INVESTIGATION ON GROUNDWATER QUALITY FROM CASSAVA EFFLUENT AT AJEGUNLE AREA, OYO STATE, NIGERIA.

Cassava processing generates large quantities of acidic, cyanide-containing wastewater, which poses a possible threat to groundwater quality in communities reliant on wells and boreholes for domestic water supply. This study investigated the groundwater quality at five sampling points: two hand-dug wells (S1, S2) and three boreholes (S3, S4, S5) in the Ajegunle area of Atiba Local Government Area, Oyo State, Nigeria, where multiple small-scale cassava processing units discharge effluent directly to surrounding land surfaces. Groundwater samples were collected during the dry season and tested for pH, turbidity, total dissolved solids (TDS), biochemical oxygen demand (BOD), chemical oxygen demand (COD), total coliform, and fecal coliform. The results were compared with the permitted limits of the World Health Organization's (WHO, 2017) permitted limits, the Nigerian Standard for Drinking Water Quality (NSDWQ, 2015), and the National Environmental Standards and Regulations Enforcement Agency (NESREA, 2011). We evaluated the weighted arithmetic water quality index (WQI), entropy-weighted water quality index (EWQI), pollution index (PI), BOD:COD ratio, and coefficient of variation (CV). The five samples met the regulation standards set by the three agencies for pH, turbidity, TDS, total coliform, and fecal coliform, and no coliform bacteria were found in any of the samples. However, the integrated assessment indices showed that the hand-dug well at S1 was the most affected sampling point. S1 had a TDS value of 493 mg/L that exceeded 98.6% of the NSDWQ allowable threshold, a mean PI of 0.8816 under NSDWQ standards that indicated a high pollution burden, and an EWQI classification of Very Poor. The BOD:COD ratio for all samples was below 0.14, indicating that the biodegradable organic fractions from the cassava effluent were weakened during vadose zone infiltration, leaving primarily nonbiodegradable residual organic matter at the water table. Boreholes had fewer contamination indicators than hand-dug wells. The study showed that the groundwater in Ajegunle presently complies with regulatory standards, but there was partial contamination pressure, especially at shallow well sites, and that the continued unregulated discharge of cassava processing effluent will lead to progressive groundwater quality deterioration. Cassava processing operators should implement basic effluent pretreatment, establish routine groundwater monitoring during the wet and dry season groundwater monitoring, and ensure that relevant regulatory authorities enforce minimum setback distances between effluent disposal sites and groundwater abstraction points. This study provides evidence-based data to promote sustainable groundwater resource management and reinforces SDG 6: Clean Water and Sanitation.