Distribution of mercury in the upper sediments from a polluted area (Ria de aveiro, Portugal)

Abstract

It is well known that chlor-alkali industries using mercury as a cathode are an important source of this heavy metal to aquatic environments. Once discharged into rivers, lakes or coastal waters, mercury accumulates in sediments and in the aquatic food web (Gonzalez, 1991). Both elemental mercury and its compounds are noxious to all living organisms, and are bioaccumulative and persistent. Metal concentrations in sediments usually exceed those of the overlying water column. As a consequence, metals originating from human activities can often be identified more readily by analysis of sediments than by the quantification of metal concentrations present in water (Haynes et al., 1995). Sediments also integrate the temporal variability that characterizes metals originating from human sources. However, metals may be remobilized during the early stages of post-depositional transformations of the sediment (Gobeil and Cossa, 1993). Ria de Aveiro is a long, narrow, coastal lagoon, of 43 km wet area with a single, artificially maintained connection to the sea (Fig. 1). The coastal plain around the lagoon supports an intensive, diversified agriculture, a variety of heavy and light industries and a population of about half a million people, a part of which discharge their untreated or partially treated sewage into the lagoon (Lucas et al., 1986). An extensive web of islands and channels make water circulation inside the lagoon difficult and complex, thus allowing the spread of any conservative contaminants before they are discharged into the costal waters through the single sea mouth. One such contaminant is the mercury, released by a chlor-alkali plant into one of the remotest branches of the lagoon—Estarreja Channel (Fig. 1) (Hall et al., 1988). The Channel ends in a basin (Laranjo Basin) where most of the discharged mercury settles. Laranjo Basin has an area of about 1.5 km and is totally emptied in most ebb tides. The average volume of water exported with each tidal cycle may be around 2 km (Lucas et al., 1986). This work reports levels of Hg, Fe and Mn and sulphides in the upper sediments of the Laranjo Basin, and examines the influence of these elements on the distribution of mercury. Short sediment cores (10 cm) were collected by hand at sites A, B and C of Ria de Aveiro (Fig. 1). Cores were sampled in March, May, June 1996 and April 1997. The sediments were sliced in situ at the following depths: 0– 2, 2–4, 8–10, 14–16, 24–26, 34–36, 44–46, 54–56, 64–66 and 94–96 mm. The samples were rapidly transferred to vials which were placed under nitrogen atmosphere and frozen. In the laboratory, part of the sediments samples Edited by Bruce J. Richardson