Brief Summary

Surface sediments from the outer Thames Estuary, UK. Extraction of MAs from the sediment solid phase was carried out using a widely used single-volume extraction with 2 M KCl (20 mL per g dry sediment) and compared with a multi-volume extraction (3 x 20 mL followed by 3 x 40 mL). The multi-volume technique recovered much higher amounts of MAs, with the single-volume step rarely recovering more than 50 % of the multi-volume total.

Experiments were carried out in which sediment samples were resuspended in overlying water in order to measure the rate of MA release from the sediments. This occurred rapidly and maximum concentrations were usually measured after 2 h, with desorption from the solid phase accounting for most of the MAs released into solution. The measured concentrations at the end of the experiment were greater than those recovered through single-volume and, in some cases, multi-volume extraction, suggesting that single-volume extraction can seriously underestimate the bioavailable MAs in sediments. SPM concentrations, which were measured through a tidal cycle at one site, increased as high tide approached and showed that the sediment concentrations used in the resuspension experiments were reasonably close to those measured in the field.

The results of this study show that the mobility and bioavailability of ON in estuarine sediments may not be accounted for using single-volume extraction, while factors such as resuspension must be evaluated in macro-tidal systems.

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