DISTRIBUTION AND TEMPORAL VARIATION OF CADMIUM IN THE ST. LAWRENCE RIVER BASIN

Samples of raw water were collected at regular intevals at two transec ts in the St. Lawremce River and four of its tributaries from March to November 1991 and from April to June 1992. Water samples were analyze d for both the dissolved and the particulate phase for cadmium (Cd), o rganic carbon, iron and manganese. Mean dissolved Cd concentration was 10 +/- 5 ng/L and no spatial variability was observed. Higher concent rations were found during high flow periods, suggesting an uptake of c admium by phytoplankton during summer. In addition, dissolved cadmium did not appear to be associated with either DOC, dissolved Fe or disso lved Mn. The mean particulate Cd concentration was 1.3 +/- 1.1 mu g/g, with almost all stations presenting the same concentration except the Yamaska River, which had a concentration of 0.5 +/- 0.2 mu g/g. Parti culate Cd showed a negative correlation with suspended particulate mat ter and a positive correlation with particulate organic carbon and par ticulare manganese. Fifty-nine percent of the cadmium was found to be in the particulate phase. Partition coefficients for cadmium (Kd), org anic carbon (Kc), iron (KdFe) and manganese (KdMn) were calculated for each sample. Log Kd varied from 3.9 to 5.9, with an average of 5.0 +/ - 0.4. Log Kd decreased with increasing particulate matter as did Log Kc and Log KdMn. No significant correlation was found between Log Kd a nd Log Kc, suggesting that the distribution of cadmium between the dis solved and the particulate phase is not influenced by the distribution of organic carbon. In contrast, positive correlations were observed b etween Log Kd, Log KdFe and Log KdMn. Cadmium distribution appears to be influenced by Fe and Mn distribution.