Lead in grass in the Scottish uplands: deposition or uptake?

Abstract

If it is assumed that the Pb collected in grass samples is derived mainly from atmospheric deposition then grass samples can be used as a convenient and easily analysed monitor for Pb deposition, in particular to establish the isotopic composition of current deposition in remote locations. As some studies have demonstrated a strong correlation between soil and grass Pb concentrations, it was considered important to establish the proportion of soil Pb in the grasses used to monitor atmospheric deposition at upland locations in Scotland. Consideration of earlier studies provided evidence that very little, if any, Pb in grass was derived from soil. Lead isotope analysis, by thermal ionisation mass spectrometry, of grasses grown on soils spiked with enriched ²⁰⁷Pb in the field situation allowed the relative contribution of atmospheric deposition and soil to the grass Pb to be calculated. In most cases, >80% of Pb in grass was derived from atmospheric deposition and in recent years this value was >90%. Recalculation of the ²⁰⁶Pb/²⁰⁷Pb ratio in grass samples showed that there was very little error in the results, published previously, which were based on the assumption that all the Pb collected with grass was derived from the atmosphere. The trends established by measuring the ²⁰⁶Pb/²⁰⁷Pb ratio in grasses were confirmed and remain valid.