Retention of trace metal contaminants in the sediment at an accumulating sewage sludge disposal site

Abstract

Since 1974 sewage sludge has been dumped in the Firth of Clyde at a position 6 km south of Garroch Head point on the Isle of Bute. The water currents in this area allow sludge solids to settle on the sea bed, resulting in the accumulation of trace metal contaminants in the sediments at the disposal site. The present study was carried out to investigate the degree to which the dumped metal contaminants are retained within the sediments. The quantity of trace metals in the sludge dumped at the site since 1974, calculated from historical records, was compared with the quantity of trace metal in the sediment which could be ascribed to the sludge disposal. This comparative study showed that 36–38% of the copper and mercury and 18–20% of the cadmium, lead and zinc from the sludge could be accounted for by elevations in trace metal concentrations in the sediments. The sensitivity of these estimates to uncertainties in the values of the various parameters of the calculations are discussed.