Selenium has been a concern in the Central Valley and San Francisco Bay for several decades, a region with selenium sources from San Joaquin Valley and various point sources including refineries. A TMDL project is underway to address selenium toxicity in North San Francisco Bay led by the San Francisco Bay Regional Board. With funding provided by WSPA, Tetra Tech is providing technical support to the Regional Board for the development of the TMDL which includes (i) an independent toxicological assessment, (ii) identification and characterization of selenium sources to the North Bay, (iii) development of a conceptual model to represent the processes that control the uptake of selenium by wildlife, and (iv) development of a numerical model of selenium for the bay. The model has been calibrated to flow, water quality, and tissue selenium data from the Delta and Bay, and was used to evaluate the role of individual loads on water column and biota concentrations in the bay. In future, the model will be used to quantify selenium loads and develop and assign waste load and load allocations among sources. Because of the high profile of selenium issues in the bay, and the long history of scientific research on the issue, all Tetra Tech work products were extensively reviewed by a technical review committee of nationally known experts. There was also an extensive outreach and review by a stakeholder group from the bay and Central Valley, that was formed for this project. Future phases of this work include additional monitoring in the bay, and the development of an implementation plan to achieve the TMDL and protect beneficial uses.