Soil and groundwater salinization in Siwa Oasis and management opportunities: twenty year change detection and assessment

The use of saline groundwater in irrigation due to the limited availability of fresh water is one of the main causes of land degradation in arid and semi-arid environments. The agro-ecosystem of Siwa Oasis was monitored over a period of two decades. A stable isotope study, oxygen-18 and deuterium, indicated that Siwa groundwater is of meteoric origin. Siwa has two groundwater aquifers: a shallow limestone aquifer, LA, with highly saline water (Total Dissolved Solids, TDS, up to 6848 mg/L), and a deep Nubian sandstone aquifer, NSA, containing fresh water (TDS 192-256 mg/L). The sodium adsorption ratio, SAR, of LA water is high with an average value of 13.9 while the SAR of NSA water is low with an average value of 2.1. Schoeller and Piper's diagrams showed that LA water was of sodium, chloride-sulfate type, while the NSA water was classified as sodium carbonate and bicarbonate water type. Saturation Index (SI) calculations showed that both the LA and NSA water were undersaturated to halite minerals. Consequently, there is room to accommodate extra Na+ and Cl- from halite. Most Siwa land is irrigated with LA water due to low drilling costs. LA groundwater and Siwa soils were found to be deteriorating dramatically and the average water and soil salinity increased by 30-36% per decade. No change in NSA water quality was recorded over the same period. Interventions for the restoration of the deteriorated Siwa ecosystem are summarized. The approach conducted in this study can be repeated in similar ecosystems.