Groundwater quality assessment of northern Harrat Rahat Aquifer, Saudi Arabia

Basaltic aquifers represent one of the major groundwater resources in the Arabian Shield, which lies in the western part of the Kingdom of Saudi Arabia. This study aimed to evaluate the quality of groundwater drawn from the Tertiary and Quaternary basaltic aquifer system of the northern part of Harrat Rahat in Al-Madinah Al-Munawwarah area for the purposes of household use and irrigation. Forty-five groundwater samples were collected and analyzed for physical and chemical parameters: pH, HCO₃^–, Na⁺, K⁺, Mg²⁺, Ca²⁺, SO₄^2–, Cl^–, CO₃^2–, electrical conductivity, total dissolved solids, and total hardness. Groundwater drinkability was assessed based on a water quality index (WQI) and the results were compared to international and local water quality standards. Sodium adsorption ratio (SAR), residual sodium carbonate (RSC), corrosivity ratio (CR), and boron toxicity (BT) were used to assess the suitability of the sampled groundwater for agricultural uses. The major cations abundance in groundwater are in the order Na⁺ > Ca²⁺ > Mg²⁺ > K⁺, while the major anions are Cl^– > SO₄^2–. A Piper diagram indicates that there are three water types in the study area: sodium chloride (NaCl), calcium chloride (CaCl), and mixed water. Based on the WQI analysis, the groundwater was classified into different categories that link its suitability for drinking purposes. Generally, 93% of the groundwater shows very poor to unsuitable quality for household uses. SAR, CR, RSC, and BT analyses indicate that the existing groundwater is considered permissible for cultivation activities.