Estuaries Vol. 20, No. 3, p. 494-503 September 1997 Trends in the Solid Phase Partitioning of Metals in the Thames Estuary Sediments During Recent Decades S. B. O'REILLY WIESE R. H. C. EMMERSON C. L. MACLEOD J. N. LESTER 1 Environment and Water Technology Group Centre for Environmental Technology Imperial College of Science, Technology and Medicine London SW7 2BU United Kingdom ABSTRACT: A sequential extraction method was employed to extractthe metals A1, Ag, Cd, Co, Cr, Cu, Pb, Fe, Li, Mn, Ni, and Zn from a 10-m sediment core taken from the Tilbury Basin on the Thames Estuary. Characteristics of the observed metal partitioning distributions were attributed primarily to the composition of the estuarine waters at the time of deposition. For s o m e metals, a decrease in the bulk sediment metal concentrations from a depth of 6.59 m ODN to the surface was also observed in one of the solid phases. This was the case for Cr, Cu, and Pb extracted from the organic phase and for Zn extracted from the carbonate phase. This decrease in sediment concentrationsis thought to reflect reported improvements to water quality in this region of the Thames Estuary in the early 1960s, following updating of major sewage treatment works (STW) approximately 20 km upstream. These findings give an indication of the influ- ence of estuarine inputs from STW on metal partitioning distributions. The order of mobility for the metals of environ- mental concern was Cd > Ag > Cr >Ni, Zn > Co, Cu, Pb. For Cd and Ag there was a tendency to partition towards the exchangeable phase, both at the surface and at depth, which indicates the potential for long-term leaching of these metals from the sediments. Introduction A sediment is a complex assemblage of hetero- geneous particles, in turn comprising different or- ganic and inorganic components. Metals of geo- chemical origin are found primarily incorporated into crystalline silicates (Jenne 1977). Those met- als originating from polluted waters are associated with more labile components of the sediment such as Fe and Mn oxides, carbonates, amorphous alu- minosilicates, and organic matter (Martin et al. 1987). The type of association between metals and sediments is of great importance because this will greatly affect their environmental significance with regard to the biota (Luoma 1983; Wang 1987; Pro- si 1989; Bryan and Langston 1992; Richard and Bourg 1991). It is, therefore, difficult to interpret data on total sediment metal concentrations with regard to their sources, mobility, and the implica- tions for the biota. To address these points, the sequential extraction of metals from sediments has been put forward as a way of assessing the strength of association between the metals and the sedi- ments (Gibbs 1973; Gupta and Chen 1975; Bran- Corresponding author; tele 44 0171 594 6013; email j.lester@ic.ac.uk. non et al. 1976; Tessier et al. 1979 and F6rstner et al. 1981). The most widely used method for the sequential extraction of metals from sediments is that of Tes- sier et al. (1979). This method presents operation- ally defined sediment fractions as those solublized by five different extraction steps; the fractions are exchangeable, carbonates, Fe and Mn oxides, or- ganic matter, and residual. The mobilization po- tential of a metal is commonly characterized by the ease with which it may be extracted from a sedi- ment. Metals enriched in the exchangeable frac- tion are considered to be more mobile that those held in the organic or residual fractions. It should be noted that the amount of metal extracted in each step will vary according to the chemical na- ture of the element, the sediment component mix- ture and the type of association between the two. Also, metals may be bound to a particular sediment component with varying degrees of strength (Kim and Fergusson 1990). However, despite the well- documented potential problems associated with this method (Kheboian and Bauer 1987; Martin et al. 1987; Nirel and Morel 1990; Carter et al. 1992), it has still proved to be a useful tool in the study of the behaviour of anthropogenic metals in sedi- 9 1997 Estuanne Research Federation 494