In 2010, King County was awarded a Puget Sound Action Agenda: Technical Investigations and Implementation Assistance Grant by the U.S. EPA to estimate loading of PCBs and PBDEs to Lake Washington, Lake Union and Puget Sound; and model potential reduction in Lake Washington fish tissue concentrations associated with select PCB loading reduction scenarios. A field study was designed and implemented from 2011 to 2012 to measure PCB and PBDE concentrations in key contaminant loading pathways to Lakes Washington and Union (i.e., rivers, streams, stormwater, CSOs, highway bridges and atmospheric deposition) and measure the concentrations in the export pathway leaving the lake system through the Ship Canal locks to Puget Sound.  By combining the contaminant concentration data with long term flow estimates for these pathways, mass loading estimates to Lakes Washington and Union and subsequent export to Puget Sound for total PCBs and total PBDEs were developed.  The approach to estimating pathway loads and loading estimates for each major pathway will be presented.