Lead lability in alluvial soils of the river Trent Catchment, U.K.

Abstract

Fluvial environments are a major pathway for the dispersal of trace pollutants, whilst alluvial soils act as historical repositories of contaminants from different sources. The UK has an extensive Pb mining history in areas such as the Pennines. We investigated the spatial changes in sources and bioavailability of Pb in alluvial soils of the River Trent floodplain. We used Pb isotope geochemistry and isotope dilution methods to identify the sources and measure reactive pools of Pb in 38 paired topsoils (0-15 cm) and subsoils (35-50 cm). Lability of soils varied between 9-56%, with little difference between top and subsoils as a result of recycling of river bank soils. Soil pH was negatively correlated with lability. Source apportionment using 206Pb/207Pb and 208Pb/207Pb ratios showed that the isotopic ratios in the total, labile and pore water pools fitted along a mixing line between “Broken Hill Type (BHT)” Pb, used as additive before the phasing out of leaded petrol, and the Midlands coal/ Pennine ore Pb. Results showed that BHT Pb migrated downwards and reached 50% of total Pb in some sites. Statistically significant differences (P<0.05) in the isotopic composition of Pb in the total, labile and pore water pools suggested an enrichment in BHT Pb in the labile and pore water pools. It is likely that there are still BHT Pb additions into the alluvial soils at present and/or that anaerobic conditions induced by flooding helps maintain an enriched pool of recently deposited BHT Pb in the labile and pore water pools