Distribution of heavy metals in sediments of the Gulf of Riga, Baltic Sea.

A large number of sediment samples (totally 138) were studied in 1991–1996 to clarify the role of sediments as a sink of heavy metals in the Gulf of Riga. The samples were analysed for total content of carbon, organic carbon, cadmium, lead, copper, zinc and mercury. Certain additional elements such as aluminium, lithium, iron, manganese, chromium, nickel, titanium and vanadium were also measured from some of the samples from the accumulation areas to enable combination with corresponding data from other parts of the Baltic Sea. The non-mineralogical portion of the heavy metals of some samples was estimated with nitric acid leaching. Heavy metal data for mean concentrations are shown separated into accumulation and non-deposition areas for 1, 2 and 5 cm sample intervals. Spatial distribution patterns are shown for the topmost 5 cm samples. The highest concentrations of metals are mainly found in the mud accumulation areas and in some specific cases, such as cadmium, in the near-shore areas. Lead, copper and zinc show a more widespread distribution over the whole Gulf. For copper and cadmium the presented vertical distributions of selected profiles show decreased accumulation trends during the past 30 years, while for other elements no similar pattern is identified. Comparisons with the Gulf of Bothnia and Gulf of Finland show that total concentrations of lead, copper and zinc are lower in the Gulf of Riga and cadmium and mercury are in the same range as those in the Gulf of Bothnia and Gulf of Finland.