Comparative evaluation of residual and total metal analyses in polluted soils

Two digestion procedures, employing aqua regia-HF (ARHF) and HNO₃-HClO₄HF (HHH), were used to analyze residual metals (following a chemical fractionation scheme) and total metal content of two soils, one moderately polluted by municipal sludge applications and the other a grossly-contaminated sample (20.8% Pb) from a battery recycling site. Although commonly used in sequential extraction analyses, the ARHF method solubilized only 53% (significant at p = 0.05) of the HHH-determined residual Pb in the battery soil. For the sludge-amended soil, residual Cd, Pb, and Zn were not statistically different by the two methods. For the battery soil, a single ARHF extraction also underestimated total Pb and Cu relative to HHH, but both methods gave statistically-similar total Cd, Cu, Pb, and Zn for the sludge-amended soil. As the sample metal concentration increased, the ability of ARHF to solubilize HHH-equivalent metal quantities generally decreased. Since the degree of contamination is often unknown for environmental samples, the HHH method is more reliable for assessing residual and total metals in polluted soils.