Mass Balance Accounting of PCBs and Lead in the Aquatic Environment

Atmospheric mechanisms have been identified as major pathways for the input of toxic contaminants to oceans and lakes. Potential routes for this include scavenging by rain and snow, dry particulate deposition and vapour-phase exchange of contaminants. Concentrations and processes in the aquatic compartment must also be considered since they affect the exchange between the atmosphere and the water. In the Laurentian Great Lakes, monitoring for a number of contaminants has been carried out for many years. An assessment of the data available was undertaken to determine whether a budget for selected chemicals entering the different lakes could be drawn up. Only PCBs among the organics and Pb from the metals had sufficient data bases to reliably attempt this for all of the lakes. The total atmospheric contribution of PCBs to the loadings of the lakes was estimated to be between 7-90% of the overall loadings, depending on the lake; direct deposition was 6-90%. The corresponding figures for pb were 46-99.5% and 30- 99.5% respectively. For PCBs, the percent of the total "lost" from individual lakes to the atmosphere ranged from 46-87%; other loss mechanisms considered were outflow and sedimentation. Most of the Pb removed from the systems went to the sediments; atmospheric loss was not considered.