Groundwater evolution and vulnerability in semi-arid regions using modeling and GIS tools for sustainable development: case study of Wadi Fatimah, Saudi Arabia

Arid and semi-arid regions suffer from multiple critical water resources' problems such as scarcity of water resources and the extensive exploitation of groundwater for different uses. These water resources problems will certainly cause declining in water tables and degradation of groundwater quality, and will raise in the near future due to the climate change and the population growth. This study will focus on Wadi Fatimah basin, which is considered the main source of water supply for many cities and villages in Makkah Province and a valuable agricultural production source. The present study aims to evaluate the hydrogeological and hydrochemical aspects of groundwater evolution and vulnerability, by the integration between Geographic Information System (GIS), hydrochemical modeling of NETPATH, and DRASTIC model. In this study, static water levels were measured in 64 wells, and 59 water samples were obtained and analyzed for concentrations of major ions, trace elements, and potentially toxic elements (PTE). The total dissolved solids (TDS) ranged from 277 to 18,518 mg/L, with a mean of 2795 mg/L, and approximately 60% of the samples were brackish water, with TDS > 1000 mg/L. The groundwater samples fall within Na-Cl-SO4 and Ca-Mg-Cl-SO4 water types. The main processes affecting the groundwater chemistry in the area tend to be evaporation and reverse ion exchange. The changes in groundwater quality that could occur because of various geochemical processes (evolution and mixing phenomena) were modeled by NETPATH. The quantities of minerals (in mmol/Kg water) that dissolved or precipitated along the flow path and/or mixed with various sources such as rainwater and seawater were calculated. Sixty-four drilling wells were also used in generating a groundwater vulnerability map based on seven parameters of the DRASTIC model. The majority of the Quaternary aquifer in Wadi Fatimah basin was represented as low-to-moderately vulnerable zones (64%, i.e., protected areas). A small portion of the aquifer in the upstream of the Wadi was identified as the very highest vulnerable zone (about 4.7%). On the other hand, a relatively high groundwater vulnerability zone was observed in the scattered sectors of the region (about 31.3%, i.e., not protected areas). The validation of the DRASTIC map using the spatial distribution map of nitrate indicated that the generated vulnerability map in Wadi Fatimah basin can be acceptable.