Heavy Metal Status of Soils Affected by Leached Ash Refuse from Black Soap-Making at Ikere-Ekiti, Nigeria

Abstract

Large quantities of leached wood ash residue, after obtaining lye needed for black soap-making at a facility in Ikere-Ekiti, Nigeria, had piled up on the surrounding land during over five decades of operation. Surface (0-15 cm) and sub-surface (15-30 cm) soil samples taken at 0, 15, 30, 60, 75, 105 and 150 m from the ash refuse pile were analyzed and compared with an area not impacted by ash. The soils were slightly alkaline to moderately alkaline silty loam to sandy clay loams with medium to high organic matter, available P and exchangeable cations. The valley bottom (150 m) contained the highest values of these chemical fertility indices while Ca and Mg were most abundant at 0 m. The control soil sample was slightly acid with low organic matter and exchangeable cations. Distribution pattern of the heavy metals differed in relation to distance from the ash refuse pile: Co increased from 0 m to 75 m and decreased thereafter; Cr decreased from 0 m to 30 m and increased to the highest value at 60 m; Cd increased with distance; Pb had highest value at 30 m of the topsoil and 60 m of the sub-soil; Ni was highest at 150 min the topsoil and sub-soil. Soil pH showed significant negative correlations with Co and Cd in the topsoil and with Co and Ni in the sub-soil. Accumulative factors- contamination factor (CF) and pollution load index (PLI) - were calculated to assess heavy metal pollution. Contamination with Co, Cd and Ni was low in the surface and sub-surface samples and moderate to considerable for Cr and Pb. Pollution Load Index shows low heavy metal accumulation in the topsoil and sub-soil contamination at 60 and 90 m from the ash refuse pile. The short- and long-term health implications of heavy metal contamination of soils in the vicinity of ash refuse pile were discussed.

Keywords: wood ash, topsoil, subsoil, heavy metals, soil analysis, contamination, index analysis approach