LEAD CONTAMINATION OF THE LONDON ORBITAL (M25) MOTORWAY (SINCE ITS OPENING IN 1986)

The Watford and South Mimms sections of the London Orbital (M25) Motorway opened in late 1986. The rate of lead contamination in vegetation, surface soil, dust and airborne fractions since those dates is presented. Lead values were determined by inductively coupled plasma-source mass spectrometry (ICP-MS) with inter-analytical method comparison using flameless atomic absorption spectroscopy (NFAAS). A consistently high level of correlation (r = +0.91) was obtained for analysis of digested reference materials and (r + +0.84) for digested environmental sample solutions using these two methods. Quality control assessment was made using seven international reference materials covering the range of matrices under investigation. Analytical precision values (n = 20 replicates) for ICP-MS measured Pb levels in soil and vegetative reference materials are 6.1 to 11.1% and 3.3 to 7.1%, respectively. Overall accuracy of the analytical procedures used in this study is very good with a high degree of agreement between measured ICP-MS values and those either certified or reported in the literature. A rapid rate of Pb deposition occurs within the first months following the opening of the motorway in relation to emissions from ∼90,000 vehicles day-1 (Watford) and ∼75,000 vehicles day-1 (South Mimms). At 12 months elevated Pb levels extend to distances of >100 m from the motorway. The average Pb content of motorway verge dust changes from 85 to 7200 μg g-1 (Watford) and 64 to 3472 μg g-1 (South Mimms) in the first 12 months. A maximum airborne Pb level of 6.0 μg m-3 (Watford) and 4.1 μg m-3 (South Mimms) is recorded at a distance of 5 m from the traffic flow of ∼120,000 vehicles day-1, Watford; and ∼98,000 vehicles day-1, South Mimms. The airborne Pb burden decreases exponentially to levels of <1.2 μg m-3 at distances of 120 m (down-wind of the motorway). Typical airborne Pb levels of <0.08 μg m-3 existed at these sites before the opening of the motorway. Lead levels at Watford and South Mimms after 24 months are approaching the values recorded at other locations around the M25 motorway which have been open for longer periods. The reasons for this are discussed. © 1990.