The role of sediment ingestion in exposing wood ducks to lead

Waterfowl on lateral lakes of the Coeur d'Alene River and on Lake Coeur d'Alene have been poisoned for many years by lead (Pb) from mining and smelting. In 1992 we undertook a study in the area to determine the importance of sediment ingestion in exposing wood ducks (Aix sponsa) to Pb. Digesta were removed from the intestines of wood ducks collected from contaminated and reference areas. The mean Pb concentration in the digesta of wood ducks from the contaminated area was 32 p.p.m. dry weight. The sediment content was estimated to average less than 2% of the dry weight of the wood duck diet. The lead concentrations in the digesta were closely correlated with the concentrations of acid-insoluble ash, Al, Ti and Fe in the digesta and these four variables are associated with sediment. Samples containing low concentrations of these variables also had low concentrations of Pb. These results suggest that most of the Pb in the digesta came from ingested sediment, rather than from plant material in the diet. The importance of ingested sediment as a source of lead was unexpected, because wood ducks are surface feeders on aquatic plants and they rarely dabble beneath the surface or feed on the bottom. However, it appears that sediment ingestion is sometimes the principal route of exposure to environmental contaminants that are not readily taken up by plants and invertebrates and this route should be considered in risk assessments of waterfowl.