In Search of Mercury Lost from Sediments in a Previously Contaminated Coastal Area, Harboøre Tange, Denmark

Concentrations of mercury in sediment and benthic invertebrate fauna of Nissum Broad, North-western Jutland, Denmark were investigated. The western coast of Nissum Broad is Harboøre Tange, along which heavy mercury contamination - caused by discharge from production of mercury containing seed dressers in the 1950 and 1960s - was documented in the 1980s. Recent investigations showed marked decreases in mercury contamination in the near shore sediments along Harboøre Tange since the 1980s and the present investigation was initiated to learn if the loss of mercury from Harboøre Tange had led to an increased mercury contamination in the neighbouring marine area, Nissum Broad. Mercury concentrations in the surface sediment correlated with the content of organic matter and the slope of the regression is a good indicator for the degree of mercury contamination. Average mercury concentrations in the upper 5 cm of the sediments ranged between 0.9 and 71 ng g^{- 1} dry weight (dw) with only 1 station exceeding the Background Assessment Concentration of 70 ng g^{- 1} dw. Average mercury concentrations in blue mussels Mytilus edulis (169-260 ng g^{- 1} dw) and periwinkles Littorina littorea (66-203 ng g^{- 1} dw) exceeded those in uncontaminated areas and the Environmental Quality Standard of approximately 100 ng g^{- 1} dw. Present sediment mercury concentrations in Nissum Broad are approximately half of what they were in the 1980s, rendering it unlikely that mercury lost from Harboøre Tange has been deposited there. Sediment and organism concentrations did not show any correlation.