Measuring reactive metal in soil: a comparison of multi-element isotopic dilution and chemical extraction

The isotopically exchangeable metal pool (E-value) of zinc (Zn), cadmium (Cd) and lead (Pb) were simultaneously measured, using stable isotope dilution, in soils contaminated by Pb/Zn mining activities and varying in properties likely to affect metal reactivity, including pH, organic matter content, metal concentration and land use. E-values were compared with single and sequential extraction schemes. Results showed a wide range of metal reactivity (approximately 1–100% of total) depending on the extent of contamination and on the prevailing soil conditions. Across the range of soils, the E-values showed no consistent correspondence to any single chemical extraction procedure (EDTA, DTPA and HNO3) although there was reasonable agreement with the extractants 0.05 m EDTA and 0.43 m HNO3 in acidic organic soils. Extraction with 0.005 m DTPA substantially under-estimated the isotopically exchangeable metal content. E-values corresponded reasonably well with the exchangeable metal (fraction 1 (F1) of the sequential extraction procedure) in calcareous soils but relatively poorly and inconsistently with F1–F2, F1–F3 or F1–F4 in acidic-neutral soils. Operational aspects associated with determination of multi-element E-values are discussed.