EFFECT OF OPEN DUMPSITE LEACHATE ON GROUNDWATER QUALITY: A CASE STUDY OF SAINT SAVIOUR DUMPSITES, BENIN CITY, EDO STATE, NIGERIA

Groundwater pollution from improper waste disposal is a growing environmental and publichealth concern in rapidly urbanizing regions such as Benin City, Nigeria. The widespread use of unlined, open dumpsites without leachate control has intensified the infiltration of decomposed organic and inorganic wastes into the subsurface, resulting in gradual deterioration of groundwater quality. This study investigates the influence of leachate from two major dumpsites on groundwater in the Saint Saviour area of Ikpoba-Okha LGA, where a dense population relies heavily on groundwater for domestic use. Groundwater samples were collected at radial distances of 100 m, 200 m, and 300 m from Dumpsite 1 and Dumpsite 2. Leachate samples were obtained directly from seepage zones within the waste cells. All samples were analyzed following APHA standard procedures. In situ measurements included temperature, pH, electrical conductivity (EC), total dissolved solids (TDS), and salinity. Nutrient parameters—nitrate, phosphate, and sulphate— were determined using UV-Vis spectrophotometry. Heavy metals (Pb, Cd, Cr, Ni, Zn, Fe, Cu, Mn, Ca, Mg, Na, K) were analyzed through acid digestion (leachate) and Atomic Absorption Spectrophotometry (AAS). Results were compared with WHO (2017; 2021) and SON (2015) drinking-water standards. Findings indicate a clear spatial gradient of contamination: pollutant concentrations were highest within the dumpsites and decreased with distance. Leachate exhibited extremely high EC (up to 6,888 µS/cm), elevated TDS (>3,300 mg/L), high salinity, and pH values ranging from slightly acidic to neutral (4.70–7.46). These patterns reflect the breakdown of organic refuse, plastics, detergents, metals, and other waste materials. The acidity promotes metal solubility, increasing the mobility of toxic elements. Nutrient concentrations were elevated near the dumpsites, with nitrate (0.481–0.585 mg/L), phosphate (0.429–1.584 mg/L), and sulphate (0.070–1.538 mg/L), indicating early nutrient enrichment from sewage, food waste, and detergents. Though below WHO limits, these values suggest a potential progression toward eutrophication and long-term ecological stress. Heavy metals provided the most significant evidence of contamination. Lead (Pb) reached 0.70 mg/L—over sixty times the WHO guideline of 0.01 mg/L. Cadmium (0.01 mg/L), chromium (0.10 mg/L), and nickel (0.06 mg/L) exceeded or approached recommended limits. Their sources include batteries, electronics, metal scraps, plastics, and paints commonly found in municipal waste. These metals are persistent, nonbiodegradable, and pose severe health risks such as neurological damage (Pb), renal dysfunction (Cd), carcinogenicity (Cr⁶⁺), and respiratory disorders (Ni). Iron levels (up to 2.50 mg/L) exceeded aesthetic limits, affecting taste and appearance of water. Overall, results show that the shallow aquifers of Saint Saviour, characterized by the permeable sandy soils of the Benin Formation, are highly vulnerable to leachate migration. The findings align with previous studies highlighting groundwater deterioration around unregulated dumpsites in developing urban centers. This study underscores the urgent need for improved waste-management policies, groundwater monitoring, and the prevention of prolonged human exposure to contaminated water sources.