MODELING CD PARTITIONING IN OXIC LAKE-SEDIMENTS AND CD CONCENTRATIONS IN THE FRESH-WATER BIVALVE ANODONTA-GRANDIS

Cadmium concentrations were measured at 49 littoral sites in 38 lakes distributed over 350,000 km2 in the provinces of Quebec and Ontario, both in the abiotic environment (oxic sediments and overlying water, diagenetic iron oxyhydroxide deposits on Teflon collectors) and in the soft tissues of the freshwater bivalve Anodonta grandis collected along a Cd contamination gradient. Concentrations of dissolved Cd above the water-sediment interface were highly undersaturated with respect to CdCO3(s). The partitioning of Cd between water and surficial oxic sediments is interpreted in terms of sorption of this metal to sedimentary organic matter and Fe oxyhydroxides, by means of surface complexation concepts. Binding intensity values for the sorption of Cd to Fe oxyhydroxides (K(Fe-Cd)) and organic matter (K(OM-Cd)) are estimated from field data (i.e. Cd concentrations in diagenetic Fe deposits, in the sediments, and in the water). The following empirical relationships between binding intensities and lake pH are found: log K(Fe-Cd), = 0.82 pH - 1.30 (r2 = 0.89) and log K(OM-Cd) = 0.9 7 pH - 2.45 (r2 = 0.78). Calculated Cd partitioning with these binding intensities indicates that Cd is bound mainly to organic matter in these sediments. Linear regression analysis indicates that Cd concentrations in the soft tissues of the bivalves, [Cd(Org)] (mug g-1 dry wt), are related to dissolved Cd concentrations: [Cd(Org)] = 44[Cd2+] + 10 (r2 = 0. 8 1) for [Cd2+ expressed in nmol liter-1. In these lakes, simple normalization of sedimentary extractable Cd with respect to sedimentary Fe oxyhydroxide or organic C concentrations proved inadequate for predicting Cd concentrations in A. grandis. Combining surface complexation concepts with the free-metal ion model of trace metal-organism interactions, we show that Cd concentrations in the soft tissues of A. grandis can be predicted with similar success as with the dissolved Cd concentration from water pH and sediment chemistry (total sedimentary Cd, Fe oxyhydroxides, and organic C concentrations).