An index-based approach to assess groundwater quality for drinking and irrigation in Asir region of Saudi Arabia

Abstract

The arid and the semiarid regions around the world mostly rely on groundwater to meet the domestic and other demands, due to absence of surface waterbodies and very inconsistent rainfall. With increasing pollution and extreme climatic influence, the quality assurance of groundwater becomes much important to determine its suitability for domestic needs and agriculture. A random sampling of 62 groundwater samples was conducted in the study region. The groundwater samples were analysed in the laboratory for all the major physico-chemical parameters. The pH of all except 4 samples was found > 7 suggesting dominance of alkaline environment. Results suggest that the major ion chemistry is greatly influenced by the extreme arid climatic condition; however, ion exchange and reverse ion exchange along with dissolution of carbonate minerals govern the major ion chemistry in groundwater. High nitrate (NO3) exceeding the WHO guideline of 50 mg/L is a major public health concern along with high alkalinity and electrical conductivity (EC). Water quality index (WQI) was estimated based on the variables analysed in the lab to determine its appropriateness for drinking. The irrigation suitability of groundwater was estimated based on several indices such as sodium percentage (Na%), permeability index (PI), residual sodium bicarbonate (RSBC), magnesium hazard (MH), Kelly’s ratio (KR), and magnesium absorption ratio (MAR). WQI values of 62.9% of groundwater samples indicate good quality of water followed by 25.8% in poor range of water quality and 6.4% and 4.8% of samples are respectively under excellent and very poor water classes. The irrigation indices, i.e. Na%, RSBC, MH, KR, and MAR, are found within the suitable range; however, 53.3% of the samples has PI values < 25% indicating that water is not suitable and it might increase the permeability of soil which leads to accumulation of ions in soil resulting in low productivity. USSL diagram suggests that 17.7% of the samples falls under C2S1 (medium salinity-low sodium hazard), 72% of the water samples falls under C3S1 (high salinity-low sodium hazard), and 10% of the samples falls under C4S1 zone (very high salinity-low sodium hazard), suggesting good, moderately suitable, and poor water quality for irrigation. The groundwater is found to be impacted mainly due to arid climate as based on SO42−/Cl− ratio, it depicts the similarity with seawater/salt pans. A proper groundwater management strategy and assessment of its suitability should be done before groundwater for domestic and agricultural consumption.