Hydrochemical Characterization and Water Quality Index Modelling of Shallow Groundwater in Igarra Town, Northern Edo State, Nigeria: Seasonal Dynamics and Drinking Water Suitability

Shallow groundwater resources in the basement complex terrain have been identified as the main source of water for millions of rural dwellers in Nigeria. However, the hydrochemical characteristics of the groundwater and its potability have been less understood in many rural areas. This study carried out a comprehensive hydrochemical characterization and Water Quality Index (WQI) assessment of twenty hand-dug wells in three municipal districts: Uffa, Itua, and Ugbogbo in Igarra town, Akoko-Edo Local Government Area, northern Edo State, Nigeria. The study was done using data collected during the wet season (July 2011) and the dry season (December 2011). Eleven physiochemical parameters: pH, electrical conductivity, total dissolved solids, turbidity, total hardness, total alkalinity, sulphate, chloride, nitrate, phosphate, and manganese were evaluated against WHO permissible limits for water quality set in 2011, SON permissible limits set in 2007, and FME permissible limits set in 1996. The weighted arithmetic method was used to calculate the WQI for all the twenty wells in the wet and dry seasons. The study found that 85% of the wells have water quality classified as 'poor' and 'very poor' for the wet season. In the dry season, the percentage increased to 95%. The study showed that high turbidity, supraoptimal electrical conductivity, and pH values beyond the recommended range of 6.5–8.5 were the main factors affecting the water quality. Hydrochemical facies were classified using the ionic ratio method. The study showed that the hydrochemical facies were predominantly Ca²⁺–Mg²⁺–HCO₃⁻. The result of this analysis revealed that rock-water interaction was a major hydrochemical phenomenon controlling groundwater chemistry. In addition, WQI spatial mapping revealed a groundwater chemistry deterioration gradient from northeast to southwest that was generally related to regolith thickness and organic matter content. This study offers an evidence-based guideline for managing Igarra groundwater chemistry in seasons and aligns with SDGs 6, 3, and 11.