Lead lability in alluvial soils of the river
Lead lability in alluvial soils of the river Trent catchment, U. K. M. Izquierdo, A. M. Tye, S. R. Chenery British Geological Survey, Keyworth, Nottingham, NG 12 5 GG, UK Pb SOURCES IN THE TRENT CATCMENT Pb IN TRENT ALLUVIAL SOILS 1. Pennine Ore Overbank sediments in the river Trent floodplains act as a focus for deposition 2. Coal Extensive Pb mining since pre-Roman times and short- to long-term storage of Pb Potential for increased Pb uptake to grazing Coal mining Coal combustion emissions Pb-rich sediments still pulse into the Trent tributaries livestock, terrestrial and aquatic ecosystems Pb can be released back into the drainage 3. ‘BHT’ Pb Floodplains downstream the Orefield are a primary Pb repository network through bank erosion Petrol-derived Diffuse source AIMS & METHODOLOGY 27 sampling sites paired topsoils (0 -15 cm) Pb source apportionment and subsoils (35 -50 cm) Pb Isotope dilution + extraction of pore waters 2. 490 SOURCE APPORTIONMENT 2. 470 solubility of Pb in alluvial soils from 2. 450 coal 208 Pb/207 Pb the River Trent and River Dove Pennine ore (galena) UK petrol lead pore water surface topsoil pore water subsoil London aerosols labile pool road dust sewage 2. 430 2. 410 2. 390 Pb LABILITY 50 % petrol Pb 40 BHT Pb in subsoils post depositional vertical migration of Pb from traffic related sources 2. 370 BHT (petrol) Pb Current UK regulations on metal contamination are based on the total concentration in soil no indication of the chemically reactive or labile UK coal Pennine Pb ore Pennine ore Pb dominant BUT BHT (petrol) Pb contribution=050% despite withdrawal in 2000 Examine the sources, lability and subsoil Pb= 40 -1280 mg/kg 30 20 10 0 -10 10 30 topsoil 2. 350 1. 070 1. 090 1. 110 1. 130 fraction of a given element in a soil system 1. 150 1. 170 1. 190 206 Pb/207 Pb ISOTOPE DILUTION TECHNIQUES (E-value) allow the determination of the reactive pool of metal in soils i. e. pool of metal in equilibrium with soil LABILE Pb=9 -56% over the total pool E-VALUE Isotopically exchangeable fraction: geochemically active – bioavailable for uptake L L L L L spike No differences in Pb contents with depth N N N L L L Strong p. H control on %E L 204 Pb N N L L N N N labile: soluble+adsorbed+solubilised in pore water N fixed Pb: occluded, inert, inaccessible No Pb >fixation with depth 0 2 3 p. H 5 4 7 6 8 p. H %E 50 evidence of continuous bank recycling Pb isotopic ratios in total, labile and pore water pools Enrichment of ‘BHT’ Pb in the bioavailable pools possible causes BHT Pb still being deposited (dry/wet deposition + present/recent dissolution of poorly soluble traffic related Pb-bearing species 208 Pb/207 Pb No source control on lability No differences in Pb lability with depth Km dowstream pore water 100 150 206 Pb/207 Pb BHT Pb bound to sub-micron Fe. O colloids in soil pore waters 250 1. 100 0 20 1. 200 40 60 Lability %E 80 100 ACKNOLEDGEMENTS 206 Pb/207 Pb M. Izquierdo, A. M. Tye, S. R. Chenery Sci Total Environ 433 (2012) 110 -122 This work has been funded by a Marie Curie Fellowship (Project no. 254983) to M. Izquierdo 50
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