Surface water models serve as an integral part of the risk assessment process when the contaminant pathway or exposure media includes streams, rivers, estuaries, coastal regions, or open ocean. These models quantify the physical and chemical characteristics of the environmental compartments through which a contaminant moves. They also quantify the spatial and temporal extent of contaminant concentrations in surface water environments. These models are analytical tools, used to support ERA planing and subsequent assessment of remedial options, or pollution prevention strategies.