Tetra Tech worked closely with EPA Region IX and a multiple-agency stakeholder group called the Central Valley Drinking Water Policy Workgroup to develop conceptual models that represent the sources, transformation, and transport of the critical constituents of drinking water concern in the waters of the Central Valley. California’s Central Valley, comprising the Sacramento and San Joaquin watersheds and the Delta, is a vital source of freshwater for more than 20 million people in the state. Although the tributaries of the Sacramento and San Joaquin Rivers that originate in the Sierras are high quality sources of drinking water, as they flow into lower elevations, they are affected by urban and agricultural pollution. The pollution threat posed to these waters is a major public policy concern. The purpose of the Central Valley Drinking Water Policy is to develop water quality objectives for constituents of concern for drinking water supply, including pathogens, nutrients, and precursors of disinfection by-products such as organic carbon. The conceptual models developed by Tetra Tech are being used to identify data gaps, serve as a tool to evaluate decision-making support tools, and as the starting point for conducting more detailed mechanistic modeling in the future.