Assessing the extent of leachate impact on surrounding groundwater quality in a low-latitude basement complex area, Southwest Nigeria

Abstract

Assessing the groundwater quality close to sources of man-made contamination, such as dumpsite (DS), is essential for ensuring sustainable groundwater management, especially when such water is used for drinking and agricultural purposes. This study aimed to evaluate the impacts of leachate generated from a nearby dumpsite on surrounding groundwater that serve as a vital resource for local communities. To achieve this, an integrated methodological approach was adopted. Leachate and groundwater samples were systematically collected: three leachate samples were obtained from various locations inside the dumpsite, and ten water samples were drawn from dug-wells located in nearby proximity to dumpsite. All samples were investigated using normal physicochemical procedures to examine the concentrations of various physicochemical attributes. Hydrochemical analysis of leachate revealed that it was stabilized and alkaline in nature (pH range of 9.6 to 9.7 (mean = 9.7)), with several measured parameters exceeding permissible limits set for direct wastewater discharge. This indicates a significant potential for environmental contamination if not properly managed. The groundwater samples were slightly acidic, with an average pH of 6.8, and classified as freshwater, with a mean TDS of 186 mg/L (range: 129–327 mg/L). Despite this, most of the measured quality parameters in sampled groundwater fell within the World Health Organization (WHO) guidelines for safe drinking water. However, subtle changes in groundwater chemistry were observed, which were attributed to natural geochemical processes such as rock-water contact, reverse ion exchange, and minor anthropogenic influences. Hydrochemical facies analysis using a Piper diagram showed that the dominant groundwater type was calcium bicarbonate (CaHCO 3 ) which is typically associated with shallow, recently recharged groundwater influenced by carbonate rock dissolution. Two categories were identified by hierarchical cluster analysis: one primarily associated with temperature, and the other reflected the lithological origin of the dissolved ions. Based on the groundwater quality index (range: 35.96–88.86), 70% of the groundwater samples belong to the "good" set, 20% to the "poor" category and 10% to the "very poor" category.