A geochemical approach for the evaluation of water availability and salinity in closed basins: the Draa Basin, Morocco

In the sub-Saharan Draa Basin of southern Morocco, winter snowmelt from the Atlas Mountains is captured in a reservoir, stored, and discharged downstream for irrigation in six oases. The combined imported surface water and shallow groundwater exploitation for sustaining local agriculture is a common practice in many worldwide arid basins.. Like in many basins, the salinization of groundwater in the Draa Basin has become a limiting factor for agricultural development and long-term sustainability. .Since most of the IPCC climate models predict a significant decrease in precipitation in Northern Africa over the next few decades, water shortage and salinization induced from climate change are expected to exacerbate the existing water crisis. Evaluating the sources and mechanisms of this salinization can provide the necessary tools for sustainable water management in the Draa Basin that may be applied to many similarly arid basins. Here we present new geochemical results from 98 shallow groundwater and four surface water samples collected in May 2009 and April 2010. The samples were collected from upstream tributaries to the reservoir, the reservoir, and groundwater from in six oases downstream of the reservoir. The goal of the survey was to identify the sources of salinity using geochemical and isotopic (87Sr/86Sr, δ2H, and δ18O) signatures in the groundwater, which could derive from three possible sources (1) evaporation and recycling of the irrigation water; (2) dissolution of salts that were entrapped in the unsaturated zone; and (3) lateral flow of adjacent groundwater. The data show a large salinity range from fresh water to highly brackish water with total dissolved salts (TDS) exceeding 12,000 mg/L. The salinity increases with downstream flow between successive oases. Br/Cl and B/Cl ratios decrease with TDS, which suggests that the main mechanism of salinization in the Draa Basin is derived salt dissolution in the unsaturated zone and salinization of the underlying shallow groundwater. The 87Sr/86Sr ratio in groundwater slightly increases with distance from the reservoir, indicating some contribution of lateral groundwater with a higher 87Sr/86Sr ratio. Initial oxygen and hydrogen isotopes measurements show that some of the downstream groundwater has lower δ2H and δ18O values relative to the reservoir, while other have respectively higher values. These variations could reflect both contribution of external 18O- and 2H-depleted groundwater sources and further evaporation of the irrigation water that resulted in 18O and 2H enrichments. Overall, the isotopic data provide some unique insights for the hydrological balance and salinization mechanisms of groundwater in the Draa basin, which could be applied to other basins in arid regimes.