Groundwater quality evaluation and vulnerability assessment of Wadi Al-Arj alluvium aquifer, Taif, Saudi Arabia

Abstract

Wadi Al-Arj is the downstream part of Wadi Wajj that flows from SW to NE through Taif city, western Saudi Arabia. The wadi is underlain by a shallow alluvium aquifer. Groundwater from the aquifer is the main source for irrigation and some domestic uses in the area. There are several agricultural and municipal activities along the Wadi that can be a source of contamination to the aquifer groundwater. These are poultry farms, private agricultural farms, a wastewater treatment plant, and settlements along the Wadi banks. This study was aimed at evaluating the groundwater hydrochemistry, quality of the aquifer and its suitability for drinking and irrigation purposes as well as assessing the aquifer vulnerability for contamination using the DRASTIC method. Forty-three water samples from wells and surface runoff and ponds were collected and analyzed for their chemical composition and bacteria content. Both groundwater and surface water have similar chemical signature, which is mostly of chloride-sulfate and sodium-potassium type. Samples collected in the wet season have less constituent concentration than the dry season samples. Concentration of most of the groundwater quality indicators exceeds the maximum allowable limits in international and local standards for drinking water. However, concentration of the trace and heavy metals in groundwater are either very low or non-detectable. The chemical analysis indicates strong affect of stream runoff and sewage water on the groundwater quality as implied by low pH, high nitrate, chloride, sulfate, and bacteria concentration. High levels of boron concentration, sodium adsorption ratio, residual sodium carbonate, and magnesium hardness suggest that the groundwater is of poor to medium quality for irrigation purposes. Using a GIS environment, the DRASTIC index values and map for the alluvium aquifer of Wadi Al-Arj were generated for both municipal and industrial pollutants, and agricultural pesticides. The index values are relatively high and indicate that the Wadi Al-Arj aquifer is vulnerable to pollution. A relative classification of the vulnerability index within the Wadi Al-Arj aquifer shows that the central and downstream parts of the Wadi are most vulnerable to contamination. The result of the vulnerability study suggests the need to modify DRASTIC method to be more suitable for arid and semi-arid conditions: As a source for domestic and irrigation water, a recreational area, and as an aquifer of high vulnerability, there is a need for close monitoring and detailed study of the groundwater quality in the aquifer to minimize the potential of groundwater contamination.