An investigation of metal partitioning and organic pollution in surface sediments from Tees Bay and the Tees Estuary, UK

Abstract

Concern about the possible contamination by metals, hydrocarbons and PCBs of Tees Bay and the Tees estuary, and potential sources of such materials has led to this study of surficial sediments. Surface sediment samples were collected from the Tees estuary and Tees Bay and were analysed for grain size, organic carbon content, metals, hydrocarbons (aliphatic and polyaromatic) and polychiorinated biphenyls (PCB5). The Tees estuary sediments are largely organic-rich clayey-silts, while the Bay sediments are organic-poor sands. All of the metals measured (Cd, Co, Cr, Cu, Ni, Pb and Zn) have significantly higher concentrations in the sediments of the Tees estuary than in those of Tees Bay (F<0.01), and the mean averages decrease in the order Zn > Pb> Cr> Cu > Ni > Co> Cd. These significantly higher concentrations remain apparent when the data are normalized to remove the effect of grain size variation, suggesting that the sediments of the Tees estuary are enriched in metals in comparison with the sediments of Tees Bay and that these differences are not the result of the different grain size distributions. Metal partitioning was determined by sequential extraction, using the scheme of Tessier et al. (1979) as recently modified by Ajayi and Van Loon (1989), in order to assess the likely behaviour of metals in the sediments studied. The results of the sequential extraction showed that the partitioning patterns of Cd, Cr, Cu and Zn differed between the marine and estuarine sediments. In the Tees Bay sediments the residual fraction hosted >50% of all of the metals, except Cd and Pb. In the Tees estuary, however, this fraction only accounted for >50% of the Cd, Co and Ni. Significant portions of Cd are contained in fractions 1 and 2 in both the Tees estuary sediments and those from Tees Bay. Because of the toxicity and availability of Cd this may pose a serious problem to the ecosystem. The total Pb in the sediments is also quite high, particularly in the Tees estuary, and even the small proportion in fraction 2 could cause deleterious effects. High concentrations of Cd and Pd, in Tees Bay and the Tees estuary sediments, and Cr and Zn, in Tees estuary sediments, were associated with fraction 3. Concern is directed to heavy metals associated with fraction 3 because changing environmental conditions can transfer these elements from the sediments into the aqueous phase. Cu was the only metal significantly associated with fraction 4. This fraction is likely to have an important role as a sink for trace metals, particularly for Cu and to a lesser extent Cr and Zn. Petroleum hydrocarbons, both aliphatic and polyaromatic (PAHs), were detected and quantified in all samples. The distributions of the n-alkanes, acyclic isoprenoids (especially pristane from coal), triterpanes and steranes, in most sediments from both areas, were indicative of pollution by fossil fuels. On the other hand, a few sediments from the estuary contain hydrocarbons from both biogenic and anthropogenic sources. Polyaromatic hydrocarbon assemblages observed in the samples were relatively rich in hydrocarbons having petrogenic origins. However, the presence of pyrolytic derived components was also indicated by several characteristic component composition ratios. Single PCB congeners were determined in the sediments studied. The PCB profile is fairly constant in sediments from both areas and similar to that of Aroclor 1260 and 1254. The similarity to the commercial mixtures, with only a slight enrichment of the lighter congeners, indicates that sources responsible for PCB pollution are located in the Tees area. The sediments of the Tees estuary tend to be more highly polluted, by metals, hydrocarbons and PCBs, than those of the Bay, particularly in the middle section of the estuary. However, there is a general decrease in the concentrations of these compounds from the middle to the mouth of the estuary, while within the Bay area offshore sites showed higher concentrations. Distributions of inorganic and organic constituents in the Tees Bay were significantly influenced by the organic carbon content and particle size of sediments.