A COMPARISON OF EXTRACTION METHODS THAT EVALUATE MOBILE AND BIOAVAILABLE TRACE METALS IN RIVER SEDIMENTS

Selective extraction methods for trace metals have been widely studied and reported including work by Tessier et al. (1979) and others. These methods all suffer from the same inability to accurately determine the concentration of metals in specific phases that might be related to an elements potential for remobilization or it’s bioavailability. Therefore, these methods are often referred to as operationally defined. Ure, et al. (1992) developed the BCR method that was subsequently adopted by the European Community Bureau of Reference. The method seeks to reduce the errors associated with previous methods. The BCR method describes three sediment phases: 1) extracted by dilute acetic acid, 2) reducible phases, and 3) oxidizable phases. While these phases may shed light on how trace metals behave under varying environmental conditions, it is not clear how they might relate to trace metal bioavailability.

In this study, the BCR method is compared to a single 1 M HCl extraction that has been used to describe the most readily bioavailable trace metal fraction in sediments. Sediment samples from the Buffalo River, NY were used in the study. These sediments have been impacted by regional industrial discharges, and show elevated levels of several trace metals including Pb, Cr, Ba and As. Focusing on Pb contamination, eight samples with varying concentrations of total Pb concentrations as determined by USEPA SW846 Method 3050B. Results show total extracable Pb values ranging from 12.5 to 743 mg/kg. Results of the BCR sequential extraction show considerable variability. Extraction 1 removed between 1.5% and 27% of the total extractable Pb, while extractions 2 and 3 removed between 69% and 92%, and 4% and 8%, respectively. Comparison of the BCR results with the 1 M HCl extraction show that the HCl extracted more Pb on a percentage of total basis than did the BCR extraction 1. The HCl extraction did not, however, correlate with the sum of any of the three BCR extractions. It is concluded that the HCl extraction removes the most, if not all, of the Pb associated with the BCR 1 fraction, and some of the reducible and oxidizable fractions. Furthermore, it seems that while the BCR method is reasonable to apply to trace metal cycling, it does not relate well to bioavailability.