keywords: Baseline concentration, geo-accumulation index, heavy metal, wet digestion
The potential threat of heavy metals to soil quality and human health has led to many studies on baseline concentrations of these metals in soils; because, knowledge of baseline concentration of heavy metals in soils has become a tool for soil quality determination in the world today. On this note, forty composite surface soil samples were collected from areas of native forest with no apparent anthropogenic influence in Kogi state. The soil samples were randomly collected at the depths of 0–20 cm, air-dried, crushed and sieved with 2 mm mesh sieve. The concentrations of five heavy metals (Cd, Pb, Ni, Cu, and Zn) were quantified using Atomic Absorption Spectrophotometer with model, AA280FS after digestion with HNO H2O HCI mixture (USEPA method 3050B). Data analysis was performed using mean and range to obtain heavy metal concentrations in the soil. Geo-accumulation indices were calculated to determine the levels of heavy metal contamination in the soil. To establish the accuracy of the method used, standard reference material (WEPAL sample 989) was used and the recovery ranged from 81 to 91%. These values were within the acceptable range of 80 to 120% expected for the metals indicating good accuracy for the analysis procedure. The results obtained revealed that the mean heavy metal concentrations (mgkg-1) were: Cd, 0.44±0.28; Pb, 23.06±7.29; Ni, 17.49±8.13; Cu, 6.34±5.38 and Zn, 18.72±15.84, respectively. This study has clearly indicated that, all of the mean concentrations are within the normal range for uncontaminated soils. Also, results for the geo-accumulation index were -0.38 for Pb,-0.03 for Cd, -2.54 for Ni,-3.41 for Cu and -2.93 for Zn. All the geo-accumulation indices were less than zero (Igeo < 0) which indicated no heavy metal contamination in the soil. Apparently, these forest soils pose no health threat to humans or ecosystems. The concentrations of the studied heavy metals can be taken as the baseline which could be useful as soil quality guideline in Kogi state, Nigeria.