogeological mapping project. The aim of the study was to obtain additional insight into the historical groundwater flow pattern, to differentiate between fossil ground water and contemporaneous recharge by Nile water. An inventory of existing chemical analyses was made (252 data points) and an additional 27 new samples were taken, which were also analysed for stable isotope ( δ 18 O and δ 2 H) content and, for nine samples, 14C and tritium. In the majority of analyses, irrespective of total salinity, an enrichment in sodium was observed relative to the concentration resulting from direct mixing of Nile water with sewater. The Na-rich water is thought to have originated in the extreme arid period from 14000 to 10000 years B.P. (last phase of the Weichsel glaciation) in which, by direct evaporation of (scarce) rain, trona minerals were formed at the surface, and the ground water in phreatic aquifers gradually became saline. The succeeding humid phase from 8000 to 5000 years B.P. when sea-level was relatively high, caused freshening on the desert fringes only, because the Delta floodplain was covered by clay. The present arid phase, which started at around 3500 years B.P., also marked the start of widespread floodplain irrigation. Groundwater levels on the desert fringes dropped as a result of both aquifer depletion by discharge to the Qattara Depression in the west and climatic effects. This in turn initiated a freshening process in the Quaternary and Moghra aquifer of the Delta. Nile water has since then reached Wadi Natrun, 55 km to the west of the western floodplain border.