Accumulation of lead and manganese in soil along the N1 highway in the City of Cape Town after the banning of leaded petrol in South Africa

Abstract

Soil contamination by metals is a serious environmental problem that has significant implications for human health. Roadside soils have been shown to have considerable contamination due to depositions of metals by vehicles. Metal pollution poses significant environmental concerns because most metals are not biodegradable and often have long half-lives, thus predicating far reaching effects on biological systems, including the soil. Unleaded petrol has been available in South Africa since 1996. The conversion from leaded to unleaded petrol has been a slow process. Leaded petrol has been totally phased out since January 2006. The manganese-containing fuel additive methylcyclopentadienyl manganese tricarbonyl (MMT) that was introduced to automobile fuel formulae as an octane boosting and “anti-knock” agent, to replace or reduce the lead content in petrol, was officially approved in some developed countries. Despite the fact that metal contamination of soil has long been known, few studies have been carried out into its vertical distribution in the soil. The objectives of this investigation were: firstly, to assess the accumulation of lead and manganese in soil along the N1 highway; secondly, to determine the vertical distribution of lead and manganese in soil along the N1 highway; and thirdly, to compare the concentrations of lead and manganese in soil along the N1 highway, before and after the banning of leaded petrol in South Africa. Six soil samples were taken at each site approximately two meters from the road verges and at a depth of approximately 0-2cm of surface soil. The sampling period commenced on the 16th of February 2011 and ended on the 28th of December 2011. Once-off soil samples were also collected at sites 1, 5 and 6 at depths of 0-2cm, 30cm and 60cm, respectively. Samples were digested with 10 ml 55% nitric acid. Lead and manganese concentrations were determined by using Inductively Coupled Plasma Mass Spectrophotometer (ICP–MS). The mean concentrations of lead found in the roadside soils of the N1 highway ranged between 7.32mg/kg and 2068.31mg/kg and these were of high concentrations when compared to studies done in other countries. Results from the vertical distribution of lead investigation showed that lead concentrations ranged from 52.72mg/kg to 215.94mg/kg at surface level, from 15.80mg/kg to 164mg/kg at a depth of 30cm and from 14.06mg/kg to 216.07mg/kg at a depth of 60cm from the surface. It was also shown that there is a positive correlation between lead concentration and the amount of organic content (although not statistical), concluding that as the amount of organic content increases lead concentrations also increase. The mean manganese concentrations found in the roadside soils of the N1 ranged between 12.17mg/kg and 221.47mg/kg. The levels of manganese in the soil were found to be relatively low when compared to other studies an indication of mild to low metal contamination of the sampled soils. Results from the vertical distribution of manganese concentrations showed that concentrations in the soil ranged from 39.23 mg/kg to 63.32 mg/kg at surface level, from 14.4mg/kg to 310.86mg/kg for depths of up to 30cm, and from 4.42mg/kg to 343.96mg/kg for depths of up to 60cm. No relationships of manganese levels in the soil were found at any of the sites when sites were compared prior to the banning of lead and after the banning of lead despite the increased traffic volumes and, thus, increased MMT usage over the years, indicating that manganese contribution from MMT is very low and does not significantly increase soil contamination along the N1 highway. However, further investigations are needed into the future to monitor manganese contamination that may possibly occur.