Integrated framework for the assessment of shallow groundwater quality in semiarid basin

Abstract

Groundwater is the water that is found below the earth‘s surface. It can be divided into two categories which are shallow and deep groundwater. Shallow groundwater (SGW) quality is threatened by both natural processes and human activities. Thus, understanding groundwater quality requires an integrated approach. This study aimed to assess SGW quality using an integrated framework which combines geochemical modelling, and statistical and seasonal analyses. Theoretically, SGW is not impacted by seasonal and geological variability. Thus, the SGW is suitable for drinking and irrigation uses. The semiarid Sokoto Basin in Northern Nigeria was selected as the study area. Due to its geological and climatic characteristics, the basin was divided into three sub-basins, i.e., Birnin kebbi, Sokoto and Gusau which are located in three different states in Northern Nigeria. Ninety SGW samples were selected and analysed in different seasons. Physical parameters were analysed in situ while the chemical parameters were analysed in the laboratory. Geochemical modelling and statistical analysis were applied in the hydrochemical evolution of groundwater. The study also used drinking and irrigation water quality indices to measure its suitability for drinking uses. The findings showed that SGW quality was controlled by rock weathering. However, seasonality and geological variation have impacted SGW quality through significant variation (p = 0,001) in Temperature, Electrical Conductivity (EC), Dissolved Oxygen (DO), Total Dissolved Solids (TDS), Bicarbonate (HCO3-), Chloride (Cl-), Ammonium (NH3) and Phosphate (PO43-). It was found that SGW quality is not suitable for drinking purposes in some locations. Water Quality Index (WQI) was 90.09, 122.52 and 51.76 in Birnin kebbi, Sokoto and Gusau during the dry season. WQI was 119.18, 148.69 and 69.79 in Birnin kebbi, Sokoto and Gusau during the wet season. Based on mean WQI SGW is unsuitable for drinking. Heavy Metal Pollution Index (HPI) during the dry season was 47.32, 133.75 and 189.73 in Birnin kebbi Sokoto and Gusau. HPI was 55.25, 122.29 and 113.75 in Birnin kebbi, Sokoto and Gusau during the wet season. HPI values above 38 indicate high pollution. Irrigation water quality was significantly impacted by seasonal and geological differences. SGW is threatened by high Magnesium Adsorption Ratio (MAR), and low Sodium Adsorption Ratio (SAR). Mean MAR was above 50 in which high magnesium destroyed soil structure and reduced crop yields. Meanwhile, a low SAR of 0.97 inferred the plant osmotic pressure. Statistically, this study inferred that seasonal and geological variability significantly influenced SGW quality in the semiarid Sokoto Basin. Hence this study has established a framework for assessing groundwater quality in semiarid environments.