VARIATION OF LEVELS OF SELECTED METAL POLLUTANTS WITH DEPTH IN PETROLEUM CONTAMINATED SOILS AND THEIR IMPLICATIONS

Abstract

Human activities have led to increased metal contamination in the environment, among these is petroleum spillage. Effect of petroleum spillage on the Fe, Pb, Ni, Zn, Cd and Cu levels extracted from petroleum-contaminated soils was investigated at different seasons and soil depth. Forty soil samples were collected from four sites (S1- S4) in both wet and dry seasons at 15 cm depth intervals from 0 - 75 cm and kept in clean polythene bags. Stones and plant fragments were immediately removed by passing the samples through a < 0.2 mm sieve and then crushed to fine powder using mortar and pestle. The concentration of metals were analyzed using atomic absorption spectrophotometer. For S1 at shallower depth (0-30 cm), the levels were 3.95 ppm, 0.51 ppm, 0.05 ppm, 0.20 ppm and 0.08 ppm for Fe, Zn, Ni, Pb and Cu, respectively. At deeper depth (45-75 cm), the corresponding values were 3.31 ppm, 0.55 ppm, 0.55 ppm, 0.27 ppm and 0.10 ppm for Fe, Zn, Ni, Pb and Cu. As for S2 at shallower depth, the values were 4.83 ppm, 0.72 ppm, 1.00 ppm, 0.47 ppm and 0.10 ppm for Fe, Zn, Ni, Pb and Cu, respectively. At deeper depth, the respective values were 3.20 ppm, 0.82 ppm, 0.42 ppm, 0.42 ppm, and 0.00 ppm for Fe, Zn, Ni, Pb and Cu. Site 3 at shallower depth had respective values 4.67 ppm, 0.69 ppm, 0.57 ppm, 0.44 ppm and 0.14 ppm for Fe, Zn, Ni, Pb and Cu, whereas at deeper depth, the corresponding values were 3.48 ppm, 0.67 ppm, 0.70 ppm, 0.11 ppm and 0.13 ppm. For S4 at shallower depth, the levels were 4.72 ppm, 0.68 ppm, 0.54 ppm, 0.47 ppm and 0.11 ppm for Fe, Zn, Ni, Pb and Cu, respectively. At deeper depth, the levels were 4.00 ppm, 0.78 ppm, 0.64 ppm, 0.45 ppm and 0.14 ppm for Fe, Zn, Ni, Pb and Cu, sequentially using one way ANOVA. There were insignificant statistical differences between the four sites with p > 0.05. Levels of Fe and Pb decreased with depth, whereas Zn and Ni levels increased with depth. Copper and Cd had little vertical movements. Variation of Ni with depth was statistically significant (p = 0.01098, F = 3.83) unlike Zn, Pb, Cu, Fe and Cd and this could threaten ground water. Wet season (April 2013) had higher mean metal levels than the dry season (Dec 2012) and this was significant for Ni, Zn and Cu (p < 0.05, t > 1, r > 0.1). A significant correlation (p <0.05, r > 0.5) existed between mean metal levels and petroleum spillage signifying spillage of petroleum elevates metal levels in soil. Mean Pb levels were above the crude oil quality group (COQG) permissible limits at sites S2, S3 and S4. Regular assessment of elements from petroleum should provide a framework for control. Heavy metal permissible limits should be included in assessment of petroleum quality by the Kenya Bureau of Standards.