Accumulation and toxicity of lead in soil along the road verges in the City of Cape Town

Abstract

The widespread use of lead (Pb) and especially Pb from vehicular emissions arising from lead additives in petrol has resulted in high levels of this metal found in various soil samples taken along the road verges of Cape Town CBD. The accumulation of lead was investigated in roadside surface soil by collecting soil samples at various sites along the three major highways (N7, N2, N1), approximately three metres from the road verges and at a depth of approximately 5 cm for a period of 12 months. After digestion with 55"70 nitric acid the Pb concentrations were determined by using an Inductively Coupled Plasma Atomic Emission Spectrophotometer (ICP-AES).

Results have shown the Pb concentrations in the soil at busy intersections to be higher than at other areas along the roadsides. Lead concentrations found in the roadside soils of the N1 ranged between 200 and 2000 mg/kg and these were of the highest concentrations found compared to the other two highways and were even higher than found in other studies. The roadside soils on the N1 highway, with the most traffic, according to car count data obtained, seemed more contaminated than the other two highways.

It was also investigated whether earthworms (Eisenia fetida) accumulated Pb after being exposed to the contaminated soil from the sampling sites. The earthworms in the highly contaminated soil accumulated on average much higher concentrations of Pb than the earthworms in the lower Pb contaminated soil.

A potential biomarker (cell membrane integrity) was applied to determine whether the earthworms experienced toxic stress as a result of the exposure to lead contaminated soil. Behavioural and morphological changes in the earthworms were also observed. The Trypan blue exclusion assay was used to measure the effect of lead exposure on the membrane stability of the coelomocytes in the coelomic fluid of earthworms. In the highest exposure groups per highway, a significant decrease in percentage viable cells were seen (N7, 36 ± 0.07%; N2 48 ± 0.09%; N1, 34 ± 0.08%). The fact that clear statistically significant responses were seen after the five week exposure period in the highest, as well as lowest exposure groups indicate that these responses could serve as an early warning system of lead exposure. The percentage cell viability (biomarker) used in this study have been useful in identifying toxic stress in earthworms caused by lead in roadside soils. The additional information obtained by using biomarkers could not be obtained by chemical analysis of soil and earthworms alone.