The use of isotopic composition in field studies of lead in upland Scottish soils (U.K.)

Experimental plots were set up originally at two research stations in upland areas of Scotland with the intention of characterising environmental samples using lead isotopic composition. The further use of isotopic composition to trace the movement of lead in the soil and along the food chain from soil to plants to animals requires well-characterised sources. The distribution of lead within soil, both in terms of concentration and isotopic composition, was however, found to be heterogeneous with accumulations at the surface characterised by lower 206Pb/207Ph ratios than at greater depth. A procedure was therefore developed to homogenise soils used in field studies and, at the same time, give the opportunity to introduce enriched stable isotopes not only of lead but also other metals of interest. Soil was removed from the plots, mixed by rolling, and spiked with enriched 207Pb. This increased the 207Pb/204Pb ratios from ∼15.5 to within the range 16.6–17.5 and decreased the 206Pb/207Pb ratio to below 1.087, both of which are well outside the range normally found in soils. The mixed soils were homogeneous in terms of isotopic composition and presented a much more precisely characterised soil for use in tracer studies. Up to 50% of the lead in unwashed grass grown on the spiked soils had its origin within the soil and at least 10% of this remained after washing. Furthermore, ∼ 10% of the lead in grass grown on undisturbed soil adjacent to, but separated from, the spiked soil was shown to have its origin in the spiked soil. The technique provides a means of introducing enriched spikes of most metals and, in combination with precise analysis by thermal ionisation mass spectrometry, offers high sensitivity in studies of metals within the soil and plant environment.