Hydrochemical characterization and seasonal variability of shallow groundwater in the Sokoto basin northern Nigeria rock weathering as a dominant control factor
Saadu Umar Wali, Noraliani Alias, Abdulqadir Abubakar Usman, Abdullahi Umar, Nasiru Muhammad, Ismail Usman Kaoje, Buhari Samaila, Basiru Aliyu, Habiba Muhammad Ladan, Mudassiru Atiku, Saad Eddin Ibrahim
Abstract
Shallow groundwater in semi-arid basins shows significant hydrochemical variability, mainly influenced by seasonal changes and controlled dominantly by rock weathering processes. Seasonal variations in the hydrochemistry of shallow groundwater can be due to the interactions between internal geochemical processes and external factors such as rainfall and human activities. This study applied seasonal and multivariate statistical analyses to understand the hydrochemical controls on shallow groundwater. The study area was divided into three sub-basins, or clusters (i.e., Birnin kebbi, Sokoto, and Gusau). Fifteen shallow groundwater samples were derived from each cluster, totalling ninety shallow groundwater samples for the dry (April) and wet (July) seasons. The sampling periods have been chosen since they represent the peaks of the two seasons. Physical parameters, including Temperature, Electrical Conductivity (EC), Total Dissolved Solids (TDS), Dissolved Oxygen (DO), and pH, were analysed in situ using handheld metres. However, chemical parameters (Ca 2+ , Mg 2+ , Na + , K + , Fe 3+ , Cu 2+ , Zn 2+ , CO 3 − , HCO 3 − , Cl − , SO 4 2− , PO 4 3− , NH 3 and NO 3 − ) were analysed in the laboratory. These ions reveal hydrochemical processes, anthropogenic inputs, water-rock interactions, and pollution levels essential for evaluating groundwater quality and seasonal variability. Subsequently, statistics were applied to study the impact of seasonality and groundwater evolution. Results of the Mann-Whitney U test revealed that seasonality exerts a considerable influence on shallow groundwater through a significant difference in Temperature, EC, DO, TDS, HCO 3 − , Cl − , NH 3 and PO 4 3− . Pearson’s correlation analysis revealed strong relationships between hydrochemical elements, which suggest natural and anthropogenic influences on shallow groundwater evolution. Correlation results were concurrent with principal component analysis (PCA), hierarchical clustering analysis (HCA), and Piper and Gibbs models. Therefore, this study inferred that seasonality and rock weathering are the primary mechanisms controlling shallow aquifers’ hydrochemistry in a semiarid Sokoto Basin, as revealed by the Gibbs model and statistical analysis. The positive correlations between nitrate and major cations in shallow aquifers suggest potential pollution from agricultural runoff or sewage sources, posing risks to water quality and human health (i.e., methemoglobinemia) and highlighting the need for integrated water resources management approaches to mitigate nutrient loading in these aquifers. The seasonal and multivariate statistics provide a framework for more accurate shallow groundwater quality analysis while considering multiple groundwater quality parameters under different environmental conditions. Seasonal and multivariate statistical analyses provide a user-friendly tool for monitoring shallow groundwater quality monitoring systems in global semiarid environments.