Integrated geophysical and hydrochemical investigations of subsoil and groundwater pollution around three cassava processing factories at Ireti-Ayo community, Ilesha, Southwestern Nigeria

Abstract

Integrated geophysical and hydrochemical investigations were carried out within the vicinity of three cassava processing factory sites at Ireti-Ayo Community in Ilesha, Southwestern Nigeria. This was with a view to assessing the impact of the cassava effluent on the soil/subsoil and the groundwater quality within the study area. Fourteen Vertical Electrical Soundings (VES) data were acquired using the Schlumberger array while 2-D Dipole-Dipole data were acquired along six traverses. Hydrochemical analysis was also carried out on the cassava effluent and well water samples for cyanide, pH, TDS, electrical conductivity, acidity, alkalinity, cations and anions concentrations. The VES data were quantitatively interpreted using the partial curve matching and 1-D computer assisted forward modeling while the 2-D Dipole-Dipole data were inverted into 2-D resistivity structures. The elemental concentrations were compared with the WHO (2006) standard for potable water. The VES derived geoelectric sections identified four lithological layers which comprised the topsoil (8 – 638 Ωm), laterite (278 – 2468 Ωm), weathered basement (32 – 1004 Ωm) and fresh basement (536 – 20883 Ωm). The thin topsoil was underlain by a relatively thick lateritic layer (up to 12.2 m). The sampled cassava effluent resistivity was6.5 Ωm. The topsoil and part of the underlying laterite at and around the vicinity of the three cassava processing factory sites had relatively low resistivity values (6 - 112 Ωm) diagnostic of cassava effluent impacted zones. The pollution plumes depth extents were estimated to range from 2.5 to 5 m with a migration rate of about 0.24 m/year. The migration of the pollution plume may have been inhibited by the relatively thick column of laterite underlying the topsoil. Cyanide was detected (0.8 mg/L) in the cassava effluent, but not in the analysed well water samples. For both the upstream and downstream well water samples respectively, the physicochemical parameters were all within the WHO permissible levels. The study concluded that the groundwater in the vicinity of the factory sites investigated had not been polluted by the cassava effluent. However, the subsoils within the sites have been polluted to depth of up to 5 m.

Keywords: Geophysical Investigation, Hydrochemical Analysis, Cassava Effluents, Groundwater Quality.