Determination of some total and bioavailable heavy metals in farmland soil around Rivers Niger and Benue in Lokoja, Nigeria

Abstract

The work assessed the distribution of Cd, Cu, Mn, Ni, Pb and Zn in the farmland soils around Rivers Niger and Benue and beyond the confluence in Lokoja, Nigeria. The samples were collected in the dry and rainy seasons of 2013 and 2014, digested with aqua regia and analysed for heavy metals using atomic absorption  spectrophotometry. The soil pH ranged from 6.4 to 7.2 for the  farmland soils collected 50 m away from the bank of the rivers and each point 10 m apart (n=72). There was no significant  difference in the mean values of the organic matter, organic carbon and moisture contents of the soils across the seasons. Cd concentration (mg/kg)in the soil samples collected in March 2013, June 2013, January 2014 and May 2014 at 0 – 15 cm depth ranged from 3.95 - 8.4; Cu 11.6 - 20.2 ; Mn 150.3 - 211.5; Ni 177.0 - 281.0 ; Pb 20.3 - 34.2 and Zn 40.5 - 77.8. The levels of the metals in the soil follow the ranking: Ni > Mn > Zn > Pb > Cu > Cd; with Cd and Ni being above European Union permissible limits. The order of the percentage of extracted metal by EDTA to the total metal content was Pb>Zn>Cu>Ni>Mn>Cd. I-geo  pollution index showed that the farmland soils were moderately contaminated with Ni and Cd. Hence, there is the need to enforce environmental laws that borders on proper solid waste and effluent disposals.

Key Words: Heavy Metals– Pollution index – River Niger and Benue – Soils – Season