Assessment of toxic heavy metal loading in topsoil samples within the vicinity of a limestone quarry in South Western Nigeria

Abstract

Loading of Pb, Cd, Cr, Cu, Co, Mn, Ni, Zn, and Fe in topsoil samples (TS) collected from the vicinity of limestone quarry in South Western Nigeria was investigated. Thirty TS were sampled within ½ km radius from exploration area and 5 background samples from undeveloped area 10 km away from the study area. Limestone rock samples (RK) were also analyzed for metals’ content. All samples were pretreated and leached with appropriate acid solutions for some properties. All leachates were analyzed with atomic absorption spectrophotomer (AAS) technique. Soil pH ranged from 6.65 to 8.23, sand from 55.8 to 75.0%, silt from 16.6 to 34.6%, clay from 8.43 to 13.6%, and organic matter from 0.97 to 4.84%, respectively. These properties compared with those of background samples. Rock samples (RK) showed high Fe and Mn enrichment. Metals levels (mg/kg) in TS with background levels in parenthesis ranged as follows: 11.5 to 27.7 (10.5) Pb, 0.28 to 3.18 (0.55) Cd, 12.1 to 17.4 (5.50) Cr, 80.6 to 55.8 (3.81) Cu, 8.93 to 23.5 (11.1) Co, 262 to 710 (637) Mn, 6.34 to 17.4 (6.10) Ni, 36.0 to 620 (24.4) Zn, and 6585 to 13440 (4563) Fe. The elevated enrichment suggests influence from exploration activities. Residual phase showed highest enrichment for all metals possibly, because of high sand content. Positive correlations were shown between all metals, except Mn and Cd that were negative. Geoaccumulation index rating showed <0 for Mn denoting uncontamination, while others ranged from 1 to 2 indicating moderate contamination.