The Souss-Massa Basin, an important agriculture area of Morocco, is located in a semi-arid area and its main water resource is the groundwater. Over the last decades, population growth, agriculture development and recurrent droughts have been increasing the pressure on the aquifers of the region.
In the this context of climate variabilities (changes) and increasing demand for water, it is crucial to identify the origin, groundwater flow dynamics and mean residence times for sustainable management of water resources. In this study, groundwater residence time and recharge conditions of the Souss-Massa aquifer system were investigated using environmental isotopes (δ18O, δ2H, δ13C, 14C, 3H) and noble gases. The results shows that the aquifer is mainly recharged by waters derived from the High Atlas Mountains, where the precipitation rate is higher than in the study area. The spatial variations of stable and radioactive isotope concentrations indicate that active mixing of recently recharged and old groundwater within the aquifer system. The recent recharge is observed mainly along the Souss River and in the irrigated areas.
According to radiocarbon activity, two groups of waters are defined: recent (14C activity >60 pmC) and old (<30 pmC). Groundwater plotting between these two types can be used as evidence of water mixing processes. Based on the mean regional temperature of 20°C, we can also distinguish two groups of samples: low and relative higher noble gas recharge temperature. Recent and sub-recent waters show recharge temperatures ranging from 13.5 to 24.5°C, with an average around 18.3°C, which is consistent with the monthly inter-annual average temperature of 18.75°C ± 1°C recorded at the meteorological stations Agadir and Taroudant. However, the estimated noble gas temperatures in areas located near to the recharge zones, are still about the annual average local temperatures reflecting relative low contributions of water from the high elevated areas of the Atlas Mountains. The NGT of all samples collected from shallow wells are generally midrange of their annual average temperature at each location. The Turonian water has a recharge temperature of about 18 °C with a radiocarbon age higher than 19000 years BP.
The study clearly showed that long term recharge conditions and groundwater flow processes, are influenced by natural climatic variability and anthropogenic impacts.
This study indicates the extent of groundwater renewability and allows establishing a scientifically-based water management model for sustainable exploitation of water resources in the Souss-Massa Basin.
Keywords: Stable isotopes, Radiocarbon, Noble gases, Paleotemperature, Groundwater age.