Current trends in the isotopic composition of lead deposited from the atmosphere in upland locations in Scotland

Over the period 1989 to the present, the steady increase in the Pb-206:Ph-207 ratio in lead collected with grass samples from two upland locations in Central and North East Scotland has not been as great as might have been expected following the decreased use of leaded petrol. Over recent years there has been little increase in the ratio (1.13-1.15 dependent on location) and a steady state appears to have been reached. The relatively low ratios suggest that either the withdrawal of leaded petrol from sale has not completely removed petrol as a source of lead in the environment or that other sources of lead also have relatively low Pb-206:Pb-207 ratios. Assuming that lead deposited from the atmosphere has only two major sources, petrol lead and a diffuse background "industrial" lead, then the proportion of petrol lead was in the range 17-69% for roadside samples and 0-39% for other samples. A substantial proportion of deposited lead has its origin other than in petrol but the nature of the other component varies according to the location of the sampling site. Lead isotope analysis of grass from within one catchment shows that the isotopic composition of the deposition varies according to a number of factors including distance from traffic, relative altitude and the degree of shelter.